Conflict management
in wild chimpanzees (Pan troglodytes)
Von der Fakultät für Biowissenschaften, Pharmazie und Psychologie
der Universität Leipzig
genehmigte
DISSERTATION
zur Erlangung des akademischen Grades
doctor rerum naturalium
Dr. rer. nat.
vorgelegt von
Diplom-Biologe Roman Wittig
geboren am 23.07.1968 in
Karlsruhe
Dekan:
Prof. Dr. Kurt Eger
Gutacher:
Prof. Dr. Christophe Boesch
Prof. Dr. Peter Kappeler
Prof. Dr. Frans B. M. de Waal
Tag der Verteidigung: 19.12.2003
Preface
to Fynn and Cathy
ii
Preface
This thesis is partly based on the following publications and manuscripts:
1. Wittig, R. M. & Boesch, C. (2003a). Food competition and linear dominance
hierarchy among female chimpanzees of the Taï National Park. International Journal
of Primatology. 24: 847-867.
2. Wittig, R. M. & Boesch, C. (2003b). 'Decision-making' in conflicts of wild
chimpanzees: An extension of the Relational Model. Behavioral Ecology and
Sociobiology. 54: 491-504.
3. Wittig, R. M. & Boesch, C. (2003d). The choice of post-conflict interactions in wild
chimpanzees (Pan troglodytes). Behaviour. 140: 1527-1559.
4. Wittig, R. M. & Boesch, C. (in review). How to repair relationships in wild
chimpanzees (Pan troglodytes). Ethology.
iii
Table of contents
TABLE OF CONTENTS
NATURE OF CONFLICTS AND CONFLICT MANAGEMENT IN ANIMALS - AN
INTRODUCTION
1
1.1 THE NATURE OF CONFLICTS
1.2 CHANGING THE VIEW OF AGGRESSION
1.3 COSTS AND BENEFITS OF AGGRESSION
1.4 BENEFIT OF CONFLICT MANAGEMENT – COST REDUCTION
1.5 CONFLICT MANAGEMENT IN WILD CHIMPANZEES OF THE TAÏ NATIONAL PARK
2
2
3
5
6
BIOLOGY OF COMMON CHIMPANZEES (PAN TROGLODYTES)
8
2.1 DISTRIBUTION OF CHIMPANZEE
9
11
2.2 SOCIAL STRUCTURE AND MATING SYSTEM OF CHIMPANZEES
2.3 DIET AND ASPECTS OF FEEDING ECOLOGY OF CHIMPANZEES
13
2.4 RECONCILIATION (AND CONFLICT MANAGEMENT) IN CHIMPANZEES – A BRIEF HISTORY
14
GENERAL METHODS OF THE STUDY
16
3.1 DATA COLLECTION
3.1.1 STUDY SITE
3.1.2 OBSERVATION PROCEDURE
3.1.3 ETHOGRAM
3.2 OPERATIONAL DEFINITIONS
3.2.1 BEHAVIOURS
3.2.2 CONFLICT VARIABLES
3.2.3 CONTEXT VARIABLES
3.2.4 DOMINANCE VARIABLES
3.2.5 SOCIAL VARIABLES
3.2.6 VARIABLE ADAPTATIONS FOR STATISTICAL COMPARISON
3.3 MULTIVARIATE DYADIC APPROACH
3.4 STATISTICS
3.4.1 MULTIVARIATE ANALYSIS
3.4.2 DYADIC PERMUTATION TESTS
3.4.3 PROGRAMS
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26
FOOD COMPETITION AND LINEAR DOMINANCE HIERARCHY AMONG FEMALES
27
4.1 INTRODUCTION
4.1.1 HIERARCHIES IN CHIMPANZEES
4.1.2 SOCIO-ECOLOGY OF DOMINANCE RELATIONSHIPS
4.1.3 RELATIONSHIPS IN CHIMPANZEES
4.2 SPECIFIC METHODS
4.2.1 DATA AND TEST CONDITIONS
4.2.2 OPERATIONAL DEFINITIONS
4.2.3 STATISTICS
iv
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29
31
31
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32
Table of contents
4.3 RESULTS
4.3.1 FEMALE HIERARCHY
4.3.2 FOOD COMPETITION
4.3.3 CORRELATES OF THE LINEAR HIERARCHY
4.3.4 SOCIAL RELATIONSHIPS
4.4 DISCUSSION
4.4.1 SUMMARY OF RESULTS
4.4.2 DOMINANCE RELATIONSHIPS
4.4.3 CONTEST COMPETITION
4.4.4 REASONS FOR LINEARITY
4.4.5 COMPARISON AMONG STUDY SITES
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41
DECISION-MAKING IN CONFLICTS: EXTENSION OF THE RELATIONAL MODEL
43
5.1 INTRODUCTION
5.1.1 THE RELATIONAL MODEL (RM)
5.1.2 EXTENSION TO THE RELATIONAL MODEL
5.1.2.1 Likelihood of winning
5.1.2.2 Conflict duration and cost of aggression
5.1.2.3 Creation of social costs
5.1.3 TESTING THE EXTENDED RELATIONAL MODEL
5.2 SPECIFIC METHODS
5.2.1 DATA AND TEST CONDITIONS
5.2.2 ADDITIONAL OPERATIONAL DEFINITIONS
5.2.3 SPECIAL ANALYSIS
5.3 RESULTS
5.3.1 STEP 1: BENEFIT
5.3.1.1 Benefit of the resource
5.3.1.2 Likelihood of winning
5.3.2 STEP 2: COST OF AGGRESSION
5.3.2.1 Conflict intensity
5.3.2.2 Conflict duration
5.3.3 STEP 3: SOCIAL COSTS
5.3.3.1 Creating social costs
5.3.3.2 Reducing of social cost
5.4 DISCUSSION
5.4.1 SUMMARY OF RESULTS
5.4.2 GRABBING THE BENEFIT
5.4.3 ECONOMIC HANDLING OF THE COSTS OF AGGRESSION
5.4.4 MINIMISING SOCIAL COSTS
5.4.5 THE EXTENDED RELATIONAL MODEL
44
44
45
45
46
46
46
48
48
49
49
51
51
51
52
53
53
54
55
55
57
59
59
59
60
61
62
CHOICE OF POST-CONFLICT INTERACTIONS
64
6.1 INTRODUCTION
6.1.1 ADVANTAGES AND DISADVANTAGES OF POST-CONFLICT INTERACTIONS
6.1.2 POOL OF PCIS
6.1.2.1 Reconciliation
6.1.2.2 Consolation
6.1.2.3 Redirected aggression
65
65
66
66
66
67
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Table of contents
6.1.2.4 Renewed aggression
6.1.2.5 Avoidance of interactions (no PCI)
6.1.3 TESTING THE CHOICE OF PCI
6.2 SPECIFIC METHODS
6.2.1 DATA AND TEST CONDITIONS
6.2.2 ADDITIONAL OPERATIONAL DEFINITIONS
6.2.3 SPECIAL ANALYSIS AND STATISTICS
6.3 RESULTS
6.3.1 ARE PCIS DEPENDENT ON THE CONFLICT?
6.3.2 THE EFFECT OF CONFLICT DURATION
6.3.3 AVOIDANCE OF INTERACTIONS (NO PCI)
6.3.4 NORMAL INTERACTIONS – BUSINESS AS USUAL
6.3.5 CHOOSING A POST-CONFLICT INTERACTION
6.3.5.1 Reconciliation
6.3.5.2 Offered consolation
6.3.5.3 Renewed aggression
6.3.5.4 Redirected aggression
6.3.5.5 Third party aggression
6.4 DISCUSSION
6.4.1 SUMMARY OF RESULTS
6.4.2 BUSINESS AS USUAL
6.4.3 AVOIDING FURTHER INTERACTIONS
6.4.4 RECONCILIATION
6.4.5 CONSOLATION
6.4.6 RENEWED AGGRESSION
6.4.7 REDIRECTED AGGRESSION
6.4.8 THIRD PARTY AGGRESSION
6.4.9 CHOICE OF PCI: GENERALISED RULES
67
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70
70
71
71
72
73
73
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83
84
84
85
HOW ARE RELATIONSHIPS REPAIRED ?
87
7.1 INTRODUCTION
7.1.1 WHY REPAIR RELATIONSHIPS?
7.1.2 WHAT DAMAGES RELATIONSHIPS?
7.1.3 HOW TO REPAIR THE DAMAGE? VARIATION IN RECONCILIATION
7.2 SPECIFIC METHODS
7.2.1 DATA AND TEST CONDITIONS
7.2.2 ADDITIONAL OPERATIONAL DEFINITIONS
7.2.3 MEASURING RECONCILIATION
7.3 RESULTS
7.3.1 OCCURRENCE OF RECONCILIATION
7.3.2 FUNCTION OF RECONCILIATION
7.3.3 VARIATION WITHIN LATENCY AND DURATION OF RECONCILIATION
7.3.4 VARIATION WITHIN COMPLEXITY OF RECONCILIATION
7.3.5 INITIATOR OF RECONCILIATION
7.4 DISCUSSION
7.4.1 SUMMARY OF RESULTS
7.4.2 FUNCTION OF RECONCILIATION
7.4.3 THE VARIATION WITHIN RECONCILIATION
7.4.4 REPAIRING THE DAMAGE
vi
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94
94
94
97
99
99
100
100
101
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Table of contents
OPEN QUESTIONS AND POSSIBLE ANSWERS - A GENERAL DISCUSSION
106
8.1 CONTRIBUTION OF THIS STUDY TO OUR KNOWLEDGE IN CONFLICT MANAGEMENT
8.2 AVOIDANCE AND PREVENTION OF CONFLICTS
8.3 CONFLICT REGULATION IN A WIDER SOCIAL CONTEXT
8.4 EFFECTIVENESS OF THE MECHANISMS OF CONFLICT MANAGEMENT
8.5 SIMILARITY OF CONFLICT MANAGEMENT IN HUMANS AND CHIMPANZEES
8.6 PROSPECTS
107
108
110
111
113
114
SUMMARY
116
9.1 SUMMARY
9.2 ZUSAMMENFASSUNG
116
118
ACKNOWLEDGEMENTS
123
BIBLIOGRAPHY
125
APPENDICES
149
CURRICULUM VITAE
159
DECLARATION OF INDEPENDENCE
160
vii
Chapter 11:
Nature of conflicts and conflict management in
animals - an introduction
Figure 1.0 Male Marius is sitting on a tree-fall after fighting a competitor over a female in
oestrous (Photo by R. Wittig).
1
The present study focuses on social (group-living) animals
Chapter 1: Nature of conflicts and conflict management – an introduction
1.1 The nature of conflicts
Living in groups holds many advantages. Social partners may increase protection
against predators (Hamilton, 1971; Alexander, 1974), they can provide support in
competition (de Waal, 1978), they facilitate food acquisition (Janson, 1988a; Boesch &
Boesch, 1989) and they may help in caring for ones’ offspring (O'Brien, 1993; Muroyama,
1994). To increase these advantages individuals build cooperative relationships
(Wrangham, 1980), that usually reflect long-term investments and can change over time
(Kummer, 1978; Hinde, 1979).
Social partners, however, can also be disadvantageous, as they compete for the
same resources or they pursue different, even contrary, goals. An individual’s mating
success, food acquisition and spatial patterns are often influenced by the behaviour of the
social partners (Hamilton, 1967; Eisenberg et al., 1972; Parker, 1974; Clutton-Brock &
Harvey, 1978). When competing for the same resource social partners face a problem:
both claim the same resource, but often only one can possess it (e.g. food competition:
Hand, 1986). When pursuing contrary goals social partners face another problem in as
much that both try to achieve different aims. In order to do so, however, they may need
to prevent the other from realising their aim (e.g. weaning conflict: Trivers, 1974, conflict
of the sexes: Dawkins, 1976). Such situations are defined as conflicts of interest
(McEnery, 1985; van der Dennen & Falger, 1990). A conflict of interest often escalates
and results in aggression. Aggressive interactions are then called open conflicts, or for the
sake of simplicity, just conflicts.
1.2 Changing the view of aggression
The traditional view of aggression implied a spontaneous, uncontrollable and
antisocial nature to conflicts. This idea derived from the concept that aggression is an
independent instinct, which is driven by a number of internal and external variables
(Lorenz, 1963; Hinde, 1967; Tinbergen, 1968). This concept suggested that the
motivation for aggression could increase up to a threshold where aggression would erupt
spontaneously. The conditions for opening such a pressure relief valve would be situations
in which aggression had not been vented for some time. Lorenz (1963) postulated that
2
Chapter 1: Nature of conflicts and conflict management – an introduction
the function of intra-species aggression, in plain terms, is a spacing mechanism. The
repulsive reaction of opponents to aggression would produce an even distribution of
individuals over the habitat, which would serve the survival of the species by
guaranteeing an optimal exploitation of all resources.
Over the last 30 years, however, the view of aggression has changed slowly as it
became apparent that aggression seems to be primarily an adaptation to cope with
competition (Maynard Smith, 1982; Huntingford & Turner, 1987; Moynihan, 1998).
Competition appears to be ubiquitous and inevitable, as an animal’s environment does not
consist solely of food supply, sleeping places and shelter from predators, but also of social
partners. Therefore, each individual competes with others (e.g. kin, friends, rivals, mating
partners, neighbours, other species) over the resources that contribute positively to their
inclusive fitness. As resources are limited, mechanisms are required to regulate access to
resources. Aggressive interactions, for example, regulate the control over resources.
However,
besides
the
advantage
of
controlling
resources,
aggression
carries
disadvantages. One of these drawbacks is that in attacking or fighting individuals run the
risk of injury or even death.
Recently the view of aggression has changed again and aggressive behaviour has
been recognised as a tool of negotiation in conflicts of interest among group-living
animals (de Waal, 1996a; Silk, 1996; Aureli & Smucny, 1998; Aureli et al., 2002).
Aggression – besides tolerance and avoidance of the social partner – is seen as one
possible strategy to regulate a conflict of interest. Conflicts of interest, therefore, can be
settled by sharing the resource among the competitors (tolerance towards the partner: de
Waal, 1989a), or by the subordinate partner leaving the resource to the dominant
(avoidance of the partner: Janson, 1985). A conflict of interest, however, can also
escalate and result in aggression.
1.3 Costs and benefits of aggression
Whether or not a conflict of interest escalates into aggression depends on three
variables of a cost-benefit estimation: (1) each competitor’s motivation to fight, which
depends on the value the resource has for each of them, (2) each competitor’s likelihood
of winning, which means each competitor’s likelihood of accessing the resource, and (3)
3
Chapter 1: Nature of conflicts and conflict management – an introduction
the costs of aggression that each competitor is prepared to accept when the conflict of
interest escalates into aggression (Maynard Smith, 1982; Huntingford & Turner, 1987;
Dunbar, 1988; Preuschoft & van Schaik, 2000).
The value of a resource is related to how much benefit the resource provides for a
competitor in terms of fitness increase. The limiting factor for fitness increase appears to
be different for each sex. While in males the limiting factor is the accessibility of mates,
food access seems to be the limiting factor in females (Crook & Gartlan, 1966; Emlen &
Oring, 1977; Wrangham, 1980; Janson, 2000). Direct fighting over these resources is one
possible way to access the benefit. However dominance relationships often regulate the
access to resources. The dominance rank of a male, for example, may improve his access
to fertile females. Many studies in the wild have found a positive correlation between
dominance rank and reproductive success for male primates (Pope, 1990; de Ruiter & van
Hoof, 1993; Dixson et al., 1993; Ohsawa et al., 1993; Altmann et al., 1996; Gerloff et al.,
1999; Constable et al., 2001; Launhardt et al., 2001; Boesch et al., in prep.).
Injury and death are obvious risks of fighting. Injuries can have tremendous costs
leading to infection, infirmity and disability, which can have detrimental consequences for
food acquisition, travelling and predation risk (Jaeger, 1981; Tuttle & Ryan, 1981; Nishida
et al., 1985; Mesnick & Le Boeuf, 1991). Despite injuries and death, there are other costs
of conflicts. Aggressive interactions induce acute stress in opponents (Maestripieri et al.,
1992). Stress is costly, as it may distract the attention of opponents from beneficial
activities and it may be energetically expensive. Furthermore aggression disturbs the
relaxed coexistence between opponents and thus interferes negatively with their
relationship (Cords & Aureli, 1996). Relationships reflect the history of many interactions
among social partners and negative consequences will be mirrored in all of their
interactions (Kummer, 1978; Hinde, 1979). Thus, an out-competed cooperative partner
may for example withhold support from the aggressor in the future. In short, the basic
dilemma of group-living competitors may be that in initiating a fight the aggressor risks
losing a cooperative partner. Such costs will be referred to as social costs.
As soon as individuals have several possibilities to resolve a problem situation, such
as a conflict of interest, they are viewed as going through a decision-making process
(Krebs & Kacelnik, 1991). The term decision does not necessarily imply any conscious
reasoning process, but rather that behaviours can switch between options. The causal
4
Chapter 1: Nature of conflicts and conflict management – an introduction
mechanisms underlying such switching as well as the functional significance are of main
interest for understanding the evolution of decision-making. The theoretical perspective of
decision-making is based on the assumption that individuals maximise the net benefit of a
conflict of interest and, therefore in the long term, increase their fitness (Hamilton, 1964).
Thus the decision can be analysed in terms of costs and benefits of alternative courses of
action, while measuring costs and benefits in terms of a more immediate metric, such as
access to food, energy expenditure, risk of injury, or number of copulations. When an
individual decides to engage in aggression, which is not cost-free, the benefits that are
accessible through the conflict must prevail over the costs of conflicts. In order to
maximise the net benefit of the outcome of a conflict, opponents could either increase the
benefits or decrease the costs of the conflict. Because benefits gained are fixed by the
value of the resource, only costs of the conflict are variable. Therefore adaptive strategies
are usually recognised when they decrease disadvantages (e.g. dominance relationships:
Bernstein, 1981). However another possibility for an adaptive strategy would be finding
an alternative way of gaining the advantages (e.g. food sharing: de Waal, 1992).
1.4 Benefit of conflict management – cost reduction
By conflict management one usually understands all actions, strategies and social
agreements that serve to reduce the costs of conflicts to one or both opponents (Mason &
Mendoza, 1993; Cords & Killen, 1998). Many actions of conflict management are postconflict interactions, which are interactions of one or both conflict partners after their
aggressive interaction. Reconciliation, the friendly post-conflict interaction between former
opponents using affiliative behaviour, can repair the disturbance of the relationship of
conflict partners in order to allow for future cooperation between them (Cords, 1992;
Cords & Thurnheer, 1993), and seems to be able to reduce acute stress (Maestripieri et
al., 1992; Aureli, 1997). Consolation, a friendly post-conflict interaction between a conflict
partner and a third party using affiliative behaviours, and redirected aggression, when a
conflict partner attacks a third party after an aggressive interaction, are both supposed to
reduce the stress of conflict partners (Das, 2000; Watts et al., 2000).
In contrast to post-conflict interactions, dominance relationships manage conflicts of
interests before any escalation by preventing a conflict of interest from escalating into
aggression (Preuschoft & van Schaik, 2000). On the other hand, ritualised aggression
5
Chapter 1: Nature of conflicts and conflict management – an introduction
enables aggression to be used in a conflict of interest whilst incurring minimal costs, as
ritualised aggression reduces the risk of injuries. Thus strategies that regulate the
outbreak or the level of escalation are included in conflict management.
Although conflict management reduces the costs of conflicts it does not necessarily
resolve the conflict of interest. Dominance relationships, acceptance of ownership or
ritualised aggression, for example, regulate the access of a resource with low or no costs
of aggression, but they do not eliminate the incompatibility of the opponents’ goals. On
the other hand, sharing resources can end the conflict of interest over sharable resources,
since both opponents obtain their goal (reviewed in: Aureli & de Waal, 2000a).
1.5 Conflict management in wild chimpanzees of the Taï National Park
This study was conducted in order to understand the economics behind conflict
management and the causes and functions of the decision-making” of wild chimpanzees
in the Taï National Park (Pan troglodytes verus). Given the complexity of the topic, many
detailed questions are of importance for the understanding of the conflict management,
which are mentioned within the result chapters (4, 5, 6 and 7). Chimpanzees are of
special interest for the study of conflict management (see chapter 22), since they are our
closest living relatives (Ebersberger et al., 2002), and they were the first non-human
species where reconciliation was observed (de Waal & van Roosmalen, 1979).
The conflict management of chimpanzees encompasses different mechanisms that
act in conflicts of interest at different times. Avoidance of dominant conflict partners or
tolerance of conflict partners by sharing are actions that are carried out before the conflict
of interest escalates into aggression. This will be referred to as pre-conflict management.
In order to understand pre-conflict management, I investigated whether or not
chimpanzees dominance relationships are linear and based on contest competition
(chapter 4; Wittig & Boesch, 2003a,c), and if the decision to initiate aggression is
influenced by the resource benefit or the dominance relationship between opponents
(chapter 5; Wittig & Boesch, 2003b). Using ritualised aggression instead of contact
aggression or fighting non-friends instead of friends are strategies that can be used
2
see 2.4
6
Chapter 1: Nature of conflicts and conflict management – an introduction
during the escalation to aggression – referred to here as peri-conflict management. Thus,
in terms of peri-conflict management, I next investigated if opponents use strategies of
cost reduction by varying the aggression and type of opponent (chapter 5; Wittig &
Boesch, 2003b). Finally, many interactions of conflict management are reactions to costs
that have been already created. These interactions are carried out after the conflict and
are part of post-conflict management. For the post-conflict management, I examined the
economic rules for choosing a post-conflict interaction (chapter 6; Wittig & Boesch,
2003d) and the way to repair the disturbance of usually relaxed relationships after the
conflict (chapter 7; Wittig & Boesch, in review).
Conflict management is naturally a system of decision-making. Although triadic
relations are also important (de Waal & van Hooff, 1981; Judge, 1991; Das et al., 1997),
conflict management mostly contains dyadic interactions. Thus my analyses were based
on dyadic interactions. Multivariate analyses were used to detect co-variances and
interactions between influencing factors (see chapter 33). This study is the first to my
knowledge that has concurrently investigated all aspects of conflict-management in a
species. Moreover this study places the patterns of conflict management in an economic
framework of costs and benefits. As Pan and Homo are sister-groups, knowing about the
conflict management of chimpanzees may help us to understand our own conflict
management and aspects of our own evolution (chapter 84).
3
4
see 3.4.1
see 8.5
7
Chapter 25:
Biology of common chimpanzees (Pan
troglodytes)
Figure 2.0 Members of the North community in Taï are feeding of and resting around a
fallen Palm-tree (Photo by R. Wittig).
5
This chapter provides the reader with information on the biology of chimpanzees that is relevant
to this study.
Chapter 2: Biology of chimpanzees
2.1 Distribution of chimpanzee
Chimpanzees (Pan troglodytes) are found in the forests and woodlands of tropical
Africa (Figure 2.1a). They group in four sub-species (in order from west to east, Figure
2.1b): (a) the West African Chimpanzee (Pan troglodytes verus), ranging in the west of
the Niger River, from Senegal to Ghana; (b) the Nigerian Chimpanzee (P.t. vellorosus),
found between the rivers Niger and Sanaga in Nigeria and the northern part of Cameroon;
(c) the Central African Chimpanzee (P.t. troglodytes), existing in the west of the Congo
River between the rivers Sanaga and Ubangi in Gabon, Central African Republic (C.A.R.),
Congo and northern part of Democratic Republic of Congo; and (d) the East African
Chimpanzee (P.t. schweinfurthii), ranging between the rivers Congo and Ubangi and
spread out east beyond the Great Rift Valley, into the triangle of Uganda, the eastern part
of Democratic Republic of Congo and Tanzania (Gagneux et al., 2001; Gagneux, 2002).
Chimpanzees and Bonobos (Pan paniscus) represent together the genus Pan, which is the
phylogenetic sister-group to humans (Homo sapiens) (Kaessmann et al., 2001;
Ebersberger et al., 2002).
Our knowledge about the biology of wild chimpanzees originates mainly from seven
long-term studies (see Figure 2.1b). The two oldest long-term studies on chimpanzees are
located in east Africa with Pan troglodytes schweinfurthii. One is in the Gombe Stream
National Park, Tanzania (since 1960: Goodall, 1986) and the other is in the Mahale
Mountains National Park, Tanzania (since 1965: Nishida, 1990), both on the shores of
Lake Tanganyika. Two further long-term studies were established in the second half of
the 1970s in west Africa on Pan troglodytes verus. One is located in Bossou, Guinea (since
1976: Sugiyama, 1984) and the other one in the Taï National Park, Côte d’Ivoire (Figure
2.1c,d; since 1979: Boesch & Boesch-Achermann, 2000; Wittig, 2003). Another three
studies also investigate Pan troglodytes schweinfurthii, of which two are placed in the
Kibale National Park, Uganda (since 1987: Kanyawara, Wrangham et al., 1991; since
1995: Ngogo, Watts, 1998) and one in the Budongo Forest Reserve, Uganda (since 1994:
Newton-Fisher, 1999a). All of these long-term studies are located in eastern or western
Africa and represent only two of the four subspecies (P.t. schweinfurthii and P.t. verus).
However, recently another two study sites were established with the potential to become
long-term studies. One study site is in the Gashaka Forest Reserve, Nigeria (Sommer et
al., 2003) on Pan troglodytes vellorosus and the other one in the Goualougo Triangle,
Nouabalé-Ndoki National Park, Republic of Congo (Morgan & Sanz, 2003) on Pan
9
Chapter 2: Biology of chimpanzees
troglodytes troglodytes (Figure 2.1b). The present study was conducted in the Taï
National Park, Côte d’Ivoire.
P.t. verus
P.t. vellerosus
P.t. troglodytes
P.t. schweinfurthii
Senegal
Guinea
Bossou
Côte
Liberia d’Ivoire Ghana
Taï
Nigeria
Gahshaka
C.A.R.
Cameroon
Goualougo
Gabon
Budongo
Uganda
Congo
Democratic
Republic of
Congo
Kibale
Gombe
Tanzania
Mahale
Africa
(b)
(a)
(5°52’ N,
7°22’ W)
Taï
Study Area of
the Taï
Chimpanzee
Project
Côte d’Ivoire
Taï
National
Park
Yamoussoukro
Liberia
Taï
National
Park
Abidjan
(c)
(d)
Figure 2.1 Approximate distribution of the four subspecies of chimpanzees (Pan
troglodytes) over Africa (a+b), following the suggested distribution by Gagneux (2002),
locations of the long-term studies (
) and the precise location of the Taï Chimpanzee
Project (c+d).
10
Chapter 2: Biology of chimpanzees
2.2 Social structure and mating system of chimpanzees
Chimpanzees live in multimale – multifemale communities (=unit-groups: Nishida,
1968; Goodall, 1983). These communities have a fission–fusion grouping pattern, as they
travel around in temporary grouping units (= parties) with constantly fluctuating
compositions (Sugiyama, 1984; Goodall, 1986; Nishida, 1990; Boesch, 1991; Wrangham
et al., 1992). All party members seen together in various parties belong to the
community. Communities consist of between 10 (middle community in Taï: Herbinger et
al., 2001) and 150 individuals (Ngogo community: Mitani & Watts, 2001) of all age
classes. Males are the philopatric sex in chimpanzees. They stay in their natal community,
while females transfer permanently to another community usually at the end of
adolescence (Nishida & Hiraiwa-Hasegawa, 1985; Watts & Pusey, 1993).
Chimpanzees inhabit a home range of between 9 km2 (Bossou: Sakura, 1994) and
35 km2 (Ngogo: Watts, 2002), with home range size increasing with number of males
(Lehmann & Boesch, in press). As only about 10% of the home range area is exclusively
used by the resident community, whilst the remaining area is also used by neighbouring
communities, chimpanzees can face up to a mean of four inter-community encounters per
month (Lehmann & Boesch, in press). The males of the resident community, sometimes
supported by the females, defend the their home range by means of drumming displays
accompanied by pant-hoot and long bark vocalisations (Crockford & Boesch, 2003). In
case of direct contact between the communities, serious fighting can emerge, which can
be lethal (Nishida et al., 1985; Fawcett & Muhumuza, 2000).
Chimpanzees have a promiscuous mating system, where females mate with several
males and males with several females (Tutin, 1979; Hasegawa & Hiraiwa-Hasegawa,
1983; Matsumoto-Oda, 1999). Female chimpanzees show a sexual swelling of their
perineum (Dixson, 1998). This sexual swelling (=oestrus swelling; Figure 2.2) lasts for
about 10 days (the time period which I refer to as being in oestrus) during each fertile
cycle in female chimpanzees (about 36 days: Wallis, 1982) and indicates for the
probability of ovulation (Deschner et al., 2003). After conception female chimpanzees
have a gestation period of about 8 months, following a lactation time of about 2 years
and a total inter-birth interval of almost 6 years (Furuichi & Hashimoto, 2002; Wrangham,
2002). Only when a younger sibling is born do children stop travelling on their mother’s
11
Chapter 2: Biology of chimpanzees
back, however they still stay together with her mother until they are adolescent (another
five years).
Figure 2.3 Male Macho and female Venus are feeding on
Figure 2.2 Female Mystère with a full sexual swelling a carcass of a red colobus (Procolobus badius), while
(Photo by T. Deschner).
females Goma and Belle are watching the carcass (Photo
by R. Wittig).
Social relationships play an important role in the social life of chimpanzees. In most
communities chimpanzee are probably male bonded, as males are philopatric and seem to
have stronger bonds than females (Mitani et al., 2002). Males, for example, build long
lasting alliances, that can be sometimes strong enough to perform a coup, overturning
the alpha male (Nishida, 1983). In Taï chimpanzees, however, relationships in females
can become so strong that a female may adopt the children of another female who dies
(Boesch & Boesch-Achermann, 2000). Chimpanzees of both sexes have many benefits to
give to or to receive from their partners. Partners can support each other in agonistic
situations (e.g. de Waal, 1984; Hemelrijk & Ek, 1991), they can share special foods (e.g.
de Waal, 1989a; Mitani & Watts, 2001), or they can groom each other (e.g. Huffmann,
1990; Wrangham et al., 1992; Watts, 2000). Chimpanzees can trade benefits in order to
built up or to maintain good relationships. Since chimpanzees can become 40 years and
older (Hill et al., 2001), many of their relationships can be long lasting and may influence
the structure of the community (e.g. grooming and association: Boesch & BoeschAchermann, 2000; Watts, 2000).
12
Chapter 2: Biology of chimpanzees
2.3 Diet and aspects of feeding ecology of chimpanzees
Chimpanzees are omnivorous, although most of their diet consists of ripe fruits
(Goodall, 1986; Yamakoshi, 1998; Newton-Fisher, 1999b; Boesch & Boesch-Achermann,
2000). Most fruits are eaten in large plentiful patches under and in the trees and
therefore are not usually a limited food source. However apart from fruits chimpanzees
feed on many other food items. They hunt other mammals, mostly smaller primates, but
in some communities also small forest antelopes and bush pigs (Boesch, 1994a; Stanford
et al., 1994; Mitani & Watts, 1999; Watts & Mitani, 2002). In Taï, group hunting is
common and hunters collaborate by taking different roles6 (Boesch, 1994b). A carcass of
a hunted monkey can be characterised as nutritious but monopolisibale (Figure 2.3).
Moreover chimpanzees in Taï extract nut kernels from their shells using pieces of wood or
stones as hammers and anvils (Boesch & Boesch, 1983; Boesch & Boesch, 1984a,b).
Some nut species possess very hard shells and chimpanzees are obliged to use stone
hammers to crack them (Figure 2.4). Since large stones are rare in the Taï forest, stone
hammers are also considered to be a monopolisable resource, needed to extract nutritious
food. Chimpanzees also feed on insects. For some types of insects they use tools, for
example when extracting maggots out of dead wood (Figure 2.5) or dipping for ants
(Figure 2.6). When dipping for driver ants (Dorylus spec.), the right position at the nest is
important to avoid painful ant bites – also a situation of possible monopolisation. Despite
fruits, meats, nuts and insects, chimpanzees feed also on mushrooms, leaves, terrestrial
herbivorous vegetation and insect products, such as honey. Some of these food items can
be rare and monopolisable, but most of them are widely spread and usually common.
Figure 2.4 Female Goma is cracking Panda-nuts (Panda
Figure 2.5 Female Belle is extracting maggots from a
oleosa) with a stone hammer, while her daughter Giselle
dead tree with a stick that she is using like a fork (Photo
is feeding on the left-over (Photo by R. Wittig).
by R. Wittig).
6
coordination in time, space and function of behaviour
13
Chapter 2: Biology of chimpanzees
2.4 Reconciliation (and conflict management) in chimpanzees – a brief history
With such a social structure of long-term cooperative relationships and competition
over food and mating partners, it is not surprising that reconciliation was discovered first
in chimpanzees. In 1979 de Waal & van Roosmalen published the first detailed study on
unexpected friendly interactions among former opponents after aggression in a group of
chimpanzees in Arnhem Zoo, The Netherlands. They found that former opponents were
more likely to be partners in the first friendly interaction after a conflict and that the time
interval between the aggressive and the first interaction was shorter than between the
first and the second interaction. De Waal and van Roosmalen called those first friendly
interactions reconciliation and intuitively used the right functional label for these
interactions (chapter 77; Cords & Thurnheer, 1993; Wittig & Boesch, in review). Although
reconciliation has been studied in many other species after de Waal’s and van
Roosmalen’s discovery (Aureli & de Waal, 2000a; Appendix A), it needed more than 10
years before the first comparison was conducted with another chimpanzee group in a zoo
(Baker & Smuts, 1994). Since then several other studies have partly re-evaluated the
results of de Waal’s and van Roosmalen’s pioneering work with data on other captive
chimpanzees (Fuentes et al., 2002; Preuschoft et al., 2002).
Figure 2.6 Male Marius is ant dipping from a nest of
driver ants (Dorylus spec.) on the ground (Photo by R.
Wittig).
7
see 7.3.2
14
Chapter 2: Biology of chimpanzees
However in wild chimpanzees reconciliation was forgotten for many years. In 1986
(Goodall, 1986) mentioned reconciliation in a short paragraph and stated: “The gestures
(note: that is reconciliation) used by the Arnhem chimpanzees were similar to those
observed at Gombe (note: her study site), but it was obviously still more important in the
captive situation (where the chimpanzees could not escape from one another) that
quarrels be speedily settled.” It needed more than 20 years before the first study on
reconciliation was published for wild chimpanzees (Arnold & Whiten, 2001). However,
only with the present study were we are able to analyse all aspects of conflict
management in wild chimpanzees.
15
Chapter 3:
General methods of the study
Figure 3.0 The author, Roman M. Wittig, fully equipped
(hand-held computer, watch, binoculars, pencil and paper) in
the forest while observing chimpanzees in a tree (Photo by C.
Crockford).
Chapter 3: General methods
3.1 Data collection
3.1.1 Study Site
Data was collected in the study area of the Taï Chimpanzee Project, Taï National
Park, Côte d’Ivoire (West Africa, 5°52 N, 7°22 W). The study was conducted between
October 1996 and April 1999 on the North-community, which has been observed
continuously since 1979 (further information: Boesch & Boesch-Achermann, 2000). In
October 1996 the community consisted of four males (three adults, one adolescent), 14
females (11 adults, three adolescents) and 13 juveniles and infants. Almost all individuals,
except for the female Ricci and her adolescent son Nino, were fully habituated to humans
and were followed from nest to nest. The end of 1997 Nino became fully habituated and
was followed for full days. During the observation period five chimpanzees disappeared or
died (one adult male, two adolescent females, two juveniles) and six infants were born.
3.1.2 Observation Procedure
I collected the following four types of data during all-day follows of the four males
and 10 adult females. (1) I did all-day focal animal sampling (Altmann, 1974) of a target
chimpanzee, recording activities, social interactions and vocalisations. (2) I recorded the
target’s party composition by scanning the presence of individuals in visibility of the target
every 10 minutes. (3) I documented the identity and number of females with genital
swellings per day. (4) I noted specific information at each feeding site regarding the foods
consumed by the target, food monopolisability and number of competitors present. All
these data I entered directly into a Psion Organiser® hand-held computer via The
Observer® (Noldus, 1989). Additionally I recorded quickly changing behaviours on a tape
recorder and entered this information the same evening into the daily computer file
(example for a daily computer file: Appendix B).
I aimed to change the target chimpanzee each day, observing females once and
males twice per month. There was however, some variability in individual observation
frequency due to the fission-fusion character of chimpanzee societies (chapter 28), death
and the habituation level. The result was 80 all-day follows of males (Macho/Marius: 31
days each, Brutus/Nino: 9 days each) and 123 all-day follows of females (between 10 and
15 days per female). 1071 conflicts with complete information were collected. Of these,
17
Chapter 3: General methods
876 conflicts were analysed in chapter 59 (Wittig & Boesch, 2003b) and 610 (Wittig &
Boesch, 2003d), as I excluded conflicts involving juveniles and infants. However in
chapter 711 (Wittig & Boesch, in review), I analysed only conflicts with the complete
information required for testing for reconciliation, which basically left 791 conflicts to
analyse.
3.1.3 Ethogram
The ethogram, I used for my study, was a subset of the ethogram of the Taï
chimpanzee Project (C. Boesch, unpublished). It included 11 activities of the focal animal,
6 behaviours related to the proximity between partners, 11 social behaviours related to
food, 6 submissive social behaviours, 21 aggressive social behaviours, 12 affiliative social
behaviours and 16 vocalisations and gestures (Table 3.1). Some of the behaviours had
additional modifiers to separate subtypes of the same behaviour (e.g. Table 3.1, No. 62,
french kiss vs. directed kiss). Names of food plants were taken from a list including all
species consumed by the Taï chimpanzee (D. Anderson, Z. Gone Bi and C. Boesch,
unpublished).
Table 3.1 Ethogram of the study on conflict management in Taï chimpanzees. Behaviours have a serial number and are ordered in the 7
categories: activity, proximity, food related social behaviour, submissive social behaviour, aggressive social behaviour, affiliative social behaviour
and gestures/vocalisations. A represents the actor (or initiator) and B the reactor (or receiver) in the description of the behaviour. Occasionally
either the French or the German name for the behaviour refers to the code, which is presented then in brackets.
No. Name
activity
1
move (déplacement)
2
eat (manger)
incl. what A is eating
incl. how many competitors
incl. monopolisability
3
play (jeux)
JEU
4
hunt (jagen)
JAG
5
wait
WAI
6
7
rest (repos) and sleep
groom
incl. direction
incl. duration of grooming
nest (nid)
contact
handhold
have
REP
GRO
8
9
10
11
Code
Description
DEP
MAN
A is moving on the ground or in the trees
A is harvesting food and/or ingesting food, nut-cracking, and/or travelling within feeding site
fruits, meat, nuts, insects, leaves, mushrooms, THV (terrestrial herbal vegetation), others
absolute number of competitors
relative amount and distribution of food source in relation to the number of competitors
A plays with social partner or object. Play contains either behavioural elements out of the
aggressive range or object manipulation, but is always accompanied by play face.
A is at the height of monkeys and/or is moving towards monkeys with the “intention” (trying) to
catch one
A interrupts travelling movement when others lag behind or do not follow, and continues to
travel once others have appeared or start following
A is lying, sitting, standing or sleeping, not involved in other activities
A grooms B
indicating whether A grooms B or grooming is reciprocal
indicating the end of grooming session
A constructs a nest to lie in
A rests in physical contact with B
A rests holding B’s hand
A possesses X, with X being a food item, a tool, or other object
NID
CON
HAN
HAV
8
see 2.2
see 5.2.1
10
see 6.2.1
11
see 7.2.1
9
18
Chapter 3: General methods
proximity
12
approach
13
hesitant approach
14
fast approach
15
leave
16
retreat
17
pass
APP
HAP
FAP
LEA
RET
PAS
A
A
A
A
A
A
food related social behaviour
18
stare at (fixer)
19
beg (mendier)
FIX
MEN
20
21
PRE
RE*
PPA
PAP
PAC
CAD
REF
TBA
SUP
A approaches within 1m of B and stares at food B is possessing
A, who is within 1m of B, lightly touches the food B is holding, or touches B with his hand (when
B is eating), or puts his hand, palm up, toward food, or puts cupped hand under the food, or
attempts to take food from B
A takes food belonging to B
A takes over food or object that B left on the ground
either after B left the spot, or when B is still around
fruits, meat, nuts, insects, leaves, mushrooms, THV (terrestrial herbal vegetation), others, tools
B takes bits of food or object which A controls. A neither defends food nor encourages B
B takes bits of food from A and A makes a gesture to encourage B
A actively gives a piece of food to B
A gives B all or more than half of food source
A moves food or himself away from B
A takes back some or all the food taken by B
A approaches B who leaves, A takes over feeding/resting site or grooming partner
submissive social behaviour
29
present
30
crouch present
31
crouch
32
bobbing
33
intense greeting
34
hand reach
PRS
CPR
CRO
BOB
IGR
HRE
A turns rump to B exposing genital area
A presents to B with flexed legs
A faces B with flexed legs
A performs up and down movement by flexing and stretching arms and legs
A sequence of fast approach, eye contact seeking, bobbing and intense pant-grunting
A stretches arm out to B without actually touching B
aggressive social behaviour
35
piloerrection
36
head tipping
37
push away
38
arm wave
PIL
HTI
PAW
AWA
39
40
41
42
throw
shaking branch
rocking
hunch
THR
SBR
ROC
HUN
43
44
45
46
47
48
shuffling
bipedal swagger
slapping or stamping
bite
dragging
display
SHU
BSW
HIT
BIT
DRA
DIS
49
50
51
52
53
54
55
short chase
chase
charge
attack
fight
support
flight
SCH
CHA
CHR
ATT
FIG
SPP
FLI
A has erected hair
A makes a slight upward and backward jerk of the head towards B
A stretches arm out and shoves B in direction away from himself
A is flinging-out his arm towards B, a hitting down, punching-down or sweeping upwards
movement
A picks up an object and throws it unaimed (no target) or aimed (hitting a target)
A takes hold of a branch, twig or sapling and sways it from side to side or forward and backward
A rocks from side to side or forward and backward
A raises shoulders accompanied by hair erection, can be bipedal, quadrupedal or in sitting
position
A walks dragging feet on the ground making the movement noisy
A stands upright, sways from foot to foot, shoulders slightly hunched, arms out
A hits substrate or B with hands or feet
A puts his mouth to B’s body and bites him (presses his teeth)
A pulls object or B over the ground
A shows a variety of conspicuous and exaggerated motor patterns (e.g. bipedal running,
stamping ground, waving arms in air) including piloerection
A makes an movement of the whole body in the direction of B
A runs after B who runs away from A
A rushes towards B without B fleeing
A suddenly, forcefully and deliberately contacts B with a series of aggressive contacts
A attacks B while B attacks A
C supports A or B via aggressive behaviours in an aggressive interaction
A runs away from B
22
23
24
25
26
27
28
take (prendre)
take over (récupération)
incl. type of take over
incl. type of food or object
food share (partage) passive
food share active-passive
food share active
present (cadeau)
refusal
take back
replacement (suppléant)
affiliative social behaviour
56
bluff over
57
reach touch
58
finger in mouth
59
touch genitals
60
rump rub
61
mount
62
kiss
incl. modifier
63
embrace
incl. modifier
64
under arm
65
pat
66
copulation
67
genital investigation
BOV
TOU
FIM
TGE
RRU
MOU
KIS
EMB
UAR
PAT
COP
GIN
enters at normal speed within 1 m of B
enters at normal speed within 1 m of B, interrupted by pauses
runs or walks fast directly over to B and enters within 1m of B
casually walks out of 1m range of B
makes sudden movement away from the vicinity (1m range) of B, but remains in visibility
enters B’s 1m range, continues walking and leaves B’s 1m range without changing direction
A walks behind B putting his arms over him
A reaches and touches any party of B’s body
A puts his finger in B’s mouth
A touches B’s oestrous or scrotum
A rubs his rump against B’s rump
A mounts B’s body, A makes pelvic thrusts, but does not ejaculate
A presses his lips to B’s body
french: kiss mouth to mouth; directed: kiss mouth to body part
A stands/squats in front/behind B and puts his arms around B’s body
reciprocal, directional
A stands beside B and puts one arm around B’s body, while A and B look in the same direction
A strokes or quickly caresses B’s body
A approaches B, mounts, achieves penetration, performs pelvic thrusts
A touches, sniffs or makes a few quick grooming movements on B’s oestrous
19
Chapter 3: General methods
vocalisations, gestures & facial expressions
68
solicited grooming
SGR
A approaches B, stops in front and turns his side to B, presents an arm or scratches vigorously
69
solicited copulation
SCO
A presents penis erection and/or pelvic thrusts, bipedal swagger, knuckle knock to B
70
grin (fear)
GRI
A shows his teeth while maxilla and mandible are closed or almost closed
71
pant-grunt
PGR
A pants while inhaling and grunts with an open mouth while exhaling
72
bark
BAR
A produces a short and sharp vocalisation, that sounds like ‘waa’, ‘waaoo’ or ‘aaoo’ to the human
ear
73
pant
PAN
A produces a panting sound whilst in- and exhaling with an open mouth
74
grunt
GRU
A produces a grunting sound with closed or open mouth
75
greeting-hooh
GHO
A produces a repeated and intense serious of ‘hoos’
76
pant-hoot
PHO
A produces a series of alternating pants and hoots with increasing volume that usually ends in a
climax scream
77
scream
SCR
A produces a loud and shrill vocalisation with an open mouth
78
crying (pleur)
PLE
A produces a series of short tonal hoos
79
drum (tambourinage)
TAM
A hits buttress of a tree with hands and/or feet
80
drumming sequence
DSE
A performs a sequence of behaviours starting with a warm up (rocking and/or leaf clipping in
front of a tree), pant-hooting, drumming and climax scream
81
alarm call (cris d’alarme)
CDA
A produces a series of very loud bark-like vocalisations that sounds like ‘waaoou’ to the human
ear
82
temper tantrum
TTA
A screams or cries and tumbles on the ground
83
smack
SMA
A makes a smacking sound with the lips while grooming B
3.2 Operational definitions
3.2.1 Behaviours
Aggressive behaviours are threats (e.g. barks, arm wave), non-contact aggression
(e.g. displays) and contact aggression (e.g. bites, hits). Affiliative behaviours are friendly
behaviours with body contact (e.g. kiss, genital touch, embrace, grooming). Submissive
behaviours are submission (e.g. greeting, crouching) and flights (e.g. fleeing, retreating).
3.2.2 Conflict variables
A conflict was defined as a dyadic aggressive interaction that started with the first
aggressive behaviour exchanged and ended with either submission, flight, reciprocal
screaming, or a non-aggressive behaviour which was not immediately followed by further
aggression. The conflict duration was measured from the first exchange of aggressive
behaviour until the defined end of the conflict. Aggressive behaviours were classified into
five ordinal intensities according to the likelihood of the aggression injuring the partner
(Table 3.2). For each conflict the most intense aggressive behaviour used determined the
intensity score for that conflict. The cross product of sex of both the initiator and the
receiver of aggression are referred to as the sex combination of conflict partners in four
categories (initiator-receiver: male-male, m-m; male-female, m-f; female-male, f-m;
female-female, f-f).
20
Chapter 3: General methods
3.2.3 Context variables
I observed conflicts within three different contexts. In sex contexts, conflicts are
with or over oestrus females either to copulate with them or to prevent others’ copulation
attempts. Food conflicts are over food resources. In the social contexts, conflicts show or
challenge dominance (e.g. subordinates approaching without greeting followed by
aggression) or conflicts arise from competition for social partners. However, it is partly a
default option (as conflicts can only occur with other social partners present), which other
studies have summarised in an unknown category (e.g. de Waal & Hoekstra, 1980). Two
further variables are the possibility to monopolise the resource as well as how many
competitors are present. The resource monopolisation is a measure of the relative
distribution and aquantity of the resource that is fought over. This is a relative scoring of
the number of competitors needed to monopolise the resource. The competitor proportion
is the proportion of competitors present in the party compared to all potential competitors
for the specific resource.
3.2.4 Dominance variables
Hierarchies in chimpanzees are classically determined by greeting vocalisations (de
Waal, 1978). I used pant-grunts among males and additionally greeting-hoo and greeting-
pant among females to determine the hierarchy (chapter 412; Wittig & Boesch, 2003a).
One rank change occurred in the males during the data collection period due to the death
of Brutus, the b-male. Since the two lower ranking males rose up in rank accordingly,
conflicts between them and the a-male had two rank differences each in the data set,
depending on whether the conflict occurred before or after the rank change. The relative
dominance of the initiator towards the recipient of aggression was scored in the initiator’s
rank variable. The winner of the conflict was defined as the one able to access the
resource. In food contexts the winner was the one possessing the food after the conflict.
In sex contexts the winner was able to assert his or her choice (e.g. a female refused
copulation, a male interrupted a copulation of another male). In social contexts the
winner was the one who neither showed submission, screams nor flight, or, in cases of
competition over the access to a social partners, the one who affiliated with this social
partner afterwards. I defined conflicts as a draw when neither conflict partner won.
Whether or not the initiator won was reflected in the initiator victory outcome, while
12
see 4.2.1
21
Chapter 3: General methods
whether or not the dominant won the conflict was marked by the variable of the winner’s
rank.
3.2.5 Social variables
Association is defined as being present in the same party (being within visibility). I
used the dyadic association index (DAI) to measure how frequently two individuals were
associated: DAIAB =
å ( A + B)
, where A is the time individual A was seen, B
å A + å B - å ( A + B)
is the time individual B was seen and A+B is the time A and B were seen together
(Nishida, 1968). I gave the relationship benefit three categories according to the presence
of food-sharing and support events within the dyads (Table 3.2). Dyads of high
relationship benefit are called high value partners or friends.
Table 3.2 Main variables with definitions used in the present study on conflict management to describe the conflict situation.
Parameters names of the variables are underlined for a better contrast. Some variables are adapted for comparisons (see
3.2.5) and thus same parameters have different ranges for e.g. different sex combinations.
Name
Conflict Duration
Conflict Intensity
Type
continuous
ordinal
multinomial
Initiator’s Sex
Recipient’s Sex
Conflict Context
binomial
binomial
multinomial
Resource Monopolisation
ordinal
multinomial
Competitor Proportion
ordinal
multinomial
Rank Difference (r.d.)
ordinal
multinomial
Initiator’s Rank
binomial
Initiator Victory Outcome
multinomial
Winner’s Rank
binomial
Association Index
ordinal
binomial
ordinal
multinomial
Relationship Benefit
Variable definitions and parameter scoring
duration from start of first aggressive behaviour to end of the conflict (in seconds)
intensity of the conflict scored by the most intensive single aggressive behaviour: 1 =
aggressive vocalisation or gesture; 2 = non-contact aggression, no movement; 3 = noncontact aggression, including movement; 4 = physical contact aggression, one action; 5 =
physical contact aggression, many actions
sex of the initiator of aggression: m = male aggressor; f = female aggressor
sex of the recipient of aggression: m = male aggressed; f = female aggressed
context in which the conflict occurred: sex = conflict about oestrus females; food = conflict
about food or possession of tool; social = conflict about hierarchy or social partners and
default category
relative scoring of the monopolisability of the resource that is fought over: monopolisable by
one = resource can be monopolised by one competitor; monopolisable by few = resource can
be monopolised by some of the competitors; not monopolisable = resource is not
monopolisable
proportion (prop.) of competitors present in the party in relation to all potential competitors
(in social context: adults in party / all adults; in sex context: males in party / all males; in food
context: adults feeding / all adults in party): few for prop.£0.33; some for 0.33<prop. £0.66;
many for prop.>0.66
difference of ranks in the linear dominance hierarchy (Wittig & Boesch 2003a; chapter 4)
between conflict partners (rank subordinate – rank dominant); different category ranges,
because of different numbers in sex classes; categories of r.d. in m-m dyads (4 males): small
for rank neighbours, middle for r.d.=2, large for r.d.=3; categories of r.d. in f-f dyads (12
females): small for r.d.£3, middle for 3<r.d.£6; large for r.d.>6; categories of r.d. in m-f
dyads (16 individuals) small for r.d.£5, middle for 5<r.d.£10, large for r.d.>10
dominance relationship of the conflict partners from the perspective of the initiator: dom =
aggressor dominant over aggressed; sub = aggressor subordinate to aggressed
outcome of the conflict from the perspective of the initiator: w = initiator wins aggressive
interaction; l = initiator loses; n = no winner
dominance relationship of the conflict partners from the perspective of the winner: d = winner
is dominant over loser; s = winner is subordinate to loser; n = no winner
relative scoring of the dyadic association index (Nishida 1968): rare = m-m: £50% of
observation time, others: £25%; frequent = m-m: >50%, others: >25%
scoring of the cooperative character of the relationship of dyads: low = no food sharing, no
support within dyad; medium = either food sharing or support within the dyad; high = both
food sharing and support within the dyad
22
Chapter 3: General methods
3.2.6 Variable adaptations for statistical comparison
Some variables are affected by the sex of chimpanzees (e.g. DAI: higher for males
than females, Boesch & Boesch-Achermann, 2000; dominance-rank: males dominant over
females, Bygott, 1979). For the multivariate analysis, I corrected for sex differences by
calculating indices. The indices relate data to average values of sex combinations in order
to enable a comparison between the sexes (detailed description: Table 3.2).
3.3 Multivariate dyadic approach
I employed a multivariate approach to detect the factors that affected the variation
within conflict management. Table 3.2 displays variables (with definitions and scoring of
parameters) that I considered either as dependent variables or as independent variables
for variation within conflict management. In addition to the variables of Table 3.2, I
tested some chapter specific variables and interactions of variables, which are explained
in detail within the methods part of each chapter. Conducting a multivariate analysis I
considered all independent variables simultaneously. As many of the variables (e.g. rank
difference, association index, relationship benefit) were different for the same individual
with different partners, I conducted the analyses on a dyadic level. However, since
repeated measurements of individuals can inflate the Type I (a) error, I controlled for this
and ruled out the influence of repeated measurements on variables (see statistical
process). The strength of the results was therefore similar to an individual based analysis,
but included the advantage that one is able to detect dyadic variability.
3.4 Statistics
Despite the usual parametric and non-parametric statistic, I conducted two less
common types of statistical tests throughout almost all four chapters. One type of tests
was multivariate analysis in two different versions (for categorical and continuous
dependent variables) and the other type included dyadic permutation tests. When a
23
Chapter 3: General methods
question required another unusual statistical approach (e.g. chapter 4: linearity tests13),
then I have explained it in the methods specific to that chapter.
3.4.1 Multivariate analysis
To detect the variables that affect the variation found within conflict management, I
executed multivariate analyses as follows:
1. In order to eliminate repeated measurements of the same conflict type per dyad, I
summarised conflicts in one data point for identical cases. Identical conflicts had the
same initiator and receiver of aggression, the same value of some chapter specific
variables and were identical in all other independent variables, apart from conflict
duration and conflict intensity, which were scored with mean and median values
respectively. However each specific question of my thesis (each of the chapters 4, 5,
6 and 7) had a different set of independent variables. Thus the previously described
balancing process lead for each question to a different final data set for the
multivariate analysis, which is referred to in each chapter again.
2. I used a generalisation of the Logistic Regression called General Linear Models (GLM:
McCullagh & Nelder, 1989; Agresti, 1996) to examine the functional relationship
between the occurrence of one dependent variable and several independent
variables. GLMs have been successfully applied in studies on reconciliation and
dominance relationships (Call et al., 1999; Côté & Festa-Bianchet, 2001).
(a) Investigating the variation of categorical dependent variables I conducted a
Generalised Linear Model (GLZ). For GLZs I chose an ordinal multinomial, or
binomial respectively, error distribution and a logit link function (McCullagh &
Nelder, 1989). The best model was selected by the best subset method, which is
an iterative method based on maximum likelihood estimation (LR: likelihood
statistics), and the Akaike’s Information Criterion (AIC), which penalises for the
number of independent variables in the model (Akaike, 1973). The significance of
the independent variables and their parameters was assessed using Wald
statistics for the best model (Dobson, 1990). The estimate-coefficient b is an
indicator for the strength of the effect that an independent variable-parameter
13
see 4.2.3
24
Chapter 3: General methods
has on the occurrence of the dependent variable. The probability that the tested
dependent variable occurred was eb more likely with one unit increase of the
independent variable, after adjusting for all other variables. This means that, for
example, a positive b indicates an effect in favour for the first parameter of the
dependent variable that enters the model, while a negative b indicates the
opposite direction. The further away b was from 0 the bigger the influence of the
independent variable-parameter.
(b) Investigating the variation of continuous dependent variables I conducted a
General Linear Model (GLM). The GLM is based on the least square method and
can estimate the effect of several variables on one dependent variable. After
calculating the all effect model, I calculated the effect size for the significant
variables by excluding trivial effects (effect size £ 0.1) from the model (Cohen,
1988). For variable interactions I used Fisher LSD post-hoc test (equivalent to ttest) with Bonferroni correction.
3. Since I was analysing on a dyadic level, I controlled for a possible inflation of the Type
I (a) error due to multiple measurements of the same actor. Therefore I included the
identity of the conflict initiator as an additional independent variable and tested the
best model again (similar procedure was used by Côté & Festa-Bianchet (2001)).
When the significant explanatory independent variables remained significant, one is
able to assume that the effect was not due to the replicated observations of the same
individual (Bland & Altman, 1995). For the sake of simplicity I have presented only the
remaining significant variables of the best models in the results, where p<0.05.
3.4.2 Dyadic permutation tests
1. In order to test differences of variables between different subgroups, I conducted a
Permutation Test, which is an extension of the network analysis test by (Dow & de
Waal, 1989). I created a matrix of the variable of all individuals and calculated the
mean difference between the subgroups I wanted to compare. Afterwards I sampled
the data set without replacements and calculated from this sample the mean
difference between subgroups again. The procedure was repeated 5000 times and I
checked if the original means were found inside or outside the 95% confidence
interval of the approximate distribution of mean differences. The tests were
25
Chapter 3: General methods
conducted two-tailed with an significance level of P<0.05 and Bonferroni corrected
when multiple tested.
2. In order to compare dyadic interactions with a baseline, I conducted a bootstrap test.
I calculated the mean value of a certain interaction for each dyad and then sampled
the means of this interaction with replacements (Manly, 1997). This bootstrap
procedure was repeated 5000 times to estimate the distribution of the samples.
Afterwards I calculated a bias corrected 95% confidence interval, which considers a
skewed distribution of samples (Efron & Tibshirani, 1994). Finally I checked whether
the baseline (=1) was found within or outside of the 95% confidence interval of the
approximate distribution of relative latencies of dyads. This bootstrap test was
conducted two-tailed with a significance level of P<0.05. Baselines were different and
therefore I refer to them in the specific methods part of the chapters.
3.4.3 Programs
All multivariate analyses (GLZ and GLM) and parametric statistics were performed in
STATISTICA© 99 edition (StatSoft, 1999). Common non-parametric statistics were carried
out in STATXACT© 5 (CytelSoftware, 2001), using exact statistics for samples of N£15
(Mundry & Fischer, 1998). The bootstrap confidence intervals and the permutation tests
were computed in S-Plus (Insightful Corporation, 2001). All tests were performed as twotailed, apart of one test in chapter 614, which is indicated as one-tailed. In order to
uniformly state all the tests statistics (e.g. N = complete sample, n = sample of variable),
I used the statistical nomenclature of Vogt (1999), .
14
see 6.3.5.2
26
Chapter 415:
Food competition and linear dominance hierarchy
among females
Figure 4.0 Conflict over food: Male Macho and the females Belle and Venus are feeding
on the carcass of a red colobus monkey (Procolobus badius). Female Fossey (below) begs
for meat by touching the carcass, while Venus screams and raises her arm to hit Fossey
(Photo by R. Wittig).
15
Corresponding with: Wittig, R.M. & Boesch, C. (2003a). Food competition and linear
dominance hierarchy among female chimpanzees of the Taï National Park. International Journal of
Primatology. 24:847-867
Chapter 4: Food competition and linear dominance hierarchy
4.1 Introduction
4.1.1 Hierarchies in chimpanzees
Dominance relationships in many primates fit to a linear hierarchy, though in some
species rank orders are unclear or ambiguous (Walters & Seyfarth, 1987). Both seem to
be true for chimpanzees. While male chimpanzees rank either in linear hierarchies
(Mahale K-group: Nishida, 1979; Mahale M-group: Nishida & Hosaka, 1996; Kibale Ngogo:
Watts, 1998; Taï North-group: Boesch & Boesch-Achermann, 2000) or at least in narrow
rank categories (Budongo Sonso: Newton-Fisher, 2002; Gombe Kasakela: Bygott, 1979;
Goodall, 1986), linear hierarchies in female chimpanzees have not been detected.
Dominance relationships among females were either ordered in broad rank categories
(Gombe Kasakela: Pusey et al., 1997; Kibale Kanyawara: Wrangham et al., 1992), or at
least two-thirds of the dyadic dominance relationships were unknown to researchers
(Mahale M-group: Nishida, 1989; Budongo Sonso: Fawcett, 2000). Nevertheless high
ranking females in Gombe had significantly higher infant survival, faster maturing
daughters and more rapid production of offspring (Pusey et al., 1997). Thus female
chimpanzees increase reproductive success in a similar way to male chimpanzees,
wherein the alpha male or high ranking males are more successful in siring offspring
(Gombe: Constable et al., 2001; Taï: Boesch et al., in prep.). Although the outcome might
be similar in both sexes, the reason for the variance in reproductive success seems to be
different. While males can monopolise mating partners and secure exclusive mating, highranking females may obtain access to the best foods. A better fed female can invest more
energy in reproduction and thereby produce more offspring, or she can supply more food
to her offspring.
4.1.2 Socio-ecology of dominance relationships
Ecological factors have far reaching consequences on the relationships formed
among primate females (Wrangham, 1987). The socio-ecological model predicts that food
distribution and predation risk shape the competitive regime and therefore the
relationships formed among females of diurnal primate species (van Schaik, 1989; Sterck
et al., 1997; Koenig, 2002). Species facing scramble competition or no competition should
have egalitarian dominance relationships, in which hierarchies are unclear and non-linear,
if distinguishable at all. In contrast, species facing contest competition should have
despotic dominance relationships, in which dominance relationships are clearly established
28
Chapter 4: Food competition and linear dominance hierarchy
and form usually linear hierarchies. Such despotic females have often formalised
dominance relationships, which are expressed in ritualised signals wherein the direction is
independent of the context (de Waal, 1986; de Waal, 1989b).
Contest-type competition occurs when food distribution allows some individuals to
exclude others from accessing the resource. Therefore, contest competition should
increase with the monopolisablity of the resource and with the number of competitors. A
linear hierarchy should be adaptive when contest is so strong that the number of
aggressive interactions needs to be reduced by clear dominance relationships among the
competitors. Indeed, intra-specific and inter-specific comparisons have proved that
females that face more contest competition have a more linear and formalised hierarchy,
e.g. Saimiri oerstedi versus Saimiri sciureus (Mitchell et al., 1991), sympatric Presbytis
thomasi and Macaca fascicularis (Sterck & Steenbeek, 1997), and 3 neighbouring groups
of Semnopithecus entellus (Koenig, 2000).
Dominance relationships develop from repeated contests within dyads (Bernstein,
1981; Drews, 1993). Many group-living animals use ritualised signals to avoid aggression
(de Waal, 1986). Such formalised submissions are one of the last behaviours to indicate
rank changes and therefore serve as an indicator for acceptance of the relationship
(Macaca fascicularis: de Waal, 1977; Macaca mulatta: de Waal & Luttrell, 1985; Papio
cynocephalus: Walters, 1980; Pan troglodytes: de Waal, 1982; Wittig, 1997). De Waal &
Luttrell (1985) concluded that social integration is a condition for a formal hierarchy as it
clarifies the social status of an individual in the group. Clear relationships among all
individuals should lead to an interaction pattern, where in all individuals can be accepted
as partners for affiliative interactions, though interaction frequencies and intensities
among dyads differ due to the quality of the relationship.
4.1.3 Relationships in chimpanzees
Chimpanzees of all study sites live in multimale-multifemale, fission-fusion societies,
where the composition of parties can change frequently (Sugiyama, 1984; Goodall, 1986;
Nishida, 1990; Boesch, 1991; Wrangham et al., 1992). When meeting each other,
subordinates greet dominants by emitting pant-grunt vocalisations, which in chimpanzees
serve the function of formalised submission, as they provide a unidirectional and contextfree assessment of dominance relationships (de Waal, 1978; Bygott, 1979; Noë et al.,
29
Chapter 4: Food competition and linear dominance hierarchy
1980). However, as an exchange of greetings is only possible when associated, the
greeting frequency should increase with association.
Most chimpanzee communities are probably male-bonded (Mitani et al., 2002), as
males are philopatric in chimpanzees, while females emigrate from their natal community
(Watts & Pusey, 1993). Furthermore, males create long-term alliances (Riss & Goodall,
1977; de Waal, 1982; Nishida & Hosaka, 1996), and female chimpanzees are usually
characterised as being egalitarian (Sterck et al., 1997). However, genetically both Taï and
Gombe males are not more related than females within the same community (Vigilant et
al., 2001), though they are still the philopatric sex. Furthermore, it has been argued that
Taï chimpanzees are bisexually bonded because females build co-operative long-term
relationships, occasionally with stronger association than males (Boesch & BoeschAchermann, 2000). As Taï females are nut-crackers (Boesch & Boesch, 1983; Boesch &
Boesch, 1984,b) and have frequent access to meat (Boesch & Boesch, 1989; Boesch,
1994a,b), contest competition might be more important among them than in other
communities. Therefore Taï females may benefit from forming a linear dominance
hierarchy.
I investigated the existence of formal dominance relationships among females of a
community of wild chimpanzees in the Taï National Park, Côte d’Ivoire. I fit the formal
dominance relationships of the females to a hierarchy and test for linearity. I next
quantify contest competition over food resources in females and hypothesise that females
increase
aggressive
interactions
when
more
competitors
are
present
or
with
monopolisable food. Additionally, dominant partners are expected to possess the food
after the conflict. I also investigated the relationship among dominance hierarchy, age
and contest aggression. As females in other study sites did not establish linear
hierarchies, though they had different ages, dominance rank should only correlate with
contest aggression. Finally I assess the association and grooming relationships among the
females and discuss possible explanations for different findings in hierarchies in other
chimpanzee populations. Taï females should associate more frequently and have more
grooming dyads, indicating a stronger integration of females in the social network. The
same principles should work within the community and female dyads with unknown
relationships should be less associated and have weaker grooming relationships.
30
Chapter 4: Food competition and linear dominance hierarchy
4.2 Specific methods
4.2.1 Data and test conditions
The data set consists of 123 full-day observations of focal females. I observed each
female between 10 and 15 days during the observation period, which provided a total of
1028 h of female focal observation time. The death of Brutus in March 1997 changed the
association pattern in the community, so I analysed the 2 periods separately for the
questions related to association.
4.2.2 Operational definitions
I assess the dominance relationships of dyads by greetings, which are specific
vocalisations emitted by the subordinate to the dominant, usually accompanied by an
approach and submissive behaviour, such as crouching or bobbing, of the subordinate.
Three vocalisations serve the function of greetings in Taï: (a) pant-grunt (PG), a repeated
grunt exhaled with an open mouth; (b) greeting-hooh (GH), a repeated and intense hooh;
and (c) greeting-pant (GP), a repeated pant accompanied by submissive behaviour.
Food is monopolisable when it was accessible only in one spot or by one tool in the
feeding site, e.g., one stone hammer. One food spot is only one fruit, one water hole, one
insect-nest or whole colobus monkey. All these situations gave one individual the
possibility to feed on the food exclusively. The number of competitors are all the adult
and independent subadult chimpanzees present in the feeding site (=feeding party),
which excludes all members of the party that stay outside of the feeding site, e.g., resting
under the food tree, but visible. I calculated the observation time for feeding parties and
the time a type of food was present from the focal female observations. An aggressive
interaction over food is a food conflict and was won by the individual that possessed the
food after the conflict. I excluded food conflicts with infants and juveniles from the
analysis.
The greeting rate (GR) is the number of greetings within dyad AB divided by the
observation time of females A and B. The conflict rate is the number of aggressive
interactions among females in a feeding party of size X divided by the total observation
time of feeding parties of size X. The grooming rate is the duration of grooming of dyad
AB divided by the observation time of female A and B. Generally, rates are the number of
31
Chapter 4: Food competition and linear dominance hierarchy
events per total observation time; however, corrected rates are divided by the DAI,
because females in fission-fusion societies can only interact with a partner when
associated in the same party. For comparisons with other study sites I had to calculate
the rates as number of events among all females divided by the total observation time of
all females, due to lack of data for relationships among female chimpanzees.
4.2.3 Statistics
I tested for linearity of hierarchies via MATMAN© (Noldus, 1998), which provides
several measures to describe the linearity. I implemented two tests:
1. The linearity test, including Landaus linearity index (h) and Kendall’s coefficient of
linearity (K), provide a measure of the degree to which a dominance hierarchy is
linear, and both range from 0 to 1, with 1 describing complete linearity. While h
basically compares the number of dyads in which A dominates B to the total number
of dyads (Landau, 1951; Chase, 1974), K basically compares the number of circular
triads with the total number of dyads (Appleby, 1983). Linearity is evidenced when
the proportion of circular triads is less than expected by chance based on a c2
distribution (Appleby, 1983). Both measurements give similar results but they are
problematic when the matrix of the dominance relationships contains unknown
relationships.
2. I conducted an improved linearity test when >10% of the relationships were unknown
or tied. Unknown relationships are dyads with no greeting exchanged. Tied
relationships are dyads with an equal number of greetings in both directions. For the
improved linearity test I used the corrected linearity index (h’), which is the average of
all h values calculated for the complete set of each possible dominance matrix. The
complete set of dominance matrices is produced by systematically switching the
direction of dominance for unknown relationships, while the tied relationships are
always assigned half dominant and half subordinate status (de Vries, 1995). The
significance of the linearity was obtained by a randomisation test, which compares the
h value of 10000 randomly chosen matrices with h’. Linearity is documented when
>95% of the randomly chosen matrices have a smaller h than the corrected linearity
index (h’).
32
Chapter 4: Food competition and linear dominance hierarchy
To compare differences of interactions in dyads, I applied the permutation test.
Kendall partial rank-order correlation coefficient was conducted using the procedure of
(Siegel & Castellan, 1988).
4.3 Results
4.3.1 Female hierarchy
I observed 187 greeting interactions among females that included PG (62%), GP
(30.5%) or GH (7.5%). Of the 55 female-female dyads, two thirds showed unidirectional
greeting (67.3%), 12.7% of the dyads had bi-directional greetings and 20% of the
dominance relationships remained unknown as no greetings were exchanged. This lack of
~
greetings might be due to the rather low greeting rate (GR) in female dyads ( GR ff=0.011
greetings/hour). Greetings among females occurred sixteen times less frequently than
~
among males ( GR mm=0.178 g/h; Permutation test: p<0.05) and 4 times less frequently
~
than in mixed sex pairs ( GR mf=0.047 g/h; Permutation test: p<0.05).
dominant
Receiver
rank categories:
Signaller
Macho Brutus Marius Mystère
Macho
0
0
0
Brutus 13
0
0
Marius 643
5
0
Mystère
96
19
20
Nino 361
21
42
0
Loukoum
51
4
13
7
Venus
42
10
11
1
Ricci
79
11
19
4
Goma
44
11
14
2
Perla
48
14
15
7
Belle
28
9
15
1
Castor
94
12
11
19
Narcisse
53
3
15
3
Dilly
86
33
30
25
Fossey
29
3
7
0
subordinate
Nino
0
0
0
0
4
0
0
1
1
0
3
0
1
0
Loukoum
0
0
0
1
0
2
5
2
6
0
5
3
8
2
Venus
0
0
0
0
0
0
2
1
1
4
2
1
4
0
Ricci
0
0
0
0
0
0
1
2
1
1
1
0
3
1
Goma
0
0
0
0
0
0
1
0
7
3
1
0
21
3
Perla
0
0
0
0
0
0
0
0
0
2
2
0
1
1
Belle
0
0
0
0
0
0
0
0
0
1
1
0
0
1
Castor Narcisse
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
0
0
0
0
2
4
4
0
0
Dilly
0
0
0
0
0
0
0
0
1
2
0
0
0
Fossey
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Figure 4.1 Dominance relationships, based on greetings, among the adult chimpanzees in the North community in Taï. The signaller,
presented vertical, is greeting the receiver, presented horizontal. The individuals are shown according to their rank in increasing order from
left to right. The six narrow rank categories of females are indicated by different bars above the females names (note that the last category,
marked white, includes the greetings of the subadult females). For dyads including males (italic–grey letters) pant-grunts are shown only
(grey numbers), whereas among females greeting-pants and greeting-hoohs are inserted as well.
When analysing the direction of greeting events I detected a linearity in the
hierarchy among the females (Figure 4.1; Improved linearity test: h’=0.67, p<0.01).
Forming narrow rank categories (Figure 4.1; two rank-neighbouring females per category,
and the three adolescent females in the lowest rank category), females showed a clear
33
Chapter 4: Food competition and linear dominance hierarchy
2
rank order (Linearity test: h=0.97; K=0.97; c 30
=51, p<0.01), which has an equivalent
strength to the linear hierarchy of the males (Figure 4.1; Linearity test: h=1; K=1). The
hierarchy of the complete community (females and males) was linear with females
subordinate to males (Improved linearity test: h’=0.80, p<0.001) except of the unknown
dominance relationship between a-female Mystère and subadult male Nino. Thus the
dominance relationships of female chimpanzees in Taï fitted a linear hierarchy.
Table 4.1. Food conflicts among female chimpanzees in the North
community in Taï. The number of conflicts is presented per initiating female
in declining dominance rank order. While the left column shows the
proportion of conflicts over monopolisable food, the right columns show the
possessor of the food depending on the dominance relationships of the
conflict partners. Average proportions and expected proportion for conflicts
over monopolisable food are presented in the last rows.
Female
Conflicts over
Possessor of food after conflict
dominant subordinate 3rd individual
conflicts
monopolisable food*
n
%
Initiator
n
n
n
22
9
0
0
Mystère 9
63
27
5
3
Loukoum 35
60
5
0
0
Venus 5
33
2
1
0
Ricci 3
33
4
0
2
Goma 6
36
8
1
2
Perla 11
100
1
1
0
Belle 2
9
6
2
3
Castor 11
100
0
1
0
Narcisse 1
59
7
5
5
Dilly 17
33
3
0
0
Fossey 3
mean
50%
66%
23%
11%
expected
8%
*conflicts over non monopolisable food complete proportions to 100%
4.3.2 Food competition
I observed 103 conflicts over food among females, giving a rate of about 0.1 food
conflicts per hour of observation time and about 0.22 food conflicts per hour in a feeding
party. Although only 8% of the community’s feeding time was on monopolisable food,
monopolisable food was the reason for 50% of the food conflicts among females (Table
4.1). Comparing the indices of difference (observed minus expected frequency) for each
female showed that females initiate conflicts over monopolisable food more frequently
than over non-monopolisable food (Wilcoxon exact: T=66, N=11, p<0.01). Females
fought over many different types of monopolisable food, such as meat, stone hammers
34
Chapter 4: Food competition and linear dominance hierarchy
(tools to crack nuts), water holes in trees, eggs of ants, honeycombs, fruits of Treculia
africana and tree-mushrooms. Although only 42% of the feeding time on monopolisable
food was attributed to meat, almost 70% (34 of 49 cases) of the conflicts on
monopolisable food were over meat. Meat was the main reason for contests over
monopolisable food among female chimpanzees in Taï (Goodness of fit exact: c 12 =15.04,
p<0.001).
2
conflict rate [conflict / h]
1.5
r 2 =0.74, p <0.001
1
0.5
r 2=0.01, ns
0
non
1
2
3
4
5
6
7
8
9
10
11
12
13
14
competitors present in feeding party [N]
Figure 4.2 Conflict rate among females over food with different numbers of competitors
present in the feeding party in Taï chimpanzees. Feeding parties attending nonmonopolisable food (D) are considered separately from those attending monopolisable food
(l). Regression lines are calculated for non-monopolisable (solid line) and monopolisable
food (dashed line). Test statistics see 4.3.2.
The conflict rate, when feeding on non-monopolisable food, strongly increased with
the number of competitors present in the feeding party (Figure 4.2; Pearson: r=0.86,
N=14, p<0.001). However, no correlation was found for monopolisable food (Pearson:
r=0.12, N=14, ns), indicating no difference in strength of competition over food between
35
Chapter 4: Food competition and linear dominance hierarchy
one and more competitors, when the food can be monopolised by one individual (Figure
4.2). Although females faced competition with between 1 and 17 chimpanzees in their
feeding parties, I only included feeding parties up to 14 competitors, as cumulative
feeding time reaches a plateau with 14 competitors and feeding time with more
competitors was rare (<1 h).
On average the dominant female of the conflict dyad possessed the food in 66%
after conflicts (Table 4.1). Being dominant over a competitor provided an advantage in
female contests, as dominant conflict partners possessed the food significantly more
frequently after conflicts than subordinates independent from the initiator (Table 4.1;
Wilcoxon exact: T=53.5, N=11, p<0.01). However in 15 cases none of the females was
successful (Table 4.1), as both lost the food to a third party (in five cases to a male, four
to a dominant female, five to a group of females and in one case to a subordinate
female). Hence, female chimpanzees in Taï faced contest competition over food, as
females fought more for monopolisable food or with increasing numbers of competitors
and dominant females possessed the food after a conflict.
Table 4.2 Correlates of dominance rank in Taï females.
The females are ordered in declining dominance rank order
and show the assigned dominance rank, contest rank and
age.
Name Dominance rank
Contest rank Age [years]*
Mystère
1
2
24
Loukoum
2
1
27
Venus
3
7
21
Ricci
4
3
36
Goma
5
5
26
Perla
6
4
23
Belle
7
8
23
Castor
8
6
23
Narcisse
9
10
16
Dilly
10
9
21
Fossey
11
11
20
*approximate age in 1999, as dates of birth are estimated except
for Belle (Boesch and Boesch-Achermann, 2000)
36
Chapter 4: Food competition and linear dominance hierarchy
4.3.3 Correlates of the linear hierarchy
I tested two correlates of the linear ranking in Taï females. First, dominance rank
order of females was related to age, with older females dominating younger ones (Table
4.2; Spearman Rank: r=-0.70, N=11, p<0.05). Second, I created a linear hierarchy based
on contest aggression interactions over food (contest rank), with winners being assigned
higher ranking (Table 4.2; Improved linearity test: h’=0.43, p=0.06), and found that
contest rank correlates with dominance rank (Spearman Rank: r=0.86, N=11, p<0.001).
However, when eliminating the influence of either contest rank or age, the correlation
between dominance rank and age disappeared (Kendall Partial Correlation: T=0.05,
N=11, ns), while the correlation between dominance rank and contest rank remained
significant (Kendall Partial Correlation: T=0.54, N=11, p<0.05). Thus the linear hierarchy
is related to the outcome of the contest, while it is independent from age.
4.3.4 Social relationships
~
Females had a median DAI
ff
of 0.27 (Appendix C). Although their association is
very high compared to other study sites (Table 4.3), female-female association in Taï was
~
about one third less than male-male association ( DAI
mm=0.42;
Permutation test:
~
p<0.05), but more frequent than female-male association ( DAI mf=0.23; Permutation
test: p<0.05). Before the death of Brutus the association among the females was about
~
25% lower than after (Appendix C; with Brutus: DAI ff=0.21; Permutation test: p<0.05).
If the lack of clear dominance relationships in other study sites is due to low greeting
rates because of a lower association (Table 4.3), one would expect a lower corrected
greeting rate (CGR) during the period of lower association. However there was no
difference in the CGR between the periods of lower and higher association (Permutation
test: ns). I even found a higher CGR during the period of lower association, when
analysing the 15 female dyads that showed greetings in both periods (with Brutus:
~
CGR ff=0.171 greetings/associated hour (25%: 0.147, 75%: 0.311); without Brutus:
~
CGR ff=0.074g/ah (25%: 0.032, 75%: 0.128); Permutation test: p<0.05).
Finally I tested whether the 11 female dyads of unknown dominance relationships
~
were rare associates and non-groomers. The median DAI
unknown
of the 11 dyads was
0.27 (combine Figure 4.1 and Appendix C) and thus the females of unknown dominance
relationships associated the same amount as females with unidirectional or bidirectional
37
Chapter 4: Food competition and linear dominance hierarchy
~
relationships ( DAI
unidirectional=0.29;
~
Permutation test: ns; DAI
bidirectional=0.31;
Permutation
test: ns). Female pairs had a median grooming rate of 2.71 seconds/hour (25%: 0.32s/h,
75%: 4.90s/h), spread over 30 bi-directional grooming, 18 unidirectional grooming and 7
non-grooming relationships. The 11 unknown dominance relationships were represented
by five bi-directional, five unidirectional and one non-grooming dyads. Thus there was no
difference in the grooming pattern between dyads of unknown and unidirectional
dominance relationships (Goodness of fit exact: c 22 =0.83, ns). Hence I note that within
the community an increase in association did not imply more greetings and that neither
association nor grooming patterns among Taï females were able to explain the lack of
dominance relationships in some dyads.
Table 4.3 Comparison of dominance relationships among female chimpanzees in five study sites in the wild. Parameters presented in the
table describe the hierarchy, social relationships and contest competition of females.
proportion of nonunidirectional greeting
dominance relationships
74%
DAI·
0.09
grooming
pattern¸
---
greeting rate¹
corrected rate »
0.022
0.24
food conflict rateº
corrected rate »
---
high, middle,
low2
---
0.053
35/26/393
---
0.0033
0.06
13
high and low
---
0.08
0/0/100
---
---
135
1-135 (linear)
69%5
(0.39)6
»0.057
45/558
0.0465
0.92
0.0165
0.32
0.113
0.42
0.1
0.37
mean female
observation time [h]¶
54
N adult
females
12
rank
categories
a and not-a
4363
11-172
Kibale4
Kanyawara
7
Mahale5,6,7,8
M-group
635
study site
Budongo1
Sonso
Gombe2,3
Kasakela
103
11
1-11 (linear)
33%
0.27
55/32.5/
Taï9
North-group
12.5
Note:
¶ mean observation time of target females in hours.
· average dyadic association index of females.
¸ grooming pattern shows proportion of bi-directional / unidirectional / non grooming female dyads (Mahale:
grooming / non-grooming).
¹ greeting rate is all greetings exchanged by females per total female observation time.
º food conflict rate is all food conflicts among females per total female observation time.
» corrected rate (in italic numbers) are rates divided by the DAI.
--- no data available.
1
Fawcett, 2000
2
Pusey et al., 1997
3
Goodall, 1986; observation time: 8 focal females for the two year period of 1978-79 from appendix c, DAI: from
1978-81 (appendix d), grooming pattern: calculated from in camp grooming sessions of 3 adult females within 31
dyads (Fig. 14.2), conflict rate: from Fig. 12.4 and multiplied with 0.66 as only two thirds of female conflicts are
with other females (chapter: aggression).
4
Wrangham et al., 1992
5
Nishida, 1989; conflict rate: from Table 8.
6
Nishida, 1968; K-group.
7
Nishida & Hosaka, 1996; DAI: with a correction factor for DAI in not artificial feeding areas (correction method
described in: Boesch & Boesch-Achermann, 2000, Table 5.9).
8
Huffmann, 1990; from focal data of females in 1985: Fig. 12.2, of 29 F-F dyads representing grooming dyads vs.
non-grooming dyads.
9
this study (chapter 4; Wittig & Boesch, 2003a).
38
Chapter 4: Food competition and linear dominance hierarchy
4.4 Discussion
4.4.1 Summary of results
The dominance relationships of female chimpanzees in Taï fit to a linear hierarchy.
While the linear hierarchy is independent of age, winning contest over food is related to
the dominance rank order. Taï females face food competition with higher competition
over monopolisable food or with more competitors in the feeding site. They maintain
affiliative relationships with almost all other females in the community and show higher
association patterns than females in other chimpanzee sites. Neither association nor
grooming patterns explained the occurrence of unknown dominance relationships within
the community.
4.4.2 Dominance relationships
Despite the fact that I found a linear hierarchy among the females in Taï, some
dyads emitted greetings in both directions. This is similar to other study sites where
females sometimes used mutual pant-grunts during greetings (Gombe: Goodall, 1986;
Mahale: Nishida, 1979). Even among males, where unidirectionality of pant-grunts is
generally reported in the literature, dominance reversed pant-grunts were found (Gombe:
Goodall, 1986; Mahale: Nishida & Hosaka, 1996; Taï: Boesch & Boesch-Achermann,
2000), indicating that pant-grunts are not exclusively unidirectional. Even though I used a
combination of different greeting vocalisations among the females, these greetings still
showed strong directionality. Therefore these greetings provide a good indicator for
females’ dominance relationships, showing that they form a stable and linear hierarchy.
Female chimpanzees in Taï appeared to show a more clear linearity in their
hierarchy than females in the five other chimpanzee sites (Table 4.3). Only two dyads
(Perla-Dilly and Dilly-Fossey) remained unclear when taking triadic relationships into
account. In contrast more than two thirds of the dominance relationships were unknown
in Mahale, about three quarters of the dominance relationships were unknown in Budongo
(Table 4.3) and in Gombe, even though broad categories were implemented, 12% of the
females did not fit to a rank (Pusey et al., 1997).
39
Chapter 4: Food competition and linear dominance hierarchy
4.4.3 Contest competition
Female chimpanzees in Taï used aggression to keep resources or to win them from
other females. Unequal access, especially to monopolisable food such as meat, might be
an explanation for the development of the linear hierarchy in Taï females. For example,
dominant females may access 500 grams of meat per successful hunt, a substantial
benefit compared to 80 grams of average females (Boesch & Boesch-Achermann, 2000).
There was even unequal access to non-monopolisable food due to contests in large
feeding parties (Figure 4.2). It seems that Taï females contest over feeding space in food
patches (e.g crowns of fruit trees), as observed in Thomas langurs (Presbytis thomasi:
Sterck & Steenbeek, 1997). Being dominant seems to enable females to have priority of
access to a higher quality and quantity of food. However the question remains whether
the frequency of contest is regular enough to actually cause the linear hierarchy.
Comparing the individual aggression rates of Taï females attending a feeding site reveals
that Taï females are in the upper range of primate species of whom several fit to linear
hierarchies (individual female food contest rate [conflict/hour feeding]: e.g.: Cercocebus
torquatus: 0.04c/h, Range & Noë, 2002; Chlorocebus aethiopis: 0.07c/h and Erythrocebus
patas: 0.04c/h, Pruetz & Isbell, 2000; Saimiri sciureus: 0.28c/h, Mitchell et al., 1991; Pan
troglodytes: 0.22c/h, this study; Wittig & Boesch, 2003a). Thus contest competition
seems to be high and beneficial for the dominant females.
4.4.4 Reasons for linearity
It seems that being older enhances the chance to win the contest. Taï females, like
females in other study sites, may increase their individual rank as they age (Gombe:
Pusey et al., 1997; Mahale: Nishida, 1989). However the dominance rank order in Taï
females is related to contest rank but not to age. Thus competition over food is the main
determinant for the development of the linear hierarchy in Taï females.
In spite of contest competition, Taï females have affiliative interactions with almost
all other female community members. Although neither association nor dyadic grooming
relationships were able to explain the variability in the Taï dominance relationships, the
comparison with other study sites reveals remarkable differences (Table 4.3). In addition
to an almost five times higher association rate among females in Taï, they presented the
highest number of dyads with bi-directional grooming and the lowest number of females
that never groomed together compared to other study sites. Taï females build long-lasting
40
Chapter 4: Food competition and linear dominance hierarchy
friendships including food-sharing and support (Boesch & Boesch-Achermann, 2000) and
they seem to take in account such factors when initiating aggression (chapter 516; Wittig
& Boesch, 2003b). Therefore the female society in Taï might be shaped through
advantages of female bonding when access to food is affected by contest competition.
4.4.5 Comparison among study sites
Why did I find such differences in the female relationships among different
communities of the same species? One argument might be that linear hierarchies have
not been detected yet in the other study sites. On one hand, linearity is easier to find in
small groups (Drews, 1993), but the number of females examined was similar across sites
(Table 4.3). On the other hand longer observations increase the chance to observe
greetings between females. Observation time is surely not the problem for Gombe, while
it is perhaps a problem in the Kibale study (Table 4.3). However observation time in Taï,
Mahale and Budongo is similar, and therefore differences among the female hierarchies
should be independent of observation time. In Mahale and Budongo greetings among
females occurred at least 50% less often than in Taï (greeting rate) which could explain
the large number of non-unidirectional relationships (Table 4.3). However in Mahale, but
not in Budongo, females actually greeted more frequently when they met each other than
in Taï (Table 4.3; corrected greeting rate). This is a similar observation to our findings of
a higher corrected greeting rate during the lower association period. It may be, that
female chimpanzees have to confirm their submissive position more often the less
frequently they meet when a dominance relationship does exist. Thus in Mahale a linear
hierarchy might exist, but may require more data for detection, whereas the dominance
relationships are ambiguous among the females in Budongo.
The other possibility might be that female dominance relationships do not fit to
linear hierarchies in other study sites. As food distribution and predation risk are
suggested to influence female gregariousness, which shapes the competitive regime of
females, contest competition should depend on the association of females (Sterck et al.,
1997). Although food conflicts occur 6 to 30 times less frequently in other study sites than
in Taï (Table 4.3; food conflict rate), the actual rate of food conflicts among two females
when associated is the same for Taï and Mahale (Table 4.3; corrected food conflict rate).
Thus Mahale females face similar levels of contest competition as Taï females, whereas in
16
see Figure 5.5
41
Chapter 4: Food competition and linear dominance hierarchy
Gombe contest competition among females is lower. However females in Mahale and Taï
cope differently with the contest competition. While Mahale females seem to disperse to
reduce contest competition, Taï females build a formal linear hierarchy to endure the
contest competition. This may reflect an higher advantage in staying together for Taï
females – perhaps to reduce predation risk (Caraco et al., 1980; van Schaik et al., 1983;
Boesch, 1991). Although mortality rates for Taï and Mahale chimpanzees are similar and
the highest under the known chimpanzee communities (Hill et al., 2001), predation by
leopards is only known to exist in Taï (Hiraiwa-Hasegawa et al., 1986; Boesch, 1991;
Boesch & Boesch-Achermann, 2000). Hence females in Taï may have evolved a clearer
linear dominance hierarchy than females in other chimpanzee study sites, as an
adaptation to a stronger contest competition (compared to Gombe) and a higher
predation risk (compared to Mahale).
The comparison among study sites (Table 4.3) revealed strong differences, not only
in the females’ dominance relationships, but also in their competitive regimes, their level
of association and affiliation, and the predation risk females and their offspring face.
Although there are some limitations to the data used here for the cross-study comparison,
the data having been collected with different methods or during different periods and
adjusted post-hoc for comparison, our comparison supports the predictions of the socioecological model (van Schaik, 1989; Sterck et al., 1997). Data on general food distribution
and abundance were not considered for the comparison, as such data are just becoming
available (e.g.: Fawcett, 2000; Anderson et al., 2002).
Our findings of a linear dominance ranking in females and a stronger female
integration in the social network, support the bisexual model of Taï chimpanzee social
organisation (Boesch, 1991), as do other findings from Taï (Lehmann & Boesch, in prep.).
This shows that social structure in chimpanzees is more flexible than previously thought
and adapts to ecological circumstances.
42
Chapter 517:
Decision-making in conflicts: extension of the
Relational Model
Figure 5.0 Alpha-male Macho (centre) displays a bipedal swagger with a shaking branch,
just before he starts to charge male Marius (Photo by R. Wittig).
17
Corresponding with: Wittig, R.M. & Boesch, C. (2003b). “Decision-making” in conflicts of wild
chimpanzees (Pan troglodytes): an extension of the Relational Model. Behavioral Ecology and
Socioniology. 54:491-504
Chapter 5: Decision-making in conflicts
5.1 Introduction
5.1.1 The Relational Model (RM)
Group-living animals face the problem of competing over resources and mating
partners with the same social partners that they cooperate with in protection against
predators and in resource acquisition (Wrangham, 1980; Walters & Seyfarth, 1987; van
Hooff & van Schaik, 1992; de Waal, 2000b). The dilemma is that an out-competed
cooperation partner might withhold assistance in the next situation when cooperation is
required. Therefore the use of aggression to resolve competitive situations carries
advantages and disadvantages. This has led to a number of mechanisms being proposed
which function as counter-disadvantage regulations (e.g. dominance hierarchy: Popp & de
Vore, 1979; post-conflict management: Kappeler & van Schaik, 1992; meat-sharing:
Boesch, 1994b). The most recent and comprehensive model for aggressive competition
including counter-disadvantage mechanisms is the Relational Model (RM: de Waal,
1996a).
The RM considers aggression as well as tolerance and avoidance as possible
reactions to a conflict of interest – a competitive situation of two individuals having
incompatible goals (van der Dennen & Falger, 1990). The model predicts that whenever
there is a conflict of interest, the tendency to take aggressive action increases with the
number of opportunities for competition, as well as the resource value and the reparability
of the relationship, while it decreases with the risk of injury and the value of the
relationship (de Waal, 2000a). An individual is viewed as going through a decision-making
process during a conflict of interest, weighing the costs against the benefits. Thus, I do
not imply any deliberate intention when referring to decision-making18. Although the RM is
a convincing model, it has never been tested as a complete model with data of one
species. In this paper I assess this decision-making process in wild chimpanzees using a
cost – benefit estimation based on the RM, where the net-benefit applies to benefits
minus costs.
In the RM the benefit is taken as the value of the resource. According to the socioecological model, food is proposed as the most beneficial resource for females whereas a
sexual partner holds the most benefit for a male (Janson, 2000). High rank, determining
18
see last paragraph of 1.3
44
Chapter 5: Decision-making in conflicts
priority of access to females, is very beneficial to male primates (Dixson, 1998). In the RM
cost of aggression is taken as the risk of injury. Injury obtained during fighting can entail
tremendous costs through infection and disability, hampering food acquisition, travelling
and defence against predators (Palombit, 1993). In the RM social costs are represented
by the negative effects aggression produces by disturbing the relationship between
partners (e.g. unwillingness to support or to share food). The more valuable the
relationship, the more partners suffer from the disturbance (Aureli & Smucny, 2000).
However, the disturbance can be reduced by reconciliation, such that relationships of
former opponents return to their baseline level and these dyads seem to interact like
before (Cords, 1992). In the RM the tendency to reconcile is referred to as the reparability
of the relationship. The opportunity for competition, suggested as a predictor in the RM,
seems to be more a predictor for the number of conflicts of interests individuals face
rather than for their aggressive tendency. Although the aggression rate increases with
competition (Janson, 1988b; Barton & Whiten, 1993; Sterck & Steenbeek, 1997), the
individual’s aggressive tendency per conflict of interest should not be affected when the
other factors stay constant. Therefore, the opportunity for competition is not considered
as a predictor in our study.
5.1.2 Extension to the Relational Model
5.1.2.1 Likelihood of winning
Despite the fact that several costs and benefits are taken into account, the RM does
not explicitly consider two other important factors. First, whilst predicting a higher
tendency for aggression when the value of the resource is high, it only evaluates the
potential benefit of the resource, without considering that the resource has to be
accessed to gain the benefit. This requires winning the contest. Different fighting abilities
of conflict partners influence the outcome of the contest (Huntingford & Turner, 1987). In
stable social groups, where the same two individuals compete repeatedly, dyadic
dominance relationships often emerge as linear rankings at group level (Jackson, 1988).
Conflict partners with small rank differences have more equally matched fighting abilities.
In consequence, the probability of winning should increase with the rank difference for
the dominant initiator, but should decrease for the subordinate who fights a stronger
individual. Except for the highest and the lowest ranking member of the community,
individuals are facing conflict situations where each can be either dominant or subordinate
to the conflict partner. In this manner, the likelihood of winning a contest, a precondition
45
Chapter 5: Decision-making in conflicts
for gaining the value of the resource (the benefit), is an important component added to
the RM.
5.1.2.2 Conflict duration and cost of aggression
Second, the RM incorporates the risk of injury as the only cost of aggression.
Although most aggressive interactions end without injuries (Bernstein et al., 1983;
Whitten & Smith, 1984), are they really free of cost? On one hand aggression requires
energy expenditure and this energetic cost of fighting has been shown to increase with
duration (Smith & Taylor, 1993; Hack, 1997; Neat et al., 1998). On the other hand, longer
conflicts are usually followed by aggressive post-conflict interactions (chapter 619; Wittig &
Boesch, 2003d). Thus, third parties might join the conflict the longer an aggressive
interaction continues, which involves additional costs of aggression. As energy
expenditure increases with conflict duration and long conflicts involve additional
aggression costs I have added duration as second necessary component to the RM.
5.1.2.3 Creation of social costs
Furthermore the RM predicts that the tendency to initiate aggression increases with
reparability of the relationship independent of whether there will be negative effects of
the aggression. However, repairing a relationship is important only when there is
something to repair. Cords & Thurnheer (1993) have shown that the occurrence of
reconciliation is dependent on the value of the relationship of former opponents.
Therefore, the value of the relationship and its reparability are dependent on their
contribution to the RM. Both factors have to be considered together in the decisionmaking process, as repairing the relationship can counteract the social costs.
5.1.3 Testing the extended Relational Model
In general animals can accept higher costs as long as the net-benefit is positive
(Parker & Rubenstein, 1981). This might lead to different strategies by the same
individual depending on the conflict situation: the same resource provides a different
benefit to different individuals and different opponents require different fighting intensities
or incur different social costs. Thus the dyadic character of conflict situations has to be
considered by a competitor when deciding about aggression.
46
Chapter 5: Decision-making in conflicts
Chimpanzees represent a good model to test the decision-making process.
Chimpanzees live in multi-male multi-female communities and face intra- and inter-sexual
competition with multiple partners. A formal linear hierarchy is found among all adult
members of the community based on dyadic dominance relationships (chapter 420; Wittig
& Boesch, 2003a). They have frequent bouts of aggression that occur in different
contexts, with different partners and different intensities (Goodall, 1986). Alternatively,
individuals can avoid aggression before it arises by just leaving the party due to the
fission-fusion character of the chimpanzee society. Finally chimpanzees built and maintain
long-term relationships and often reconcile after aggressive interactions (Arnold & Whiten,
2001; Preuschoft et al., 2002).
The aim of this chapter is to test the extended RM with aggressive interactions of
chimpanzees in the Taï National Park, Côte d’Ivoire. This test investigates how
chimpanzees handle the conflict situation, taking into account the conflict’s costs and
benefits. The extended RM includes as benefit a combination of the benefit of the
resource – assessed by the value of the resource – and the likelihood of winning, whereas
costs include the cost of aggression and the social costs – assessed by the value of the
relationship – minus the part of the social costs that gets reduced by relationship repair.
My extended version of the RM predicts that aggression is expected when the net-benefit
is positive.
I tested the extended model using a step by step procedure, adding more
parameters in each step, to show schematically how the decision-making process may
operate for chimpanzees. First, the resource arouses the individuals interest. The
individual assesses its likelihood of winning and how much aggression to invest, taking
into account the identity of the competitor. Finally the individual estimates the disturbance
the aggression will cause to its relationship with the competitor and if this is repairable.
Thus the potential gain in net-benefit may be estimated, allowing an individual to decide
whether to initiate aggression in this particular situation and how to handle the conflict.
Each decision is highly dependent on the identity of the individuals in the dyad. Therefore
our analysis is based on dyadic data and not on individual data. For simplicity from now
on the term conflict is used instead of aggressive interactions.
19
20
see Figure 6.2
see 4.3.1
47
Chapter 5: Decision-making in conflicts
Step1: Estimating the benefit
a. The benefit of the resource: individuals are predicted to initiate conflicts more
frequently in accordance with the value of the resource to them. I expect females to
initiate more conflicts over food and males over oestrous females.
b. The likelihood of winning: individuals are predicted to initiate conflicts more
frequently the higher their likelihood of winning. I expect dominants to initiate and to win
more conflicts than subordinates.
Step 2: Estimating the costs of aggression
Individuals are predicted to risk more the higher the benefit of the resource is to
them as long as they have a chance to access it. I expect chimpanzee initiators to have
longer and more intense fights the higher the value of the resource is to them and when
they are dominant in the dyad. However, although dominant initiators are able to risk
more than subordinates, in order to minimise costs individuals are predicted to avoid
escalation of aggression. I expect therefore short and less intense fights when the rank
difference to the opponent is large.
Step 3: Estimating the social costs
a. The social costs: individuals are predicted to keep the disturbance of their
relationship with the conflict partner small. I expect chimpanzees to initiate less conflicts
with high benefit partners. If they initiate conflicts, then they are shorter and less intense
(to minimise the costs of aggression) and mainly for high benefit resources in dyads
which they are likely to win (to maximise the benefit).
b. Reducing social costs: individuals are predicted to reduce social costs more
frequently when the social costs are high. I expect chimpanzees to have a high chance of
reconciling when they initiate conflicts with high benefit partners (to reduce social cost).
5.2 Specific methods
5.2.1 Data and test conditions
The result of the balancing process to eliminate repeated measurements was a data
set of 409 conflicts. I tested the predictions following a stepwise model design, which
changed the number of variables in each step (Table 5.1). During step 1b (likelihood of
48
Chapter 5: Decision-making in conflicts
winning), cases ending in a draw were not considered, as samplesize was too small (less
than 4% of conflicts).
Table 5.1 Variables considered for the tests in the different steps of the stepwise model design.
step
1
2
3
? = set
set W:
set X:
set Y:
set Z:
independent variables
part factor of the extended RM
dependent variable set W
set X
set Y
set Z
a benefit of the resource
Conflict Context
?
b likelihood of winning
Initiator’s Victory Outcome
?
?
a ü
Aggression Type
?
?
b ý cost of aggression
Conflict Intensity
?
?
c þ
Conflict Duration
?
?
a social cost
Relationship Benefit
?
?
?
b reduction of social cost
Corrected Conciliatory Tendency Index
?
?
?
?
of variables considered as independent variables for the test
Initiator’s Sex, Receiver’s Sex, Sex Combination
Conflict Context, interaction (Conflict Context x Initiator’s Sex), Initiator’s Rank, Rank Difference, interaction
(Initiator’s Rank x Rank Difference)
Conflict Intensity, Conflict Duration
Relationship Benefit, Association Index
5.2.2 Additional operational definitions
Reconciliation was defined as the first post-conflict interaction with socio-positive
body-contact among former opponents. I used the corrected conciliatory tendency (CCT)
for the probability that a dyad will reconcile after a conflict (Veenema et al., 1994). CCT is
the quotient of the number of reconciliation events occurring sooner than the baseline
time minus the number of reconciliation events occurring later than the baseline time,
divided by number of conflicts. I calculated the baseline time as the average time interval
between two consecutive friendly interactions of the same dyad on the same day. Since
CCT is higher for males than females (Preuschoft et al., 2002), I corrected CCT for sex
differences by calculating an index for comparative reasons (like for DAI and rank
difference: see 3.2.5 and Table 3.2).
5.2.3 Special analysis
To calculate expected frequencies I corrected for the frequency of occurrence of
each context. As conflicts could occur only when there was a partner present, I excluded
data when chimpanzees were alone. Social conflicts could happen anytime (expected
frequency: 1.0). However food conflicts could occur only when chimpanzees were
feeding. Males like females showed an average feeding time of about 45% of their time
budget (expected frequency=0.45). Conflicts in a sex context could occur only when at
least one female had a genital swelling. For male focals, oestrous females were present
on 48 of the 80 days and focal females were in oestrous on 23 of the 123 days. As a
49
Chapter 5: Decision-making in conflicts
result, expected frequencies are different for male-male conflicts (0.6), mixed sex conflicts
(0.35) and female-female conflicts (0), since only conflicts with the focal animal were
recorded and competition over sex among females was not expected. To enable a
comparison between expected and observed frequencies, I standardised the frequencies
by setting the sum of expected frequencies within each sex combination to 1. When
comparing among five and less individuals, standard non-parametric statistics is not
applicable (step 1a for male initiated conflicts). Therefore, I applied the permutation test
(see 3.4.2).
Table 5.2 Frequency (in %) of conflicts initiated by females and males per
conflict context and sex of conflict partner in Taï chimpanzees.
females
context
Belle
Castor
Dilly
Fossey
Goma
Kana
Loukoum
Mystère
Narcisse
Perla
Ricci
Venus
average
expected
female – female conflicts
n
social
food
sex*
14
86
14
19
42
58
26
35
65
7
57
43
10
40
60
3
33
67
40
13
87
19
53
47
6
83
17
15
27
73
8
62
38
12
58
42
49
51
69
31
n
2
4
3
2
2
0
18
3
4
1
5
5
female – male conflicts
social
food
sex
50
50
0
25
50
25
67
33
0
0
0
100
50
50
0
------39
61
0
33
67
0
100
0
0
0
100
0
20
80
0
60
0
40
40
45
15
56
25
19
males
male – male conflicts
male – female conflicts
context
n
social
food
sex
n
social
food
sex
Brutus
19
58
37
5
31
42
55
3
Macho 356
80
9
11
100
41
30
29
Marius
29
90
3
7
77
51
4
45
Nino
0
------36
58
25
17
76
16
8
48
28
24
average
expected
49
22
29
56
25
19
expected are the standardised expected frequencies within each sex combination;
*females did not compete over sex.
50
Chapter 5: Decision-making in conflicts
5.3 Results
5.3.1 Step 1: Benefit
5.3.1.1 Benefit of the resource
Did chimpanzees fight more in particular contexts (Table 5.1: step 1a)? Table 5.2
compares the frequency of initiated conflicts in the different contexts of each individual
with the random expectation. The average frequencies suggest that females in same sex
and mixed sex dyads used more aggression in competition over food. On the other hand
aggression among males was more common in social contexts and males attacked
females more in food and sex contexts. I compared the relative difference between
observed with expected frequencies for each individual. Females fought more frequently
over food than in social contexts with other females (Wilcoxon exact: T=10, N=12,
p<0.05), and more frequently over food than in social and sex contexts when attacking
males (Friedman exact: T2=6.049, N=11, p<0.05). For males I applied the permutation
test due to the low number of individuals (Table 5.2). The six male-male dyads fought
more often in social contexts (Permutation test: p<0.01) and less often over food
(Permutation test: p<0.05), while they fought like expected over sex (Permutation test:
ns). On the other hand the 47 dyads of males attacking females did not show a
preference in any context (Permutation test: social, ns; food, ns; sex, p<0.1). Thus
females fought more over food and males fought males more in social contexts.
Table 5.3 Factors affecting the likelihood of winning a conflict in Taï chimpanzees
(Table 2: step 1b). The table presents the significant variables and the estimatecoefficient (b) of the parameters.
independent variables
dependent variable
Initiators Victory Outcome
Wald
P
parameters
n
dom
sub
dom-small
sub-small
dom-middle
sub-middle
dom-large
sub-large
302
78
110
45
108
24
84
9
1
Initiator’s Rank
40.15
<0.0001
2
Initiator’s Rank x
Rank Difference
16.76
<0.001
b
Wald
2.53 40.15
-2.53
-1.24
16.75
1.24
-0.45
1.94
0.45
1.69
-1.69 13.18
P
<0.0001
<0.0001
ns
<0.001
Model: Winner vs. loser
independent parameters: dom = dominant initiator, sub = subordinate initiator, small = small
rank difference, middle = middle rank difference, large = large rank difference.
51
Chapter 5: Decision-making in conflicts
5.3.1.2 Likelihood of winning
How was the chance of winning a conflict affected by the dominance relationship
and the context of the conflict (Table 5.1: step 1b)? Likelihood of winning a conflict was
significantly affected by two variables (GLZ: LR c 52,380 =84.19, p<0.0001; Table 5.3).
Dominant initiators were more likely to win conflicts (92%, b=2.53) than subordinate
initiators (60%, b=-2.53). However the likelihood of winning decreased with rank
difference for dominant initiators whereas the opposite was true for subordinates (Figure
5.1). The initiator’s likelihood of winning was greater the larger the relative dominance of
the initiator (5. 1: from very subordinate to highly dominant). In consequence dominants
initiated aggression more frequently with increasing rank difference (see n in Figure 5.1,
as the categories dom-large and sub-large etc. represent dyads from two different
perspectives). Thus, the more dominant an initiator was over his opponent, the higher
was his likelihood of winning the conflict and the more conflicts he initiated.
100%
90%
80%
conflicts
70%
60%
50%
40%
30%
20%
10%
0%
dom-large dom-middle dom-small
(n=108)
(n=84)
(n=110)
sub-small
(n=45)
sub-middle
(n=24)
sub-large
(n=9)
all conflicts
(N=380)
interaction: Initiator's Rank x Rank Difference
Figure 5.1 Likelihood of winning a conflict in Taï chimpanzees. The proportion of initiator
winning (o) compared to initiator losing (n) the conflict is displayed as a function of the
interaction between Initiator’s Rank and Rank Difference. The interaction is presented from
the highest to the lowest likelihood of winning (dom = dominant initiator, sub = subordinate
initiator, large = large rank difference, middle = middle rank difference, small = small rank
difference, test statistics see Table 5.3).
52
Chapter 5: Decision-making in conflicts
5.3.2 Step 2: Cost of aggression
5.3.2.1 Conflict intensity
How was the intensity of the fight affected by the resource and the partner’s
dominance relationship (Table 5.1: step 2a)? Aggression type was only significantly
affected by rank difference between opponents (GLZ: LR c 42, 409 =17.11, p<0.01; Table
5.4a). Dyads with a large rank difference were more likely to use non-contact aggression
(large: 81%, b=0.63) than dyads with closer rank conditions (middle: 63%, b=-0.34;
small: 64%, b=-0.29), independent of the context. Did chimpanzees then prefer low
levels of intensity when subordinate and high levels when dominant to their conflict
partner (Table 5.1, step2b)? Conflict intensity was only significantly affected by initiator’s
rank (GLZ: LR c 32,409 =23.62, p<0.0001; Table 5.4b). Looking at non-contact and contact
aggression separately (Figure 5.2), I found that subordinates initiated conflicts with lower
levels of intensity for both non-contact (threats, level 1) and contact aggression (single
physical contact, level 4). Dominants, however, preferred to initiate conflicts with higher
45
40
non-contact aggression (1-3)
contact aggression (4-5)
conflicts [%]
35
30
25
20
15
10
5
0
level 1 (n=95)
threat
level 2 (n=86)
level 3 (n=96)
display no locomotion display with locomotion
level 4 (n=72)
single
contact aggression
level 5 (n=60)
multiple
contact aggression
Conflict Intensity
Figure 5.2 Intensity of conflicts in Taï chimpanzees. The percentage of conflicts for
dominant (n) and subordinate (o) initiators as well as for all conflicts (n) is presented for
each level of intensity. Intensity levels 1–3 are non-physical contact conflicts, intensity levels
4+5 are physical contact conflicts (Table 3.2, test statistics see Table 5.4b).
levels of intensity for both aggression types (non-contact aggression: displays, level 2+3;
53
Chapter 5: Decision-making in conflicts
contact aggression: multiple physical contact, level 5). Thus both types of aggression
occurred in the repertoire of subordinates and dominants, but subordinates mainly
initiated the lower and dominants the higher intensity variants of both types. In addition
both preferred non-contact aggression when the rank difference was large.
Table 5.4 Factors affecting intensity and duration of conflicts in Taï chimpanzees
(Table 2: step 2, costs of aggression). The table presents the significant variables
for three multivariate analyses (a: model of aggression type, b: model of conflict
intensity, c: model of conflict duration). In addition it shows the GLZ estimatecoefficient (b) of the parameters and GLM post-hoc statistics.
independent variables
dependent variables
(a)
1
Rank Difference
(b)
1
Initiator’s Rank
(c)
1
2
Aggression Type
Wald
p
8.41
<0.05
parameters
small
middle
large
n
b
Wald
p
174
138
97
-0.29
-0.34
0.63
2.87
3.75
8.40
<0.1
<0.1
<0.01
Conflict Intensity
Wald
p
parameters
n
b
Wald
p
5.30
0.0213
dominant
subordinate
314
95
-0.39
0.39
5.30
<0.05
Conflict Duration
Initator’s Sex
Conflict Context
F
p
Post-hoc
P
8.89
5.95
<0.01
<0.05
Fisher LSD
Fisher LSD
<0.05
<0.01
Model a: Aggression type (non-contact aggression vs. contact aggression).
Model b: Conflict intensity (1 – 2 – 3 – 4 – 5).
Model c: GLM
5.3.2.2 Conflict duration
In addition to intensity, duration of the conflict was considered as a cost of
aggression (Table 5.1: step 2c). Two variables significantly influenced the duration of a
conflict (GLM: F10,347=3.44, p<0.001; Table 5.4c). Males initiated longer conflicts than
females and conflicts in social contexts were significantly longer than in food contexts
(Figure 5.3). Since males fought more in social contexts and females more in food
contexts, but the interaction of both significant effects was not significant in the GLM
(F1=2.27, ns), I investigated the interaction separately again. I found that male initiated
54
Chapter 5: Decision-making in conflicts
social conflicts were longer and had the largest range, whereas none of the other
combinations were significantly different (Figure 5.3; Fisher LSD with Bonferroni
correction: significance level of p<0.016; male-social: range=1-61s, male-food: range=121s, female-social: range=1-22s, female-food: range=1-33s). Thus, the effects of
initiator’s sex and conflict context was partly due to males initiating long conflicts in social
contexts.
5
conflict duration [s±SE]
4
3
2
p <0.01
p<0.01
1
p <0.01
0
male
female
social
food
(n=176)
(n=182)
(n=193)
(n=165)
Initiator's Sex
male
social
male
food
female
social
female
food
(n=108)
(n=68)
(n=85)
(n=97)
Conflict Context
interaction:
Initiator's Sex x Conflict Context
Figure 5.3 Duration of conflicts in Taï chimpanzees. The mean duration (s±SE) is
presented for each parameter of the significant variables of the model of conflict
duration and also for the interaction. Post-hoc statistic for the interaction is shown in
the graph in case of significance (significance level was Bonferroni-corrected to
p<0.016, test statistics see Table 5.4c and 5.3.2.2).
5.3.3 Step 3: Social costs
5.3.3.1 Creating social costs
How did chimpanzees distribute their social costs (Table 5.1: step 3a)? Conflict
induced social costs were significantly affected by three variables (GLZ: LR c 82, 409 =90.97,
p<0.0001; Table 5.5a). Dominant initiators initiated more conflicts with low benefit
partners (low: 37.9%, b=0.48), while subordinate initiators initiated more conflicts with
opponents of higher benefit (medium+high: 67.4%, b=-0.48). Moreover, the interaction
of dominance and rank difference had the strongest effect of all significant variables on
55
Chapter 5: Decision-making in conflicts
social costs (Table 5.5a, b-coefficients). Considering dominant and subordinate initiators
separately showed that dominants initiated more aggression with friends when the rank
difference was small and more with non-friends when rank difference was large (Figure
5.4). Conversely subordinate initiators attacked friends more when the rank difference
was large and non-friends when rank difference was small (Figure 5.4). Thus dominant,
like subordinate, individuals started conflicts with friends more frequently as their relative
likelihood of winning decreased (dom-small: b=-0.95, sub-large: b=-0.94). In addition,
the sex combination of opponents had an effect (Table 5.5a). Mixed pairs fought relatively
less with low benefit partners and relatively more with partners of a high benefit
relationship, whereas in conflicts among opponents of same sex it was the opposite
(conflicts per sex combination with low benefit / high benefit partners: mm=36.3%/15.9%;
f-f=49.0%/6.9%;
m-f=28.8%/24.7%;
f-m=22.0%/19.5%;
conflicts=36.7%/16.1%).
80
dominant initiators
70
subordinate initiators
conflicts [%]
60
50
40
30
20
10
0
dom - large
(n=85)
dom - middle
(n=109)
dom - small
(n=120)
sub - small
(n=54)
sub - middle
sub - large
(n=29)
(n=12)
all conflicts
(N=409)
interction: Initiator's Rank x Rank Difference
Figure 5.4 Relationship benefit of opponents representing the social costs of conflicts in Taï
chimpanzees. The percentage of conflicts between high benefit (n) and low benefit (o)
partners is displayed as a function of the interaction between Initiator’s Rank and Rank
Difference. For simplicity medium benefit partners are not presented in the figure, but when
including medium benefit partners conflicts total a 100%. Social costs are presented from
high to low likelihood to win (dom = dominant initiator, sub = subordinate initiator, large =
large rank difference, middle = middle rank difference, small = small rank, test statistics see
Table 5.5a).
56
all
Chapter 5: Decision-making in conflicts
5.3.3.2 Reducing of social cost
When were chimpanzees more likely to reduce social costs (Table 5.1: step 3b)?
Reduction of social costs was only significantly affected by relationship benefit of the
opponents (GLZ: LR c 62, 409 =58.81, p<0.0001; Table 5.5b). The tendency to reconcile
between opponents increased strongly with relationship benefit, such that among friends
more conflicts showed high CCT index (high relationship benefit: 63.6% with high CCT
index, 36.4% with low CCT index, b=-1.20) and among lower benefit partners more
conflicts showed a low CCT index (medium relationship benefit: 23.8% high CCT index,
76.2% low CCT index, b=0.57; low relationship benefit: 23.3% high CCT index, 76.7%
low CCT index, b=0.63). Thus social costs were more likely to be reduced among friends.
Table 5.5 Factors affecting social costs in Taï chimpanzees (Table 2: step 3). The
table presents the significant variables for two multivariate analyses (a: model of
social costs, b: model of reduction of social costs) and the estimate-coefficient (b) of
the parameters.
independent variables
(a)
1
2
3
Initiator’s Rank x
Rank Difference
Sex Combination
Initiator’s Rank
(b)
1
Relationship Benefit
dependent variables
Relationship Benefit
Wald
p
parameters
n
25.69
<0.0001
dom-small
sub-small
dom-middle
sub-middle
dom-large
sub-large
m-m
f-f
m-f
f-m
dom
sub
120
54
109
29
85
12
77
145
146
41
314
95
-0.95 23.55
0.95
0.01
0.01
-0.01
0.94
16.60
-0.94
0.46
0.46
8.57
-0.46
-0.46
0.48
-0.48 6.55
n
b
150
193
66
0.63 4.83
0.57 4.73
-1.20 13.03
8.57
6.55
<0.01
<0.05
b
Wald
p
<0.0001
ns
<0.0001
<0.01
<0.05
CCT Index
Wald
p
13.07
<0.01
parameters
low
medium
high
Wald
p
<0.05
<0.05
<0.001
Model a: Relationship Benefit (low – medium – high)
Model b: Corrected Conciliatory Tendency Index (low vs. high)
independent parameters: dom = dominant initiator, sub = subordinate initiator, small = small
rank difference, middle = middle rank difference, large = large rank difference, m = male, f =
female
57
Chapter 5: Decision-making in conflicts
frequency
of initiation
of conflicts
Figure 5.5 Schematic illustration modelling
the factors affecting two fighting strategies
in Taï chimpanzees. The illustration follows
our proposed order of the ‘decision-making’
benefit of resource
frequency
of initiation
of conflicts
process. Diagrams ¶-¹ show how the
costs or benefits considered in each step
affect the variation in initiation frequencies
of conflicts, while diagram º shows how
strategy
frequency of reduction of social costs. The
dominant initiator
positive or negative slopes of the graphs
B
frequency
of initiation
of conflicts
Two strategies were found: Strategy A is
¸
used by dominant initiators (fat white
subordinate initiators (small black arrow).
The outcome (») of the two strategies is
represented by the net benefit in relation
cost of aggression
frequency
of initiation
of conflicts
arrow) whereas strategy B is used by
¹
social cost
to the relative initiation frequency of
frequency
of reduction
of social cost
conflicts for dominant (A,o) or subordinate
initiators (B,n). The outcomes of both
strategies are presented in a simplified
form (the most likely outcome) and are
by subordinate
º
social cost
outcome
shown together in one graph to clarify the
trade-off used
subordinate initiator
A
are based on the observed frequencies of
5.1-5.4), using a straight line for simplicity.
·
likelihood
to
likelihood
of winning
social costs affect the variation in the
conflicts in Taï chimpanzees (see Figures
¶
dominant initiator
conflict
A
partners: subordinate initiators trade their
subordinate initiator
lower frequency of winning a conflict for a
winning, and a smaller negative netbenefit, when they are losing, compared to
dominant initiators (see 5.4).
A
frequency of
initiated conflicts
greater positive net benefit, when they are
B
»
negative
B
B
A
0
net benefit
58
positive
Chapter 5: Decision-making in conflicts
5.4 Discussion
5.4.1 Summary of results
Taï chimpanzees consider costs and benefits when determining whom, when and
how to fight – how to handle a conflict situation. All variables considered in the RM (value
of the resource, risk of injury, value and reparability of the relationship) were found to
significantly affect the decision-making process of Taï chimpanzees. Our results also
revealed the limits of the RM. While the RM does not consider different aggression
tendencies for dominant and subordinate initiators, I found that these differed in Taï
chimpanzees, as they considered their likelihood of winning the conflict in their conflict
decisions. Moreover, the RM could not directly explain the within individual variation in
costs of aggression and social costs that I observed in Taï chimpanzees. The extension of
the RM, I propose, remedies these shortcomings.
5.4.2 Grabbing the benefit
Adult chimpanzees fought more in contexts that had a high potential benefit to
them. Female chimpanzees in Taï initiated more aggression over food than in social
conflicts. This result corresponds with the findings in Gombe chimpanzees, where females
also fought frequently over food (Goodall, 1986). The females’ focus on food is not
surprising as better fed primate females have a higher overall fitness (e.g. baboons:
Altmann et al., 1985; Japanese macaques: Mori, 1979; rhesus macaques: Small, 1981). In
contrast, males preferentially engaged in social conflicts. Considering the fact that access
to oestrus females represents the key resource for reproductive success of males, this
result is surprising. However, males might achieve high ranks by winning conflicts in nonsexual contexts, which in itself might be sufficient to reach their reproductive goals, either
through female choice (Matsumoto-Oda 1999) or because subordinate males are reluctant
to challenge dominants in a sexual context. Therefore, engaging in social conflicts might
be a more effective strategy to maximise the reproductive success in male chimpanzees,
since high ranking males in Pan species sire more offspring than low ranking ones (Gerloff
et al., 1999; Constable et al., 2001; C. Boesch & L. Vigilant, unpublished data). Thus
individuals initiated more conflicts as the benefit of the resource increased (Figure 5.5,
¶).
59
Chapter 5: Decision-making in conflicts
In accordance with the general mammalian pattern, of dominant individuals being
more aggressive than subordinates (e.g. carnivores: Creel et al., 1997; perissodactyla:
Clutton-Brock et al., 1976; primates: Hrdy & Hrdy, 1976; ungulates: Côté, 2000), Taï
chimpanzees of both sexes initiated more conflicts the higher their dominance position.
Furthermore, Taï chimpanzees also won more conflicts the more dominant they were over
their opponent. Neither results are surprising as they reflect the character of a dominance
hierarchy. Thus individuals initiated more conflicts as the likelihood of winning increased.
(Figure 5.5, ·).
Contrary to expectation, subordinate initiators often won, suggesting that Taï
chimpanzees use a sufficient decision-making process. This may be due to the egalitarian
character of chimpanzee societies which enables subordinate individuals to compete
successfully (van Schaik, 1989). Alternatively, subordinate winners may have had
potential dominant supporters in the party, as the mere presence of supporters can
change the outcome of a dyadic conflict (de Waal, 1982). A third explanation may be that
sex related differences in fighting decisions help females to win food conflicts against
males, since males seem to avoid food conflicts more often than females, although they
are generally dominant over females. Thus, subordinate initiators may choose their
opponents more carefully than dominants do by monitoring the situation more precisely.
The factors enabling subordinates to win require further examination.
5.4.3 Economic handling of the costs of aggression
Taï chimpanzees handled their costs of aggression economically, their investment in
duration and intensity of aggression being shaped by the potential benefit of the context
and their likelihood of accessing the benefit. Individuals used non-contact aggression
more frequently when the winner was predictable due to a large rank difference. Males
fought for a longer time in contexts that were more beneficial for them. While it may seen
questionable that fighting for an extra second represents a significant increase of
energetic costs, the range of conflict duration shows that male chimpanzees fought up to
one minute in social contexts, almost three times as long as the maximum conflict
duration over food. A continuous performance of muscles in humans can lead to energetic
problems within one minute due to usage of ATP reserves (Keul et al., 1967),
phosphocreatine shortage (McKenna et al., 1999) or neuronal firing fatigue (Ross et al.,
2001). In contrast, females did not fight much longer over food than in social contexts,
even though food is the most important context for them. This might reflect female
60
Chapter 5: Decision-making in conflicts
experience that longer fights draw more combatants (R.M. Wittig & C. Boesch,
unpublished data). Thus a food specific risk arises, as food owners can lose control over
the resource to a third party during a conflict, a risk that individuals seem not to face
when competing for rank. Taï chimpanzees risk high costs of aggression only when this is
required to achieve a positive outcome.
Subordinate initiators used lower levels of contact and non-contact aggression than
dominants. It must be adaptive for subordinate initiators to minimise costs of aggression
as they are less often able to offset their costs by accessing the benefit. (Crowley, 2000)
detected a similar mixed evolutionary stable strategy (EES) for asymmetric contest
situations. He investigated a general form of the hawk-dove game (Maynard Smith &
Parker, 1976) which included the fighters’ knowledge about their own fighting ability and
that of their conflict partner. The main EES combined a cautious strategy (dove) for
subordinate partners with a daring strategy (hawk) for dominants (Crowley, 2000).
Similarly dominant opponents in Taï chimpanzees initiated more conflicts with high costs
of aggression, whereas subordinate opponents initiated more conflicts with low costs of
aggression (Figure 5.5, ¸).
5.4.4 Minimising social costs
Taï chimpanzees risked different social costs depending on their likelihood of
winning. Social costs originate from the disturbance of a cooperative relationship when
one partner withholds cooperation (de Waal, 1996a). Thus, a disturbed relationship with
non-cooperative partners is unlikely to create social costs. In Taï, the effect of the
initiator’s rank on the relationship benefit of opponents clearly showed that subordinates
initiated more conflicts with cooperative partners. Consequently subordinate initiators
generally risked more social costs than dominant ones (Figure 5.5, ¹). This general trend
was also reflected by the fact that both types of initiators, dominant as well as
subordinate, risked more social costs the lower was their relative likelihood of winning. It
was probably unnecessary for a dominant to fight a friend when the outcome of the
conflict was predetermined, but dominants risked a fight when the outcome was less
predictable. Why, however, did subordinate initiators also not fight more with friends with
a small rank difference? A partnership does not provide the same value to each partner
(Cords & Aureli, 2000). Dominant individuals are likely to provide better quality food and
support to subordinates than the other way around. Therefore, subordinate initiators
should have been more careful with highly dominant friends. Instead, however, they
61
Chapter 5: Decision-making in conflicts
initiated less conflicts with closely ranked opponents. Opponents of a small rank
difference fought harder and therefore those conflicts may have induced a stronger
disturbance to the relationship. Subordinates thus were probably more careful with friends
when facing the risk of a difficult relationship repair. Subordinate initiators generally
risked more social costs than dominants, except when reparation of the relationship could
be difficult – suggesting more gentile slopes of social costs (Figure 5.5, ¹) compared to
other graphs in Figure 5.5.
Such a strategy is only adaptive if high social costs were reduced more frequently
than low ones after the conflict. Indeed Taï chimpanzees were more likely to reduce
higher than lower social costs (Figure 5.5, º). This higher reconciliation frequency with
friends has also been seen in other primates (reviewed in: Aureli et al., 2002). Since social
costs seemed to be easily reduced afterwards, why did Taï chimpanzees not fight friends
more frequently? Reconciliation, however, is not free of risk, as individuals have to
approach former opponents which may lead to further aggression (e.g. Aureli & van
Schaik, 1991b; Cords, 1992). Moreover reconciliation might fail if the same relationship is
repeatedly disturbed, with a bigger disturbance possibly requiring longer and more
intense reconciliation.
5.4.5 The extended Relational Model
How did Taï chimpanzees decide to fight for a resource? They were more likely to
initiate conflicts for resources that were highly beneficial to them, but they used less
aggression when the chance of winning was lower. The likelihood of winning therefore
served as the key factor in the decision-making process, and two different alternative
strategies can be identified (Figure 5.5):
(A) Dominant initiators: fought harder, but they did not risk undue costs of
aggression. Additionally they risked less social costs by fighting less with partners of
higher benefit. Thus, dominant conflict partners fight over highly beneficial resources,
investing more in aggression costs but less in social costs. Dominant initiators gain a
medium positive net-benefit when winning and face a medium negative net-benefit when
losing, since costs of aggression are non-reducible and have to be subtracted from the
benefit. In consequence, as dominant initiators almost always win, their strategy has a
predictable outcome: often obtained medium positive net-benefit (Figure 5.5, »)!
62
Chapter 5: Decision-making in conflicts
(B) Subordinate initiators: had lower costs of aggression, but took higher social
costs, except when reparation of the relationship could be difficult. Hence subordinate
conflict partners also fight over high benefit resources, but invest less in aggression costs
and more in social costs. Subordinate initiators gain a high positive net-benefit when
winning and face low negative net-benefit when losing, since they have low costs of
aggression and the social costs are reducible. In consequence subordinate initiators trade
their lower chance to win the conflict for a higher positive and a lower negative netbenefit compared to dominant initiators. This trade off is a risky strategy: rarely obtained
high positive net-benefit (Figure 5.5, »)!
The extended RM describes the decision-making process in Taï chimpanzees well ,
although one can never be sure that all potential effects have been identified. I did not
investigate situations directly when chimpanzees reacted non-aggressively during a
conflict of interest. Nevertheless, I found that females initiated less social conflicts,
whereas males initiated less conflicts over food. Furthermore, one partner initiated the
conflict while the other one had not yet decided to initiate aggression. Therefore, Taï
chimpanzees seem to avoid aggression when the resource is not beneficial to them and
when their decision-making process does not offer a positive net-benefit. However, the
decision-making process is not perfect as sometimes initiators misjudged the situation and
lost the conflict. Still the overall net-benefit in both strategies was positive, as subordinate
initiators showed a trade-off for their lower winning rate by going for a larger net-benefit
(Figure 5.5, »).
Taï chimpanzees demonstrate a very flexible decision-making process before
initiating a conflict. Clearly more factors are taken into account than initially suggested by
the RM. Decisions are done in an economical fashion to increase the benefit for the
initiator while diminishing the costs. The extended RM proposed here fits well with the
social complexity observed in wild chimpanzees. The extended RM, providing a powerful
way to investigate conflict management, may help to answer further questions about
when aggression is avoided and how effective the mechanisms are for managing a
conflict (de Waal & Aureli, 2000). Hence, comparative studies are requested, to test this
model on other primate species, to make within and between species differences in
conflict decisions apparent.
63
Chapter 621:
Choice of post-conflict interactions
Figure 6.0 Female Venus has the choice of several social partners in
the party for a grooming interaction (Photo by R. Wittig).
21
Corresponding with: Wittig, R.M. & Boesch, C. (2003d). The choice of post-conflict
interactions in wild chimpanzees (Pan troglodytes). Behaviour. 140:1527-1559
Chapter 6: Choice of post-conflict interctions
6.1 Introduction
6.1.1 Advantages and disadvantages of post-conflict interactions
Aggression disturbs the relationship of conflict partners and induces social tension
within social groups (Cords, 1992; Aureli et al., 1999; Matsumura & Okamoto, 2000).
Such costs of conflicts remain present even after the aggression has ended. Post-conflict
management is believed to reduce the costs of conflicts for social living animals. Postconflict interactions (PCI), which in brief is the first interaction of a conflict partner
subsequent to aggression, can either reduce those costs (e.g. stress reduction: Cords &
Aureli, 2000; limit damage to a disturbed relationship: Aureli & Smucny, 2000) or can
prevent further aggression (Aureli et al., 2002). Yet, while PCIs represent an
advantageous mechanism for social living animals, serious disadvantages may also be
accrued (Watts et al., 2000; Aureli et al., 2002). Conflict partners may become trapped in
further aggression when, for example, approaching former opponents for reconciliation
(Aureli & van Schaik, 1991b; Cords, 1992). Conflict partners can choose from a pool of
PCIs that consist of several affiliative or aggressive PCIs, carried out with former
opponents or third parties (Aureli & de Waal, 2000a). Therefore selecting an optimal PCI
requires an evaluation of both the advantages and disadvantages. Individuals choosing a
PCI are viewed as going through a decision-making process. The terms decision and
choice do not necessarily imply a conscious reasoning process, rather individuals can
switch between different behavioural possibilities22.
Within group aggression erupts when individuals compete over food, mating
partners, social partners or privileges of access to resources (Huntingford & Turner, 1987;
Mason & Mendoza, 1993). Subsequently to aggressive interactions, which I will refer to as
conflicts, PCIs can be implemented. Conflict partners can choose from a pool of PCI
including reconciliation, solicited consolation, renewed aggression and redirected
aggression, while in addition conflict bystanders can offer consolation or attack conflict
partners (Aureli & de Waal, 2000a). Alternatively conflict participants may avoid any
further interaction (no PCI). Each of the seven options (six PCIs + no PCI) has a potential
pay-off, since they carry certain advantages and disadvantages.
22
see last paragraph of 1.3
65
Chapter 6: Choice of post-conflict interctions
6.1.2 Pool of PCIs
6.1.2.1 Reconciliation
Reconciliation, the affiliative PCI between former opponents, repairs the relationship
of former opponents by restoring inter-opponent tolerance levels to baseline (Cords &
Aureli, 1996). Partners of highly valuable relationships should restore tolerance levels to
normal, since otherwise beneficial cooperation might be withheld (Cords & Thurnheer,
1993; de Waal, 1996a). Reconciliation also reduces stress indicators (Aureli, 1997; Aureli
et al., 1999) and prevents further aggression (Aureli & van Schaik, 1991b; Watts, 1995a;
Silk et al., 1996). Moreover approaching former opponents might offer another chance at
accessing the resource, which precipitated the fight. Relaxed partners could, for example,
share food after reconciliation has reduced stress levels. Although reconciliation appears
to reduce most costs of conflicts, it certainly is not implemented after each conflict (Aureli
& de Waal, 2000a). This may be due to the fact that reconciliation requires former
opponents to approach one another thereby giving the opportunity of further aggression
(Aureli & van Schaik, 1991b; Cords, 1992). Subordinate partners may thus profit
disproportionately from relationship repair, as dominant partners are likely to share better
quality food and better support than subordinates (Cords & Aureli, 2000). On the other
hand subordinate cooperation partners may tip the scales in certain situations of social
leverage (Lewis, 2002), that the need for relationship repair could be similar for both
partners. Nonetheless subordinate partners incur comparatively higher risks by
approaching former opponents, and might thus be less likely to initiate reconciliation
when dominant partners are highly motivated to fight again.
6.1.2.2 Consolation
In addition to reconciliation, consolation, the affiliative PCI with third parties, is also
proposed as a mechanism to alleviate stress and reduce the risk of further aggression
(Das, 2000; Watts et al., 2000). Consolation is unlikely to repair the relationship of conflict
partners, since former opponents do not interact directly, however it avoids an
opportunity of further aggression. Consolation thus may substitute for reconciliation after
conflicts with a high likelihood of further aggression or among low benefit partners, as
both serve similar functions of reducing stress and preventing further aggression (Watts
et al., 2000). Consolation may even restore tolerance levels of former opponents to
baseline levels when carried out with kin of former opponents (Cheney & Seyfarth, 1989;
Judge, 1991). Conflict partners profit from consolation by means of stress reduction, while
66
Chapter 6: Choice of post-conflict interctions
third parties may profit from offering consolation when they are potential recipients of
redirected aggression (Das et al., 1997; Das et al., 1998). Since there might be a
functional difference, I have distinguished between offered consolation (Table 6.1),
initiated by third parties, and solicited consolation, initiated by conflict participants (de
Waal, 1993; de Waal & Aureli, 1996; Watts et al., 2000).
6.1.2.3 Redirected aggression
In contrast to affiliative PCIs, all aggressive PCIs include the complete spectrum of
disadvantages that are associated with aggression such as risk of injury, energy use,
disturbed relationships and potential loss of access to the resource (chapter 523; de Waal,
1996a; Wittig & Boesch, 2003b; Table 6.1). Redirected aggression, in which a conflict
partner initiates an aggressive PCI with third parties, nonetheless carries advantages. It
may reduce aggression-induced stress and frustration (Aureli & van Schaik, 1991a), as
losers of conflicts with a high rate of redirected aggression showed lower levels of
physiological (Levine et al., 1989; Sapolsky, 2000) and behavioural stress indicators
(Aureli & van Schaik, 1991b; Maestripieri et al., 1992). Since winners of conflicts are also
stressed by the conflict (Aureli, 1997), they might use redirected aggression to calm their
own stress response. Furthermore, individuals may use redirected aggression to deflect
the aggressive attention of an opponent to a third party (de Waal & van Hooff, 1981).
Sometimes, when conflicts are long or intense, this might be one of the best possibilities
for a victim to leave the focus of aggressive attention. In contrast to redirected
aggression, third party aggression, in which a third party initiates an aggressive PCI, is
neglected in most studies. Advantages for a third party to attack a conflict participant may
be either to seize possession of an especially profitable resource (chapter 524; Preuschoft
& van Schaik, 2000; Wittig & Boesch, 2003b) or to defeat a frequent opponent caught in
a weak position (e.g. contra-intervention: de Waal, 1978).
6.1.2.4 Renewed aggression
In addition to these most often discussed PCIs, two additional types of post-conflict
behaviour, renewed aggression and avoidance of further interactions, are of interest as
well (Table 6.1). While renewed aggression, the aggressive PCI between former
opponents, entails costs of conflict, it provides the loser of a conflict with a second
opportunity to gain access to the disputed resource. Therefore it can be seen as a re23
see 5.1.1 & 5.1.2
67
Chapter 6: Choice of post-conflict interctions
escalation of the same conflict. Nonetheless renewed aggression is only advantageous for
losers with a sufficient likelihood of winning. On the other hand renewed aggression may
help winners of conflicts in strengthening their dominant position against an already
weakened opponent (Johnstone & Dugatkin, 2000).
6.1.2.5 Avoidance of interactions (no PCI)
Finally, conflict partners may avoid any interactions (no PCI) in order to prevent any
further confrontation. However, avoiding the disadvantages of PCIs means that the
advantages are also inaccessible, like stress reduction or a second attempt at accessing a
resource. Since stress also reduces over time (Aureli & van Schaik, 1991b), conflict
partners, that are only mildly stressed, might not need stress reducing PCIs. Nonetheless,
avoiding interactions with party members makes monopolisable resources, that are
possessed by them, inaccessible – a clear disadvantage of no PCI for the loser.
6.1.3 Testing the choice of PCI
Following a conflict social living animals can select a option from the pool of PCIs
(six PCIs and no PCI). Since some advantages can be gained from several different
options, e.g. conflict partners can reduce stress by means of at least three different PCIs
(reconciliation, consolation and redirected aggression), advantages and disadvantages
that are unique to a PCI should play a decisive role in the evaluation process for the best
choice of PCI. Table 6.1 summarises the advantages and disadvantages of all PCIs and
the conditions under which each PCI is expected. Chimpanzees represent an excellent
model to investigate the decision-making process behind the choice of PCI. They are
highly sociable (Goodall, 1986; Nishida, 1990; Boesch & Boesch-Achermann, 2000) and
most of the described options in the pool of PCIs are known to exist in chimpanzees (de
Waal & van Roosmalen, 1979; de Waal & van Hooff, 1981; de Waal, 1984; de Waal &
Aureli, 1996; Arnold & Whiten, 2001; Preuschoft et al., 2002). However this study is the
first to check on most of them for wild populations and in any case to consider all options
of the pool of PCIs simultaneously. I investigate the selection process of PCIs in wild
chimpanzees, in terms of the factors that influence the choice of PCI and who initiates the
PCI. Our underlying hypothesis is that after a conflict chimpanzees use the PCI that
provides the most advantages while carrying the fewest disadvantages. Our main
questions are:
24
see Table 5.2
68
Chapter 6: Choice of post-conflict interctions
(1) Do wild chimpanzees apply all seven options from the pool of PCIs (six PCIs and
no PCI) for post-conflict management?
(2) Under which conditions do wild chimpanzees use each type of PCI and does this
choice of PCI follow the proposed evaluation process that takes both advantages and
disadvantages into account?
(3) Which are the general rules that reflect the choice of PCI?
Table 6.1 The pool of post-conflict interactions (PCIs): Overview of advantages and disadvantages of PCIs and the conditions under which each
is expected.
Type of PCI
Reconciliation
(a)
Consolation
(b) offered +
(c) solicited
Renewed
aggression
(d)
Redirected
aggression
(e)
Third party
aggression
(f)
No PCI
(g)
Advantages (A) and Disadvantages (D)
for either conflict participants (CP) or third parties (TP)
A* (CP) stress reduction
A* (CP) prevent further aggression
A (CP) relationship repair
A (CP) access resource (non-aggressive attempt)
D (CP) possible renewed aggression
A* (CP) stress reduction
A* (CP) prevent further aggression
A (CP) no approach of former opponent
A (TP) calm tension
D (CP) no relationship repair
A (CP) access resource (second aggressive attempt)
A (CP)
D* (CP)
A* (CP)
A (CP)
A (CP)
D* (CP+TP)
A (TP)
A (TP)
D* (CP+TP)
A (CP)
D (CP)
defeat frequent opponent
costs of aggression
stress reduction
defuse frustration
deflect aggression
costs of aggression
defeat weakened frequent opponent
access resource (first aggressive attempt)
cost of aggression
avoid risks of disadvantages of PCIs
no possibility to access resource
Conditions under which PCI is expected:
PCI implemented more often when ----relationship benefit is high between CP
loser only: resource is limited
renewed aggression is unlikely between CP
----renewed aggression is likely between CP
third party only: TP possible target of redirected aggression
relationship benefit is low between CP
conflict is a draw, resource is limited or highly valuable
loser only: outcome of conflict is unexpected
winner only: CP is rank neighbour
----loser only: conflict is strong (long and intense)
non-aggressive PCI unlikely to stop aggression for CP
--conflict is strong (long and intense)
resource is limited or highly valuable
--no advantages of PCIs are needed by CP (e.g. no stress)
resource is not limited
Conditions relating to the advantages or disadvantages are shown in the same row. Overlapping* advantages and disadvantages have no expected conditions (--), since they do not distinguish between the different types of PCI.
6.2 Specific methods
6.2.1 Data and test conditions
A data set of 595 conflicts resulted from the balancing procedure. I modelled the
likelihood that a particular PCI occurs against the non-occurrence of this PCI ( =ˆ all other
PCIs). To detect the variables that affects the choice of PCI, I executed multivariate
analyses. All variables of Table 3.2, plus the interactions of initiator’s sex with recipient’s
sex, rank difference with initiator’s rank and conflict context with competitor proportion,
were considered simultaneously as independent variables in each of the multivariate
analyses. Additionally, conflict duration was analysed separately on an individual level due
to limitations of our binomial testing procedure on continuous predictors. A distinct midlength conflict duration for a PCI would not be detected, since I tested one PCI against all
69
Chapter 6: Choice of post-conflict interctions
the others (which would have longer and shorter conflict duarations). Non-parametric
statistics was applied, for example, Friedman-ANOVA with missing values, which is
basically a 10000 times permutated ANOVA (Mundry, 1999).
6.2.2 Additional operational definitions
A post-conflict interaction (PCI) was defined as the first interaction of the focal
conflict partner with another individual subsequent to an aggressive interaction. Six
possible PCIs were recorded: (a) reconciliation, affiliative PCI between former opponents
(n=188); (b) offered consolation, affiliative PCI initiated by a third party (n=164); (c)
solicited consolation, affiliative PCI with a third party initiated by a conflict partner
(n=176); (d) renewed aggression, aggressive PCI between former opponents (n=174);
(e) redirected aggression, aggressive PCI with a third party initiated by a conflict partner
(n=88); and (f) third party aggression, aggressive PCI initiated by a third party (n=28).
Avoidance of any further interaction was recorded as (g) no PCI, when the focal conflict
partner did not interact with any other chimpanzee for the rest of the day (n=58).
Affiliative PCIs consisted of friendly behaviours with body contact (e.g. kiss, genital touch,
hand holding, embrace, grooming), while aggressive PCIs consisted of threats (e.g. barks,
arm wave), non-contact aggression (e.g. displays) and contact aggression (e.g. bits, hits).
In the results I distinguish between post-conflict reactions, which are all possible actions
after a conflict (from a to g), and post-conflict interactions, which are only the PCIs
related to post-conflict management.
6.2.3 Special analysis and statistics
I applied an altered version of the time-rule (Aureli & van Schaik, 1991a; Castles &
Whiten, 1998a), to determine whether or not the occurrence of PCIs depended upon the
preceding conflict. The time rule implies that a PCI needs to be initiated faster after a
conflict than a control interaction. Therefore, I computed four different baselines that
represent the normal interaction-intervals in our study group. Baselines for inter-opponent
PCIs consisted of a value for each dyad, while baselines for PCIs with third parties had a
value for each individual: (a) baseline for reconciliation is the mean interaction-interval of
consecutive affiliative interactions for each dyad; (b) baseline of renewed aggression is
the mean interaction-interval of any consecutive interaction for each dyad; (c) baseline of
consolation is the mean interaction-interval of consecutive affiliative interactions of each
individual with any other member of the community; (d) baseline of aggression with third
70
Chapter 6: Choice of post-conflict interctions
parties is the mean interaction-interval of any consecutive interactions of each individual
with any other member of the community. To compare the latencies of PCIs with their
particular baselines, I calculated for each PCI event the relative latency (=
latency
), with
baseline
a relative latency <1 representing PCI events occurring faster than baseline. For each PCI
I calculated the mean relative latencies for each dyad (inter-opponent PCIs) or the mean
relative latencies for each individual that participated in the conflict and the PCI (PCIs
with third parties) and conducted the bootstrap test.
6.3 Results
6.3.1 Are PCIs dependent on the conflict?
Chimpanzees had the choice of six types of PCI and the option of no PCI. In order
to determine if the PCIs were implemented as a consequence of the preceding conflict, I
2,5
relative latency
2
ns
1,5
Baseline
*
1
*
*
0,5
*
*
0
Solicited
Consolation
(=Normal
Interaction)
Offered
Consolation
Redirected
Aggression
Third Party
Aggression
Reconciliation
Renewed
Aggression
Figure 6.1 Mean relative latencies for each of the six possible post-conflict interactions
(PCIs) compared to baseline in Taï chimpanzee. Relative latencies (
latency
) <1 show
baseline
that the PCI occurred faster than baseline. Error-bars mark the two-tailed 95% confidence
interval which was computed from the relative latencies of either dyads (reconciliation
and renewed aggression) or individuals. When the baseline was placed outside the
confidence interval of latencies, the PCI was significantly different from normal
interactions (significance level * is p<0.05, test statistics see 6.3.1).
71
Chapter 6: Choice of post-conflict interctions
tested the time-rule of whether or not baseline was included in the confidence interval
(95% two-tailed) of the latencies of each PCI. Results revealed that all PCIs except
solicited consolation were initiated more quickly than baseline (Figure 6.1). The
occurrence of offered consolation, redirected aggression, third party aggression,
reconciliation and renewed aggression was triggered by the preceding conflict (all
bootstrap tests: p<0.05), while solicited consolation was not different from the baseline
(bootstrap test: ns). Thus, solicited consolations were independent of the preceding
conflicts. As they were indistinguishable from normal interactions, solicited consolations
are no longer referred to as a PCI in this paper. The remaining five PCIs can however be
considered as post-conflict management.
6.3.2 The effect of conflict duration
The duration of the preceding conflict, calculated on individual levels, varied among
the seven different post-conflict reactions (five PCIs, no PCI and normal interaction;
Friedman-ANOVA with missing values: k=7, N=90, permutation=10000, p<0.01).
Therefore I ordered the seven possible reactions to a conflict with increasing mean
conflict duration (Figure 6.2). I found a distinct relation between conflict duration and
duration of preceding conflict [s±SD]
18
16
14
12
10
8
6
4
2
0
Normal
Reconciliation
Interaction
(n=15)
(n=15)
No PCI
(n=10)
Offered
Consolation
(n=15)
Renewed
Aggression
(n=12)
Redirected
Aggression
(n=12)
Third Party
Aggression
(n=11)
Figure 6.2 Average individual conflict duration (s±SD) preceding the different
types of post-conflict reactions in Taï chimpanzees. The post-conflict reactions
are ordered from the shortest (left) to the longest (right) average duration of
preceding conflicts (differences of conflict duration: a: among all post-conflict
reactions **, b: same increasing effect for each individual from left to right **,
c: affiliative vs. aggressive **, significance level ** is p<0.01, test statistics see
6.3.2 ).
72
Chapter 6: Choice of post-conflict interctions
type of post-conflict reaction. This was not due to individual effects, as each individual
that was involved in each type of interaction (5 PCIs and normal interaction) revealed the
same effect between conflict duration and type of interaction (Page exact: L=651, k=6,
N=8, p<0.01). This suggests that conflict duration influenced the choice of PCI.
Comparison between aggressive and affiliative PCIs revealed that shorter conflicts were
followed by affiliative PCIs while longer conflicts resulted in aggressive PCIs (Wilcoxon
exact: T=14, N=15, p<0.01). Multivariate analysis confirmed this result on a dyadic level
and revealed conflict duration to be the only predictor (of list in Table 3.2) that influenced
whether aggressive or affiliative PCIs followed the conflict (GLZ: LR c 42,545 =15.1, p<0.01;
Table 6.2a). Thus, the risk of further aggression increased with conflict duration and Taï
chimpanzees preferred affiliative PCIs to manage shorter conflicts while aggressive PCIs
were used to manage longer conflicts.
6.3.3 Avoidance of interactions (no PCI)
First, I investigated which conditions resulted in no PCI. Conflict partners were more
likely to avoid interactions, when resources were not monopolisable by a single
competitor, when conflict partners were rare associates or opponents had a small rank
difference (GLZ: LR c 52,595 =15.9, p<0.01; Table 6.2b). Chimpanzees were thus more likely
to implement interactions after conflicts over limited resources and after fighting familiar
partners, while fighting over a resource, that was available somewhere else in the party,
did not require following social interactions with community members. Subsequently I
analysed under which conditions the different interactions were implemented.
6.3.4 Normal interactions – business as usual
Since
the
time-rule
analysis
showed
that
solicited
consolations
were
indistinguishable from normal interactions, I tested under which conflict conditions conflict
partners continued with business as usual. Multivariate analysis proved that conflict
partners continued with normal interactions after very short conflicts or after conflicts
among same sex partners (GLZ: LR c 52,545 =27.18, p<0.0001; Table 6.2c). Thus Taï
chimpanzees were able to have business as usual after very short conflicts, while longer
conflicts needed conflict management related PCIs. Subsequently I investigated the
choice amongst the five PCIs that were dependent on the preceding conflict.
73
Chapter 6: Choice of post-conflict interctions
Table 6.2 Factors affecting the occurrence of aggressive versus affiliative post-conflict
interactions, avoidance of interactions and normal interactions in Taï chimpanzees.
Presented are the significant variables of the best models, their parameters (including the
relative frequency ( f p =%) and estimate-coefficients b) and the Wald-statistics.
independent (a) Aggressive PCIs vs. affiliative PCIs
variables Wald df
p
parameter n
effect
Conflict
Duration
6.02
1
<0.05
continuous 545
­¶
independent (b) Avoidance of interactions (no PCI)
variables Wald df
p
parameter n
[%]
Resource 6.79
2
<0.05
by one 283
5.7
Monopolisation
by few 159
9.4
by non 153 12.4
b
Wald
0.83
6.02
p
<0.05
b
Wald
-0.54 6.08
0.14 0.38
0.40 3.56
p
<0.05
ns
<0.1
Association
Index
4.34
1
<0.05
rare 384
frequent 211
9.9
5.7
0.38
-0.38
4.34
<0.05
Rank
Difference
6.03
2
<0.05
small 211
middle 247
large 125
10.1
8.5
4.8
0.55
0.06
-0.61
5.78
0.06
3.92
<0.05
ns
<0.05
independent (c) Normal interaction
variable Wald df
p
parameter n
Conflict 8.81
1
<0.01
continuous 545
Duration
[%]
¯·
b
Wald
-1.29 8.81
p
<0.01
Sex 5.11
Combination
1
<0.05
m-m 148 23.6
0.29
f-f 160 33.1
0.29
5.11
<0.05
m-f 194 17.0
-0.29
f-m 43
23.3
-0.29
Model a: Aggressive PCIs vs. affiliative PCIs; ¶ longer conflicts preceding aggressive PCIs
than preceding affiliative PCIs.
Model b: No PCI vs. all interactions; f no PCI=8.4%.
Model c: Normal interaction vs. PCIs, f normal interaction=24.0%; · shorter conflicts preceding
normal interactions.
% = frequency in percent of the dependent variable after conflicts with particular parameter
( f p ); f = overall frequency of dependent variable.
6.3.5 Choosing a post-conflict interaction
6.3.5.1 Reconciliation
Conflict partners were most likely to reconcile with opponents of opposite sex, with
high benefit partners or with frequent associates (GLZ: LR c 42, 414 =20.1, p<0.001; Table
74
Chapter 6: Choice of post-conflict interctions
6.3a). Reconciliation was equally initiated by winners and losers of conflicts (winner:
41.7%, loser: 58.3%; Goodness of fit: c 12,115 =3.14, ns). After non-contact aggression
losers significantly increased their proportion of initiation of reconciliation with conflict
intensity, while after contact aggression (parameters 4 and 5) the proportion of losers as
initiators was not significantly different from average (GLZ: LR c 72,115 =24.2, p<0.01; Table
6.3b). Furthermore, while losers were more likely than winners to initiate reconciliation
after conflicts over food, winners showed a tendency to initiate more reconciliation in
social and sex contexts (Table 6.3b).
Reconciliation was thus preferred among mixed sex partners and among opponents
with highly valuable relationships (Figure 6.3). Losers initiated reconciliation following
conflicts over food, and their initiation frequency increased with conflict intensity, yet
neither conflict partner was more initiative in reconciliation after contact aggression.
Reconciliation
Consolation
60
relative difference in frequency [%]
50
40
30
20
10
0
-10
-20
-30
-40
m-m
m-f
f-m
Sex Combination
f-f
low medium
high
Relationship Benefit
rare
frequent
few
some
many
Association Index Competitor Proportion
Figure 6.3 Comparison between the tendencies to initiate reconciliation and consolation in
Taï chimpanzees. The relative difference in frequency of each parameter (
fp - f
f
) is shown
for the significant variables, separately for reconciliation (n) and consolation (o). Positive
relative differences indicate a positive effect of the parameter on the frequency of the PCI,
while negative relative differences point out a negative effect (test statistics see Table 6.3).
75
Chapter 6: Choice of post-conflict interctions
Table 6.3 Factors affecting the choice of affiliative post-conflict interactions in Taï
chimpanzees. Presented are the significant variables of the best models, their
parameters (including the relative frequency ( f p =%) and estimate-coefficients b)
and the Wald-statistics.
independent (a) reconciliation
variables Wald df
p
parameter n
Association 4.64
1 <0.05
rare 267
Index
frequent 147
Relationship
Benefit
[%]
25.5
36.1
Wald
b
-0.26
4.64
0.26
p
<0.05
6.76
2
<0.05
low 142
medium 201
high 71
23.2
27.9
45.1
-0.29
-0.22
0.51
2.74
1.85
6.76
<0.1
ns
<0.01
Sex 4.02
Combination
1
<0.05
m-m 113
f-f 107
m-f 161
f-m 33
26.5
19.6
34.8
42.4
-0.24
-0.24
0.24
0.24
4.02
<0.05
Ê[%]
winner
b·
Wald
p
(b) Initiator of reconciliation
independent
variables Wald df
p
parameter n
Conflict
Context
9.18
2
<0.05
social
sex
food
57
26
32
49.1
50
21.9
0.66
0.70
-1.36
3.74
2.95
8.93
<0.1
<0.1
<0.01
Conflict 10.66
Intensity
4
<0.05
1
2
3
4
5
26
27
32
22
8
57.7
40.7
28.1
36.4
62.5
1.34
-0.22
-1.44
-0.08
0.40
5.54
0.22
7.97
0.02
0.29
<0.05
ns
<0.01
ns
ns
independent (c) Offered consolation
variables Wald df
p
parameter n
Competitor 7.78
2 <0.05
few 61
Proportion
some 209
many 144
[%]
34.4
24.4
18.8
Wald
b
0.36 5.84
-0.02 0.02
-0.34 6.98
30.1
19.9
21.1
0.28
-0.24
-0.04
p
<0.05
ns
<0.01
<0.05
<0.1
ns
Relationship
Benefit
7.54
2
<0.05
Sex 3.96
Combination
1
<0.05
low 142
medium 201
high 71
4.96
3.77
0.08
m-m 113 29.2
0.18
f-f 107 26.2
0.18
3.96 <0.05
m-f 161 19.9
-0.18
f-m 33
18.2
-0.18
f
Model a: Reconciliation vs. other PCIs; reconciliation=29.2%.
Model b: Winner vs. loser initiation of reconciliation; f winner=41.7%; f loser=58.3%; ¶
f
f
loser = 100% winner; · positive b = effect in favour of winner.
Model c: Offered consolation vs. other PCIs; f offered consolation=23.9%.
%= frequency in percent of the dependent variable after conflicts with particular parameter
( f p ); f = overall frequency of dependent variable.
76
Chapter 6: Choice of post-conflict interctions
6.3.5.2 Offered consolation
Taï chimpanzees received consolation more often after conflicts with same sex
partners, with partners of low benefit or when only a few competitors were present (GLZ:
LR c 52, 414 =12.8, p<0.05; Table 6.3c). Offered consolation and reconciliation occurred after
almost complementary conflict situations (Figure 6.3). Moreover, consolation was received
after significantly longer conflicts as compared to reconciled conflicts (Wilcoxon exact onetailed: T=93, N=15, p<0.05; Figure 6.2).
6.3.5.3 Renewed aggression
Renewed aggression did not show any distinct predictors in the multivariate analysis
(GLZ: LR c 12, 414 =2.68, p<0.2). However, after conflicts ending in a draw, conflict partners
were more likely to initiate renewed aggression than any other PCI (9 of 21 draws
resulted in renewed aggressions, while 96 times renewed aggression was chosen of 418
conflict partner initiated interactions; Goodness of fit: c 12, 21 = 4.76, p<0.05).
Renewed aggression was more likely to be initiated by winners than losers of
conflicts (winner: 74.4%, loser: 25.3%, Goodness of fit: c 12,87 = 21.25, p<0.0001). Losers,
however, renewed aggression when they had initiated the conflict, or when they were the
dominant partner (GLZ: LR c 102 ,87 =57.1, p<0.0001; Table 6.4a).
Therefore, renewed aggression seemed to be the preferred PCI following draws,
while dominant initiators renewed aggression when they unexpectedly lost a conflict.
6.3.5.4 Redirected aggression
Aggression was redirected slightly more often after initiators won the conflict (GLZ:
LR c 22, 414 =6.73, p<0.05; Table 6.4b). However this predictor was only marginally
significant.
6.3.5.5 Third party aggression
Third parties reacted more often with aggression after very long or very intense
conflicts or when the conflicts were in a social context (GLZ: LR c 112 , 414 =48.2, p<0.0001;
Table 6.4c). However this PCI was less frequently observed after the most intense noncontact aggression.
77
Chapter 6: Choice of post-conflict interctions
Table 6.4 Factors affecting the choice of aggressive post-conflict interactions in Taï
chimpanzees. Presented are the significant variables of the best models, their
parameters (including the relative frequency ( f p =%) and estimate-coefficients b) and
the Wald-statistics.
(a) Initiator of renewed aggression
independent
[%]¶
variables Wald df
p
parameter n
winner
b·
Wald
p
¸Initiator’s 14.49
Victory
Outcome
1
<0.001
winner
loser
77
10
83.1
10
3.76
14.49 <0.001
-3.76
3.96
1
<0.05
dominant
subordinate
70
17
84.3
35.3
1.35
-1.35
n
winner 347
loser 42
draw 25
[%]
independent (c) Third party aggression
variable Wald df
p
parameter n
Conflict 14.15 2 <0.001
social 203
Context
sex 73
food 138
Winner’s
Rank
independent (b) Redirected aggression
variables Wald df
p
parameter
Initiator’s
Victory
Outcome
4.68
2
<0.1
3.96
<0.05
b
Wald
18.8
9.5
4
0.83
0.05
-0.88
4.46
0.01
1.56
p
<0.05
ns
ns
[%]
10.3
1.4
4.3
b
Wald
1.19 9.22
-0.60 0.77
-0.59 1.56
Conflict 11.12
Duration
1
<0.001
continuous 414
­¹
Conflict 10.07
Intensity
4
<0.05
1 84
2 97
3 101
4 67
5 65
6.0
5.2
2.0
7.5
16.9
p
<0.01
ns
ns
2.08 11.11 <0.001
-0.01
0.36
-1.38
0.04
0.99
0.01
0.71
5.13
0.01
7.56
ns
ns
<0.05
ns
<0.01
initiation of renewed aggression; f winner=73.9%;
f winner; · positive b = effect in favour of winner; ¸
loser = 100% draws were excluded, since we tested winner vs. loser.
Model b: Redirected aggression vs. other PCIs; f redirected aggression=16.9%.
Model c: Third party aggression vs. other PCIs: f third party aggression=6.8%; ¹ longer
conflicts preceding third party aggression.
% = frequency in percent of the dependent variable after conflicts with particular
parameter ( f p ); f = overall frequency of dependent variable.
Model a: winner
f loser=26.1%; ¶ f
vs. loser
78
Chapter 6: Choice of post-conflict interctions
CONFLICT
DRAW
CONFLICT DURATION
SHORT
LONG
AFFILIATIVE
POST-CONFLICT
MANAGEMENT
AGRESSIVE
POST-CONFLICT
MANAGEMENT
RELATIONSHIP BENEFIT
HIGH
LOW
SEX COMBINATION
MIXED
SAME
FURTHER AGGRESSION
UNLIKELY
LIKELY
RECONCILIATION
CONSOLATION
‘EMERGENCY
EXIT’:
STRESS
REDUCTION
AFTER LONG
FIGHT, LEAVE
FOCUS OF
AGGRESSION
VERY LONG
AND INTENSE
FIGHT
UNEXPECTED
LOSER
REDIRECTED
AGGRESSION
THIRD PARTY
AGGRESSION
RENEWED
AGGRESSION
Figure 6.4 Schematic depiction of the evaluation process for post-conflict management in Taï
chimpanzees. Several factors influence the optimal choice of PCI between the conflict (starting point:
black, on top) and five post-conflict interactions (end points: black, at bottom). Affiliative PCIs are
arranged on the left side, while aggressive PCIS are grouped on the right side of the figure (marked
in grey). Some post-conflict interactions can be reached following several different effects. Dashed
box indicates that effect is one possible conclusion.
6.4 Discussion
6.4.1 Summary of results
Taï chimpanzees applied five of the initially six identified PCIs and no PCI for postconflict management. I have summarised the results of the choice of PCI in Figure 6.4, in
order to provide an accessible overview of the post-conflict management related PCIs.
79
Chapter 6: Choice of post-conflict interctions
This suggests that Taï chimpanzees show a clear-cut evaluation process in selecting a
PCI. Conflict participants avoided further interactions after conflicts over nonmonopolisible resources or among rare associates. Very short conflicts did not require
post-conflict management, as Taï chimpanzees continued with business as usual. Below, I
discuss the influence of the advantages and disadvantages on the choice of PCI and
compare the post-conflict management of Taï chimpanzees with those of other mammals.
Finally I extract general rules for the evaluation process and discuss their validity.
6.4.2 Business as usual
Solicited consolation was to the only possible PCI that did not serve a function in the
post-conflict management of Taï chimpanzees. As solicited consolation followed very short
conflicts and it was indistinguishable from normal interactions it seems likely that any
costs were minimal after very short conflicts. Moreover, conflict partners were able to
interact non-aggressively after shorter conflicts while longer conflicts led to aggressive
PCIs, and the longest conflicts usually resulted in attacks by third parties. In fact there
was almost a linear relationship between increasing length of conflict and likelihood of
further aggression as well as the level of escalation. On the one hand this may reflect the
motivation for escalation to aggression within the dyad and therefore within the party.
Competition over beneficial resources was more likely to lead to aggression than less
beneficial ones (chapters 425; Janson, 1988a; Preuschoft & van Schaik, 2000; Wittig &
Boesch, 2003a) and the length of the conflict was dependent on the value of the resource
(chapter 526; Wittig & Boesch, 2003b). On the other hand this may also suggest that
stress and tension created by the conflict increased with conflict duration. Although some
studies have tried to find relationships between post-conflict stress and conflict intensity
(contact vs. non-contact aggression), but failed to detect them (Macaca fascicularis:
Aureli, 1997; Macaca fuscata: Kutsukake & Castles, 2001; Papio anubis: Castles & Whiten,
1998b), they all neglected conflict duration as a possible predictor. It remains unclear
whether or not there is also a positive correlation between conflict duration and stress or
tension, besides the correlation with escalation to aggression. I suggest that conflict
duration is possibly a good predictor for the level of induced stress.
25
26
see 4.3.2
see 5.3.2.2
80
Chapter 6: Choice of post-conflict interctions
6.4.3 Avoiding further interactions
While Taï chimpanzees continued with business as usual after a seemingly negligible
stress response, they avoided any further interaction after fighting over non-limited
resources. Although non-limited resources do not usually cause conflicts, aggression can
arise in situations where many competitors are present (chapter 427; Janson, 1988b;
Wittig & Boesch, 2003a). Usually, with a non-limited resource, the possible disadvantages
of engaging on a PCI are likely to out weigh the possible benefits gained. Therefore,
moving to a different feeding spot or engaging in other activities (e.g. resting, travelling)
seems to be a reasonable post-conflict reaction. Whether or not the avoidance of any
further interaction was dependent on the preceding conflict, was not possible to test with
the time-rule method. Thus I can neither definitely include nor exclude no PCI from the
conflict management. Although Taï chimpanzees sometimes avoided interactions or
continued with business as usual after conflicts, most of the conflicts required a PCI.
6.4.4 Reconciliation
Taï chimpanzees reconciled more often with cooperative partners and frequent
associates. This is strong evidence that the choice for reconciliation was due to its
advantage of relationship repair. Reconciliation with high value partners is common in
many primate species, such as among cooperative partners (e.g. Cords & Thurnheer,
1993), alliance partners (e.g. Watts, 1995a), kin (e.g. York & Rowell, 1988; Cheney &
Seyfarth, 1989; Kappeler, 1993; Castles & Whiten, 1998a), partners of affiliation (e.g.
Cords & Aureli, 1993; Watts, 1995a; Castles et al., 1996; Schino et al., 1998; Call et al.,
1999), and frequent associates (e.g. de Waal & Yoshihara, 1983; Aureli et al., 1989). Wild
chimpanzees in Budongo, Uganda, preferably reconciled with mating partners (Arnold &
Whiten, 2001). Taï chimpanzees might even consider potential mating partners as
valuable since they reconciled more often in mixed sex dyads.
Reconciliation was also more likely when further aggression seemed to be less
likely. The result suggests that the disadvantage of risking recurring aggression, when
approaching the former opponent, also shaped the choice for reconciliation. Similar
results were found in other studies, where low intensity conflicts (non-contact aggression)
were reconciled more often than high intensity ones (e.g. Eulemur fulvus: Kappeler, 1993;
Pan troglodytes (Budongo): Arnold & Whiten, 2001). Furthermore, when Budongo
27
see Figure 4.2
81
Chapter 6: Choice of post-conflict interctions
chimpanzees accepted the outcome of conflicts by emitting greetings, they were more
likely to reconcile afterwards (Arnold & Whiten, 2001). Tufted capuchins (Cebus apella)
reconciled only after non-food conflicts although almost 90% of their conflicts were over
clumped food (Verbeek & de Waal, 1997). This might show that tufted capuchins do not
dare to reconcile when the chance of further aggression is high. Since macaques and
baboons reconciled less after food than after non-food conflicts (Macaca fascicularis:
Aureli, 1992; Macaca maurus: Matsumura, 1996; Papio anubis: Castles & Whiten, 1998a),
Aureli et al. (2002) argued that food conflicts may not damage the relationship of
opponents and therefore there is less need for reconciliation. However our multivariate
analysis for Taï chimpanzees showed that the occurrence of reconciliation was not
different between food and social context.
Losers initiated more reconciliation after food conflicts, while the initiation rates of
winners and losers were equal in social and sex contexts. After food conflicts losers
appeared to be the main profiteers of reconciliation. Taï chimpanzees, as well as Budongo
chimpanzees (Arnold & Whiten, 2001), probably tried to access food with a second
attempt after they calmed the former opponent. In contrast after social and sex conflicts
the social leverage of subordinates seemed to be similar to those of dominants (Hand,
1986; Lewis, 2002). Since both potential mating partners can avoid copulation, and the
support of a high value partner might be needed in future conflicts with other individuals,
both partners should have an interest in repair the relationship after social and sex
conflicts. However losers initiated more reconciliation with increasing conflict intensity, but
after contact aggression neither conflict partner showed an initiation preference. This
suggests that losers are more restricted by increasing conflict intensity than dominants.
Since subordinate partners risk more in approaching the former opponent than their
dominant partners, they may decrease their initiation rate when the dominant partners is
highly motivated to fight again. A conflict partner using contact aggression, however,
might signal a willingness to risk more to gain the benefit of the resource. Losers,
therefore, may be more hesitant to approach the former opponent for reconciliation than
after non-contact aggression. The results suggest that
increasing conflict intensity
increasingly disturbed the relationship of opponents in Taï chimpanzees. The advantages
of relationship repair and of accessing the resource through affiliation as well as the
disadvantage of further aggression shaped the proportion of loser to winner initiated
reconciliations. Different proportions of victim initiated reconciliation among species might
be explained by inter-specific differences in the risk of further aggression or the
82
Chapter 6: Choice of post-conflict interctions
advantage of cooperation or affiliation (more aggressor initiated: e.g. Cebus capucinus:
Leca et al., 2002; Papio anubis: Castles & Whiten, 1998a; Carpa hircus: Schino, 1998; no
difference: e.g. Macaca fuscata: Aureli et al., 1993; Macaca silenus: Abegg et al., 1996;
Papio papio: Petit & Thierry, 1994b; more victim initiated: e.g. Colobus guereza:
Björnsdotter et al., 2000; Macaca arctoides: de Waal & Ren, 1988; Crocuta crocuta:
Wahaj et al., 2001).
6.4.5 Consolation
Consolation was offered in almost the contrary conflict situations as compared to
reconciliation (Figure 6.3). Taï chimpanzees seemed to offer consolation when
reconciliation was either not beneficial or was too risky for conflict participants. Since low
benefit partners were those that did not share food and did not support each other,
opponents would gain little from relationship repair. Similarly same sex dyads would not
provide much benefit in terms of potential mating partners. Additionally, same sex
opponents probably have a higher tendency for further aggression than mixed sex
opponents, as competing aggressively over the same resource again might be very
unlikely when the resource provides different benefits to each opponent. Indeed same sex
dyads in Taï chimpanzees fought more often than mixed sex dyads (chapter 528; Wittig &
Boesch, 2003b). Since the risk of further aggression increased with length of conflict and
consolation followed longer conflicts more than reconciliation, consolation was probably
offered when approaching former opponents was too risky. Thus consolation may have
been substituted for reconciliation when further aggression was more likely. In gorillas
(Gorilla gorilla) consolation might also be a substitute for reconciliation for the same
reason. Consolation mostly occurred after conflicts among gorilla females (Watts, 1995b),
three quarters of which were conflicts ending without any submissive sign that showed
the acceptance of the outcome (Watts, 1994). This preference for consolation when
conflicts could be followed by further aggression might explain why female gorillas mainly
sought consolation from males (Watts, 1995b).
6.4.6 Renewed aggression
Undecided conflicts in Taï chimpanzees were usually followed by renewed
aggression. In addition losers of conflicts mostly initiated renewed aggression when they
were dominant losers or initiators that lost, and thus they had a good chance of winning
28
see Table 5.2
83
Chapter 6: Choice of post-conflict interctions
the new conflict (chapter 529; Wittig & Boesch, 2003b). The advantage of accessing the
resource in a second aggressive attempt thus seemed to be a main factor for the choice
of renewed aggression. However, why did so many winners renew the aggression? In
contrast to Taï chimpanzees, redfronted lemurs (Eulemur fulvus) had a higher tendency to
reconcile, when the conflict was undecided (Kappeler, 1993). Reconciliation is the only
other PCI that allows possible access to the resource but in addition excludes costs of
aggression. Redfronted lemurs might face a lower risk of further aggression than
chimpanzees, which enables them to arrange undecided conflicts peacefully while
chimpanzee winners might react aggressively when the former opponent, aiming to
reconcile, approaches too early. For other mammals there is no evidence that undecided
conflicts result in any specific PCI (e.g. Cercopithecus athiops: Cheney & Seyfarth, 1989;
Lemur catta: Kappeler, 1993; Capra hircus: Schino, 1998).
6.4.7 Redirected aggression
Redirected aggression was the only PCI in the post-conflict management of Taï
chimpanzees that showed an ambiguous pattern. Taï chimpanzees redirected aggression
marginally more often when the initiator won, so after expected outcomes (see: renewed
aggression). This casts doubts on the idea that individuals would redirect their frustration
to third parties. However, redirected aggression is the only aggressive PCI with evidence
for stress reduction (Aureli & van Schaik, 1991b). Since preceding conflicts of aggressive
PCIs were longer than those of affiliative PCIs, Taï chimpanzees might use redirected
aggression as a kind of emergency-exit to reduce their stress after long conflicts, where
either reconciliation or consolation were too risky, and to sneak out of the focus of
aggression. Thus the emergency-exit strategy pays for all dyads that engaged in long
conflicts regardless of their relationship. Other primate species might also employ the
emergency-exit, since redirected aggression is usually not affected by the relationship of
opponents (not affected: Gorilla berengei: Watts, 1995b; Macaca fuscata: Aureli et al.,
1993; Macaca sylvanus: Aureli et al., 1994; Papio anubis: Castles & Whiten, 1998a; but,
affected: Cercopithecus athiops: Cheney & Seyfarth, 1989).
6.4.8 Third party aggression
Third party aggression occurred after the longest and the most intense conflicts in
Taï chimpanzees. On one hand, individuals might support their coalition partners after the
29
see 5.3.1.2
84
Chapter 6: Choice of post-conflict interctions
conflict has already been decided (winner-support: de Waal, 1978) or general high
arousal within the party might induce general escalation of aggression (e.g. Aureli & van
Schaik, 1991b; Cords, 1992; Preuschoft & van Schaik, 2000). On the other hand conflict
partners, especially losers, were probably weakened after such long and intense fights.
These vulnerable individuals would be easy targets of aggression for their frequent
competitors, for example rank neighbours. Since third party aggression also occurred
more often after social conflicts, I was unable to rule out any of these possibilities.
6.4.9 Choice of PCI: generalised rules
Following the post-conflict management found in Taï chimpanzees (Figure 6.4) I try
to extrapolate generalised rules for the choice of PCIs: Reconciliation appeared to be
chosen when a disturbed relationship is costly and further aggression is unlikely. However
when initiation of reconciliation appears to be too risky, consolation seems to substitute
for reconciliation. In situations where losing partners perceive a chance to access the
resource by further aggression, they might renew aggression to gain the benefit of the
resource in a second attempt. If the social tension has reached a level where attempting
peaceful post-conflict management is too risky, conflict partners might take the
emergency-exit and redirect aggression to deflect aggressive attention to third parties.
When conflict partners seem to be weakened, third parties might seize the opportunity to
defeat a frequent competitor.
The interaction of advantages and disadvantages of PCIs might explain why
despotic macaque species reconcile less often than egalitarian ones (Thierry, 2000). As
both select usually high value partners for reconciliation (Macaca arctoides: de Waal &
Ren, 1988; Macaca fascicularis: Aureli et al., 1989; Macaca fuscata: Aureli et al., 1997;
Macaca mulatta: de Waal & Yoshihara, 1983; Macaca nemestrina: Judge, 1991),
relationship repair seems to be important in both hierarchy types. However the approach
of former opponents in despotic macaques is likely to carry higher risks of further
aggression, as tolerance levels in despotic societies are less clear-cut. Therefore
approaching a former opponent can be easily mistaken as a counterattack and lead to
further aggression. Additionally aggression in egalitarian macaques is milder and less
costly for the victim (Thierry, 1986; Thierry, 2000). Therefore a failed reconciliation
attempt in despotic macaques is more costly than in egalitarian ones. Following the
generalised rules, one would expect consolation to substitute for reconciliation in despotic
macaques. However consolation seems not to be part of the macaque repertoire (Watts
85
Chapter 6: Choice of post-conflict interctions
et al., 2000), probably due to social or cognitive constraints (de Waal & Aureli, 1996).
Despotic macaque species probably use redirected aggression instead to deflect the
disadvantages of aggression to third parties and perhaps to reduce their own stress (e.g.
Macaca fascisularis: Aureli & van Schaik, 1991b; Aureli, 1992; Macaca fuscata: Aureli et
al., 1993). In egalitarian species, however, redirected aggression is either less frequent
(Thierry, 1985; Thierry et al., 2000a) or is not observed at all (e.g. Macaca sylvanus:
Aureli et al., 1994), as they reconcile more likely.
In consequence our results indicate that Taï chimpanzees have a clear-cut
evaluation process as they seem to weigh carefully advantages against disadvantages to
select the best PCI to the experienced conflict situation. Conflict management in Taï
chimpanzees appears to take into account both cost and benefit before escalating a
conflict of interest (chapter 530; Wittig & Boesch, 2003b) and selecting the best PCI to
handle remaining costs. Since both conflict partners have the possibility to vary their postconflict cost, conflict management opens the door for negotiation to lessen the
consequences of conflicts. Thus post-conflict management seems to be an important
negotiation tool for social mammals to enable advantageous social living despite the
existing conflicts of interest.
30
see 5.4.5
86
Chapter 731:
How are relationships repaired ?
Figure 7.0 Strong relationships between siblings. Subadult male Lefkas embraces his
younger brother Leonardo (Photo by R. Wittig).
31
Corresponding with: Wittig, R.M. & Boesch, C. (in review). How to repair relationships in wild
chimpanzees (Pan troglodytes). Ethology.
Chapter 7: How are relationships repaired ?
7.1 Introduction
7.1.1 Why repair relationships?
Aggressive interactions damage the relationship of conflict partners, since they
create stress and interrupt tolerance levels between opponents (Maestripieri et al., 1992;
Aureli et al., 2002). Damaging a relationship is costly as it jeopardises the benefits related
to relationships (e.g. food-sharing, grooming, support). Reconciliation, the friendly postconflict interaction among former opponents, appears to repair relationships by restoring
tolerance to normal levels (Cords, 1992) and reduces stress induced by aggression (e.g.
Aureli & van Schaik, 1991b; Castles & Whiten, 1998b; Kutsukake & Castles, 2001).
Therefore reconciliation provides considerable benefit for high value partners. Although
reconciliation is beneficial, it nonetheless entails costs, as for example the risk of further
aggression (Aureli & van Schaik, 1991b; Cords, 1992). Such potential costs may even
prevent former opponents from reconciling conflicts under certain circumstances (chapter
632; Wittig & Boesch, 2003d).
Although a consensus exists over the benefits of reconciliation, almost nothing is
known about how opponents achieve the benefit that is provided by reconciliation. (Call et
al., 1999) found two clusters of conciliatory behaviours, one that seemed to serve
relationship repair between high value partners, while the other was used during
reconciliation by opponents of unspecific relationships. Moreover other studies have
reported that opponents preferred socially intense behaviours for reconciliation, though
the repertoire of reconciliation also contained socially less intense behaviours (Petit &
Thierry, 1994a; Abegg et al., 1996). These results suggest that socially intense behaviours
are more effective in achieving reconciliation than others. Thus it could be that some
conflict situations need more intense or complex repair than others and former opponents
vary the behaviours used to achieve reconciliation accordingly.
7.1.2 What damages relationships?
When comparing the pattern of reconciliation within and between species, I found
great variation (Appendix A). Intra-specific variation is of special interest and probably
varies for several reasons. For example it may indicate that individuals reconcile
differently under different levels of relationship damage. A strongly damaged relationship
88
Chapter 7: How are relationships repaired ?
might require a long and intensive reconciliation in order to restore tolerance levels and
reduce stress. Therefore, the investment of opponents in the repair of their relationship
should positively correlate with the strength of the damage done by the preceding
conflict.
Two factors, conflict intensity and relationship quality, potentially affect the damage
to relationships. Rhesus macaques (Macaca mulatta) showed a stronger behavioural
stress response to heavy aggression as compared to light aggression (de Waal &
Yoshihara, 1983). Moreover, severe aggression in longtailed macaques (Macaca
fascicularis) was reconciled more often than milder aggression (Koyama, 2001), and the
intensity of conflicts affected the tendency to reconcile in Japanese macaques (Macaca
fuscata: Schino et al., 1998). These results indicate that damage to a relationship
increases with the intensity of the conflict and probably other conflict characteristics.
In contrast, other studies have failed to detect a relationship between conflict
intensity and behavioural stress responses (Macaca fascicularis: Aureli, 1997; Macaca
fuscata: Kutsukake & Castles, 2001; Papio anubis: Castles & Whiten, 1998b). Instead,
opponents were more stressed either after conflicts with frequent as opposed to rare
associates (Aureli, 1997) or after conflicts with kin as opposed to non-kin (Kutsukake &
Castles, 2001). These results indicate that damage to a relationship increases with
relationship quality or other relationship characteristics. Since the results are not
consistent across studies as to which effects are causing damage to relationships, a
multivariate approach is required.
7.1.3 How to repair the damage? Variation in reconciliation
Variation is also found in the timing of reconciliation, probably due to the fact that
opponents sometimes have to compromise high benefits of reconciliation with
accompanied high costs. For example opponents should quickly re-establish mutual
tolerance as a precondition for beneficial interactions (Cords & Thurnheer, 1993), or
minimise the time being exposed to the costs of stress (Sapolsky, 1998). Therefore
reconciliation should be initiated soon after a conflict. However, since stress and tension
decrease with time (Aureli & van Schaik, 1991b), aggression can revive when
reconciliation is initiated too quickly after the conflict (chapter 633; Wittig & Boesch,
32
33
see 6.4.9
see 6.3.5.3
89
Chapter 7: How are relationships repaired ?
2003d). Thus both extremes of latency of reconciliation contain high costs and benefits
such that the timing of initiation of reconciliation is likely to be a compromise. Therefore
both costs and benefits need to be considered for the understanding of the variation
within reconciliation.
Reconciliation has been studied in almost three dozen different mammal species.
Conflict partners of most species reconciled preferably with high value partners (Aureli et
al., 2002), and most species also showed evidence for intra-species variation within
reconciliation in different features (Appendix A). First, for most species variability was
found for the duration between conflict and reconciliation and some authors specifically
stated that the latency of reconciliation was not equal (Gust & Gordon, 1993; Arnold &
Whiten, 2001). Second, many species showed several typical behaviours for reconciliation
(Appendix A), ranging from short touches to long bouts of allogrooming, indicating that
the duration of reconciliation varies greatly (de Waal & Ren, 1988; Aureli et al., 1993).
Second, opponents sometimes employed several different behaviours in the same
reconciliation, therefore increasing what I refer to here as the complexity of reconciliation.
Stumptailed macaques (Macaca arctoides), for example, combined allogrooming with
contact sitting (Call et al., 1999), sooty mangabeys (Cercocebus torquatus) joined
allogrooming and embracing in the same reconciliation (Gust & Gordon, 1993) and female
gorillas (Gorilla gorilla) reacted with combinations of several behavioural elements to
aggression by males (Watts, 1995a).
Finally, neither victims nor aggressors were exclusively initiating reconciliation in any
of the conducted studies (Appendix A). Both partners can profit from reconciliation, since
both face stress after aggression (Aureli, 1997). However, a subordinate opponent might
profit more from a good relationship with a dominant partner than the reverse, as
relationships have asymmetrical benefits for each partner (Cords & Aureli, 2000).
Wild chimpanzees are a good model to investigate the function of and the reasons
for intra-species variation within reconciliation, as they use reconciliation (chapter 634;
Arnold & Whiten, 2001; Wittig & Boesch, 2003d) and they optimise conflicts based on
cost and benefit strategies (chapter 535; Wittig & Boesch, 2003b). The aim of this study is
to test if the function of reconciliation in wild chimpanzees is about relationship repair.
34
35
see Figure 6.1
see Figure 5.5
90
Chapter 7: How are relationships repaired ?
Furthermore, I aim to understand how the variation within reconciliation, in terms of
latency, duration, complexity and initiator of reconciliation, is influenced by the preceding
conflict and the damage to relationships.
7.2 Specific methods
7.2.1 Data and test conditions
From the 1071 collected conflicts 791 were conflicts between adults and had
complete information on the conflict and the latency, duration and complexity of postconflict interactions. The resulting balanced data set of 146 out of a total of 178 conflicts
with reconciliation was used for the multivariate analyses. For each balanced data point I
calculated the mean of latency and duration of reconciliation and then I assigned each
mean to the appropriate category of quartiles (Table 7.1). In addition I used the median
for the complexity of reconciliation. All variables of Table 3.2, plus the interactions of
initiator’s sex with recipient’s sex, rank difference with initiator’s rank and conflict context
with competitor proportion, were considered simultaneously as independent variables in
each of the multivariate analyses.
Table 7.1 Three variables of the variability of reconciliation including the parameters.
Variable
Distribution
Latency of
Reconciliation
ordinal
multinomial
Duration of
Reconciliation
ordinal
multinomial
Complexity of
Reconciliation
ordinal
multinomial
Definition and parameters
time(t) between the end of the conflict and the start of
the reconciliation split into categories which
corresponded with the four quartiles: very short for
t£20s; short for 20s<t£144s; long for 144s<t£774s; very
long for t>774s
time(t) between the start and the end of the
reconciliation split with into categories which
corresponded with the four quartiles: very short for
t£4s; short for 4s<t£12s; long for 12s<t£70s; very long
for t>70s.
number of different behavioural elements used during
reconciliation: simple = one behavioural element;
advanced = two behavioural elements; complex = three
and more behavioural elements
91
Chapter 7: How are relationships repaired ?
7.2.2 Additional operational definitions
The first affiliative interaction between former opponents after a conflict was
defined as reconciliation when no other interaction with participation of the focus
individual was observed between conflict and reconciliation. I took three measurements to
describe variation found within reconciliation: (a) the latency of reconciliation, which was
the time elapsing between conflict and reconciliation, (b) the duration of reconciliation,
which was the time between the start of the first affiliative interaction to the end of the
last affiliative interaction of reconciliation, and (c) the complexity of reconciliation, which
was the number of different behavioural elements used during the reconciliation. Since
both latency and duration of reconciliation showed a strongly skewed distribution (Figure
7.1), I defined for each of them four ordinal categories that corresponded with their
quartiles (Table 7.1).
(a) Latency of reconciliation
100
% reconciliation <= time
90
80
70
60
50
40
30
20
10
0
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
time [min]
(b) Duration of reconciliation
100
% reconciliation <= time
90
80
70
60
50
40
30
20
10
0
0
12
24
36
48
60
72
84
96 108 120 132 144 156 168 180
time [s]
Figure 7.1 Distribution of latency and duration of reconciliation
over time in Taï chimpanzees (N=178). Graph (a) shows the
percentage of reconciliation that was initiated within a certain
time, while graph (b) shows the percentage of reconciliation that
continued for a certain time.
92
Chapter 7: How are relationships repaired ?
7.2.3 Measuring reconciliation
I described the occurrence of reconciliation using two methods. Firstly, the
Corrected Conciliatory Tendency (CCT: Veenema et al., 1994), in order to show that
reconciliation did not occur by chance. Therefore I computed a baseline that represents
the mean interval among the consecutive affiliative interactions for each dyad. Then I
calculated the CCT for each dyad using the following formula: CCT =
A-D
, with A
# conflicts
(attracted pairs) = #reconciliation with latency < baseline, and D (dispersed pairs) =
#reconciliation with latency > baseline. The relative preponderance of attracted pairs
(CCT>0) showed that the dyad had a tendency to reconcile.
Secondly, I applied an altered version of the time-rule (Aureli & van Schaik, 1991a;
Castles & Whiten, 1998a), to determine whether or not the occurrence of reconciliation
depended on the preceding conflict. The time rule implies that reconciliation needs to be
initiated faster after a conflict than baseline. To compare the latencies of each dyad with
their baseline, I calculated for each dyad the relative latency (=
latency
). Relative
baseline
latencies of reconciliation smaller than one indicated that the occurrence of reconciliation
was dependent on the preceding conflict.
To demonstrate the function of reconciliation I calculated relative latencies per dyad
for two other types of interactions: (a) the first affiliative inter-opponent interaction after
reconciliation (post-conciliatory interaction, n=113 in 43 dyads), and (b) the first affiliative
inter-opponent interaction after unreconciled conflicts (post-non-conciliatory interaction,
same 43 dyads), where for example consolation or redirected aggression occurred.
Relative latencies of post-conciliatory interactions are expected to equal one, if
reconciliation sets tolerance levels to normal. In contrast, relative latencies of post-nonconciliatory interactions should be greater than one, as disturbed tolerance levels were
not repaired.
93
Chapter 7: How are relationships repaired ?
7.3 Results
7.3.1 Occurrence of reconciliation
I observed 791 conflicts in 90 of the 105 adult dyads, of which 48 dyads used
reconciliation at least once, while the other 42 dyads never implemented reconciliation
(Table 7.2). Almost one out of four conflicts (178 of 791) was followed by reconciliation.
The mean conciliatory tendency (CCT) of dyads was 15.85% for Taï chimpanzees .
Conciliatory tendencies varied greatly between the sexes. In general, dyads including
males showed a higher CCT than dyads including females (CCTm=25.34%; CCTf=15.94%;
Permutation test: nm=46, nf=84, p<0.05). Female-female dyads had a lower CCT than
male-female dyads (CCTff=5.92%; CCTmf=26.96%; Permutation test & Bonferroni: nff=44,
nmf=40, p<0.016). In contrast male-male dyads (CCTmm=14.56%) did not differ from
male-female dyads (Permutation test & Bonferroni: nmm=6, nmf=40, ns) and femalefemale dyads (Permutation test & Bonferroni: nmm=6, nff=44, ns).
Table 7.2 Conciliatory Tendency (CCT) of adult Taï chimpanzees.
males
CCT [%]
a/d/n interactions*
Brutus
Macho
Marius
Nino
Brutus Macho
9
1/0/10
females
Marius
Nino
Belle
38
13
0
3/0/5
2/0/14
12
1
0/0/1
Castor
---
Dilly
100
1/0/0
Fossey Goma Loukoum Mystere Narcisse
0
0/0/1
33
1/0/2
33
1/0/2
100
1/0/0
---
Perla
0
0/0/2
15
-11
0
20
33
100
36
14
0
1/1/0
0
2/0/11
1/2/6
0/0/5
1/0/4
2/0/4
1/0/0
5/0/9
1/0/6
0/0/7
0
25
2/0/6
10
13
0
50
16
20
25
0
1/0/7
0/0/2
2/0/2
4/1/14
1/0/4
1/0/3
0/0/3
46/16/190 9/8/87
38
6/0/10
0/0/3
Belle
Castor
20
2/0/8
1/0/9
0
0/0/10
0
0/0/1
0
0/0/1
Dilly
Fossey
25
2/0/6
0
0/0/2
0
0/0/1
50
1/0/1
0
0/0/1
---
44
4/0/5
0
0/0/6
0
0/0/5
33
50
0
1/0/2
2/0/2
1/1/3
0
0
0/0/2
0/0/2
0
Goma
0/0/3
Loukoum
23
4/1/8
0
0/0/1
-33
0/1/2
--50
1/0/1
0
0/0/1
20
1/0/4
---
17
43
60
43
3/0/4
3/0/2
9/0/12
0
0/0/1
--0
0/0/1
---
20
2/0/8
0
0/0/2
0
0/0/3
-25
0
33
0/0/1
3/0/6
-----
--0
0/0/2
0
0/0/1
0
0/0/1
0
0/0/2
0
0/0/2
0
-11
0
0
33
0/0/5
0/1/8
0/0/7
0/0/3
1/0/2
0
0/0/6
0
0
0
0/0/2
0/0/2
---
---
0
0/0/2
Ricci
2/0/7
0/1/3
0/0/1
Perla
22
0/0/3
---
2/1/3
67
2/0/1
0
1/0/0
Narcisse
---
Venus
0/0/1
100
Mystere
0
Ricci
0
0/0/2
---
Venus
Top line (bold) = CCT.
Bottom line = number of attracted (a), dispersed (d) interactions and conflicts without reconciliation (=neutral interactions, n) of each dyad for the
observation period (see 7.2.3).
--- = no conflict observed in this dyad.
7.3.2 Function of reconciliation
Reconciliation was demonstrated following the time-rule (Figure 7.2), since dyads of
all three sex combinations were reconciling faster than baseline (Bootstrep tests: m-m,
94
Chapter 7: How are relationships repaired ?
n=5, p<0.05; m-f, n=29, p<0.05; f-f, n=9, p<0.05). In contrast, post-reconciliatory
interaction occurred as fast as baseline for dyads with male participation (Bootstrep tests:
m-m, n=5, ns; m-f, n=29, ns), while for female-female dyads post-reconciliatory
interactions were faster than baseline (Bootstrep test: f-f, n=9, p<0.05). Friendly
interactions after unreconciled conflicts however occurred slower than baseline within
each sex combination (Bootstrep tests: m-m, n=5, p<0.05; m-f, n=29, p<0.05; f-f, n=9,
p<0.05). Hence, aggression disturbed the usual interaction pattern among opponents,
since opponents of unreconciled conflicts needed longer than usual to interact again.
Reconciliation reinstated normal interaction rates for dyads with males but not in femalefemale dyads (Figure 7.2).
relative latency of interaction (interval/baseline)
reconciliation
post-conciliatory interaction
post-non-conciliatory interaction
8
7
6
5
4
3
2
Baseline
1
0
male-male
male-female
female-female
Figure 7.2 Comparison between latency of reconciliation (n), latency of the first postconciliatory interaction (n) and latency of the first unreconciled interaction (¨) with baseline
in Taï chimpanzees. Mean relative delays (interval between interactions divided by baseline)
of the dyads are presented for each sex class, while relative baseline is one. Confidence
intervals (95%) are shown for a two-sided distribution, calculated with bootstrap method
(test statistics see 7.3.2).
Finally I checked whether conciliatory tendencies increased with the relationship
benefit of opponents (Figure 7.3). I merged small and medium relationship benefit into
one category of low benefit and compared it with high benefit partners. Male opponents
of high relationship benefit showed higher CCTs than those of lower relationship benefit
(Permutation test: nlow=4, nhigh=2; p<0.05) and the same was true for female opponents
95
Chapter 7: How are relationships repaired ?
(Permutation test: nlow=38, nhigh=6; p<0.05). For mixed sex dyads, however, no
difference was observed (Figure 7.3; Permutation test: nlow=32, nhigh=8; ns).
35
high benefit
low benefit
average
dyadic CCT
30
*
25
20
*
15
10
5
0
m-m
m-f
f-f
Sex class
Figure 7.3 Comparison of Conciliatory Tendencies
between high (n) and low (¨) benefit partners within
each sex class of Taï chimpanzees. Low benefit
partners have a small and medium relationship benefit.
*indicate significant differences between the samples
(p-values see 7.3.2).
Table 7.3 Factors affecting (a) the latency and (b) the duration of reconciliation in Taï chimpanzees. The table shows the
significant variables and the estimate-coefficient (b) of the parameters.
(a)
independent
variables
Receiver’s Sex
Wald
df
p
9.96
1
<0.01
Rank Difference 10.73
2
<0.01
1
<0.05
Conflict Duration
4.61
parameter
male
female
Latency of reconciliation¶
very short short long
n
[%]
[%] [%]
63
11.1
25.4 25.4
83
36.1
24.1 24.1
small 76
middle 46
big 24
continuous 146
(b)
30.3
19.6
20.8
¯
28.9
17.4
25
«
21.1
26.1
33.3
­
df
p
1
<0.01
male
female
Duration of reconciliation·
very short short long
n
[%]
[%] [%]
63
23.8
19.1 22.2
83
26.5
32.5 22.9
Rank Difference 12.75
2
<0.01
small
middle
big
76
46
24
Conflict Context 12.66
2
<0.01
independent
Wald
variables
Receiver’s Sex 7.68
Model a:
quartile):
Model b:
quartile):
parameter
23.7
28.2
25.0
32.9
26.1
8.3
22.4
21.7
25.0
very long
[%]
38.1
15.7
b
Wald
p
-0.59
0.59
9.96
<0.01
19.7
36.9
20.9
­
0.70
-0.41
-0.29
-0.98
9.41
3.13
0.97
4.61
<0.01
ns
ns
<0.05
b
Wald
p
-0.61
0.61
7.68
<0.01
very long
[%]
34.1
18.1
21.0
24.0
41.7
1.08 12.66 <0.001
-0.24 0.49
ns
-0.84 4.75 <0.05
social 81
22.2
18.5 23.5
35.8
-0.29 1.29
ns
sex 29
6.9
55.2 27.6
10.3
-0.68 4.36 <0.05
food 36
47.2
22.2 16.7
13.9
0.97 12.61 <0.001
Latency of reconciliation (very short – short – long – very long); ¶ very short (1st quartile): t£20s; short (2nd
20s<t£144s; long (3rd quartile): 144s<t£774s; very long (4th quartile): t>774s.
Duration of reconciliation (very short – short – long – very long); ·very short (1st quartile): t£4s; short (2nd
4s<t£12s; long (3rd quartile): 12s<t£70s; very long (4th quartile): t>70s.
96
Chapter 7: How are relationships repaired ?
7.3.3 Variation within latency and duration of reconciliation
More than 50% of the reconciliations were initiated within 3 minutes (median=144s)
after the conflicts (range=2s-251min; Figure 7.1a). Reconciliation was initiated more
quickly after conflicts with female victims, after conflicts between conflict partners of a
small rank difference and after short conflicts, while conflict partners took longer to
initiate reconciliation after conflicts with male victims and long conflicts (GLZ:
Median
LR c 52,146 =30.0, p<0.0001; Table 7.3a).
duration of reconciliation [s] (n)
-12-10
-8 -6 -4 -2 12
0 2 4 6 8 10 12
24 26 28 30 32 34 48
36 38 40 42
24 14 16 18 20 22 36
0
22
food
20
Conflict
18
context
16
sex
social
14
12
Y Data
Sex of
10
receiver
female
male
8
6
Rank
4
difference
2
large
middle
small
0
Median
0
576 468 504
-144-108
-72 -36 144
0 36 72 108 288
144 180 216 252 432
288 324 360 396 432
latency of reconciliation [s] (¨)
Figure 7.4 Median latency (¨: bottom scale) and duration (n: top scale) of reconciliation
for the influencing variables in Taï chimpanzees. Median latencies and durations of the
variable parameters are shown as deviations of the overall median values for latency
~
~
( X =144s) and duration ( X =12s) of reconciliation, indicated by the median line. Deviations
to the left mark that latency or duration for the parameter were shorter than median, while
deviations to the right show latency or duration were longer than median (test statistics see
Table 7.3).
97
Chapter 7: How are relationships repaired ?
About 50% of reconciliations were shorter than 12s (range =1s-1026s; Figure 7.1b).
Reconciliation was shorter after food conflicts, after conflicts between opponents with a
small rank difference and after conflicts with female victims, while reconciliation was
longer after conflicts over sex (with medium duration), after conflicts between opponents
with a large rank difference and after conflicts directed to males (GLZ: LR c 92,146 =38.3,
p<0.0001; Table 7.3b).
Consequently either opponents used short reconciliation soon after the conflict,
when they had a small rank difference and female victims, or opponents initiated long
reconciliation with some delay, when they had a big rank difference and male victims
(Figure 7.4). Additionally long fighting resulted in late reconciliation while food conflicts
were reconciled more quickly than conflicts over sex.
relative occurrence of reconciliation [%]
20
gentle aggression (level 1)
hard fighting (level 5)
15
10
5
0
simple
advanced
complex
-5
-10
-15
-20
-25
Figure 7.5 Complexity of reconciliation depending on the
intensity of the preceding conflicts in Taï chimpanzees. The
relative occurrence of reconciliation is shown for the two
significant parameters of gentle aggression (level 1: n) and
hard fighting (level 5: ¨). Positive values indicate higher
frequencies and negative values indicate lower frequencies of
reconciliation than average (test statistics see Table 7.4).
98
Chapter 7: How are relationships repaired ?
7.3.4 Variation within Complexity of reconciliation
Most reconciliation (60.3%) was carried out in a simple form using only one
behavioural element for the interaction, while 30.1% were advanced (two behavioural
elements) and 9.6% were complex reconciliations (three and more behavioural elements).
Intensity of the preceding conflict was the only predictor of reconciliation complexity
(GLZ: LR c 82,146 =19.6, p<0.05; Table 7.4). Gentle aggression (conflict intensity level 1) led
to simple reconciliation, while hard fighting (conflict intensity level 5) was followed by
advanced and complex reconciliation (Figure 7.5). Conflicts with medium intensity (levels
2-4) did not differ in their variability of reconciliation complexity from the average
proportion, as expected for an ordinal variable with a linear influence. Hence, opponents
increased the complexity of reconciliation with the intensity of the preceding conflict.
Table 7.4 Factors affecting the complexity of reconciliation in Taï chimpanzees. The table shows the significant
variables and the estimate-coefficient (b) of the parameters.
Complexity of reconciliation
simple advanced complex
df
p
parameter n
p
b
Wald
[%]
[%]
[%]
4
<0.01
level 1 30
76.7
23.3
0.0
0.94 9.11 <0.01
level 2 31
67.7
19.4
12.9
0.32 1.33
ns
level 3 42
50.0
40.5
9.5
-0.28 1.36
ns
level 4 30
60.0
30.0
10.0
0.06 0.04
ns
level 5 13
38.5
38.5
23.0
-1.04 7.82 <0.01
Model: Complexity of reconciliation (simple – advanced – complex); simple: one behavioural element; advanced:
two behavioural elements; complex: three and more behavioural elements.
independent
Wald
variables
Conflict 14.87
Intensity
7.3.5 Initiator of reconciliation
Since the sex of the victim was an important determinant for latency and duration
of reconciliation, I tested which kind of reconciliation (latency, duration and complexity)
was initiated by aggressors (39.7%) and by victims (60.3%). Aggressors initiated more
simple reconciliation, as the estimate coefficient b showed a preference of victim initiators
for advanced and complex reconciliation (GLZ: LR c 22,146 =8.13, p<0.05; Table 7.5a). When
merging advanced and complex reconciliation to one category, victims preferentially
initiated advanced and complex reconciliation (GLZ: LR c 12,146 =8.91, p<0.01; Table 7.5a).
For victim initiated reconciliation, sex of the victim (male: 47.7%; female: 52.3%)
played a role on the latency and duration of reconciliation (GLZ: LR c 62,88 =20.71, p<0.01,
99
Chapter 7: How are relationships repaired ?
Table 7.5b). Female victims started reconciliation soon after the conflict while male
victims waited longer and used long reconciliation.
Table 7.5 Factors affecting initiator of reconciliation in Taï chimpanzees. Part (a) distinguishes between aggressor and victim initiated
reconciliation, while part (b) distinguishes male or female victim initiated reconciliation. The table shows the significant variables and
the estimate-coefficient (b) of the parameters. In the far right the adapted estimate-coefficients (b*) are shown for the parameters,
which were merged.
Initiator of reconciliation
(a)
independent
variables Wald
Complexity of 7.60
Reconciliation
df
p
2
<0.05
(b)
independent
Wald
variables
Latency of 11.94
Reconciliation
Duration of 7.98
Reconciliation
df
p
3
<0.01
3
<0.05
parameter
n
adapted parameters*
aggressor
[%]
victim
[%]
b
Wald
p
48.9
27.3
21.4
51.1
72.7
78.6
0.73
-0.21
-0.52
6.64
0.24
1.33
<0.01
ns
ns
b
Wald
p
simple 88
advanced 44
complex 14
Victim initiates reconciliation
male
female
n
victim [%] victim [%]
very short 19
21.1
78.9
short 20
45
55
long 21
42.9
57.1
very long 28
71.4
28.6
parameter
very short 24
short 22
long 16
very long 26
Model a: Initiator of reconciliation (aggressor versus victim).
Model b: Victim initiator (male versus female).
45.8
31.8
37.5
69.2
54.2
68.2
62.5
30.8
-0.99 4.59
0.13 0.12
-0.39 0.87
1.25 10.34
<0.05
ns
ns
<0.01
-0.01
-0.68
-0.41
1.10
ns
ns
ns
<0.01
0.00
2.63
0.80
7.19
b*
Wald*
p*
7.50
<0.01
0.50
-0.50
7.4 Discussion
7.4.1 Summary of results
Aggression in Taï chimpanzees decreased the interaction rate of former opponents
and reconciliation restored interaction rates to normal levels. A high relationship benefit
increased the tendency to reconcile between same-sex partners, while reconciliation in
mixed sex dyads was independent from the cooperative benefits of relationships.
Characteristics of reconciliation varied widely in Taï chimpanzees. Latency of reconciliation
increased with the duration of the preceding conflict and duration of reconciliation was
short after food conflicts. Both latency and duration of reconciliation were short after
conflicts among rank neighbours or when females were attacked. Furthermore,
complexity of reconciliation increased with the intensity of the preceding conflict. Finally
our analysis revealed that victims of aggression initiated more complex reconciliation
compared to aggressors and female victims initiated shorter reconciliation and sooner
after a conflict compared to male victims.
100
Chapter 7: How are relationships repaired ?
7.4.2 Function of reconciliation
While aggression among Taï chimpanzees seemed to disturb the tolerance levels
between opponents, reconciliation seemed to restore the tolerance levels of conflict
partners to normal. These results support the proposition that reconciliation functions to
repair relationships following a conflict. This was also demonstrated by Cords (1992)
experimental study which showed that tolerance to proximity with former opponents was
higher after reconciled conflicts in longtailed macaques (Macaca fascicularis). Results
indicating that reconciliation restores tolerance levels in a wild population have not
previously been shown.
Reconciliation seems to be adaptive for same sex partners with highly cooperative
relationships, as friendly interactions were remarkedly up to eight times less frequent than
when no reconciliation occurred. Same sex dyads in Taï reconciled more with partners
with whom they usually shared food and support. The reconciliation pattern of Taï
chimpanzees shares similarities with that of longtailed macaques (Macaca fascicularis),
where cooperation over food determined reconciliation (Cords & Thurnheer, 1993), and
with that of mountain gorillas (Gorilla gorilla), where alliance partners reconciled more
than others (Watts, 1995a). Mixed sex dyads showed the highest conciliatory tendencies
of all dyads, but, contrary to same sex dyads, they did not reconcile more with high
benefit partners. This indicates that mixed sex dyads gain different benefits from
reconciliation compared with same sex dyads. Budongo chimpanzees had a higher
reconciliation rate between mating partners compared to dyads that never copulated
(Arnold & Whiten, 2001). A relaxed relationship with a potential mating partner could be
beneficial for males, as females may be more willing to mate with them, and for females,
as sexual harassment by males may be avoided (Niemeyer & Anderson, 1983; Paul,
2002).
7.4.3 The variation within reconciliation
Reconciliations in Taï chimpanzees were highly variable, with latency, duration,
complexity and initiation of reconciliation varying according to the costs and benefits
accompanied by reconciliation. Opponents, for example, delayed reconciliation after long
conflicts. The risk of further aggression is known to increase with conflict duration
(chapter 636; Wittig & Boesch, 2003d) and tension is known to reduce over time (Aureli &
36
see 6.3.2
101
Chapter 7: How are relationships repaired ?
van Schaik, 1991b). This suggests that former opponents require some time after long
conflicts before they can approach to reconcile without risking further aggression.
Moreover, reconciliation in Taï chimpanzees was shorter after food conflicts
compared with sex and social contexts. When competing over limited food sources,
delaying reconciliation is costly and conflict partners should resume feeding soon after the
conflict before the food is depleted. Thus, the potential loss of time accessing limited
resources appears to shorten the duration of reconciliation. In varying reconciliation,
chimpanzees take into account not only the risk of further aggression – higher after
longer conflicts – but also the cost of interrupting beneficial activities. So that in conflicts
over food, which are often monopolisable sources (chapter 437; Wittig & Boesch, 2003a),
individuals used shorter reconciliation than after conflicts over less restricted resources.
Taï chimpanzees showed a strong interaction between latency and duration of
reconciliation, as they performed either a short reconciliation soon after the conflict or a
long reconciliation after waiting for some time. The fact that females fight preferably over
food while males usually engage in social conflicts (chapter 538; Wittig & Boesch, 2003b),
could be the cause for female victims initiating shorter reconciliation and sooner after
conflicts compared with male victims. Female victims appeared to accept an increased risk
of further aggression when reconciliation carried the potential benefit of rapid access to a
limited food source. This may explain why post-conciliatory interactions among females
occurred faster than baseline. In contrast male victims that fight mainly in social contexts
waited longer to reconcile with a former opponent. With no apparent benefits to off-set
the risk of further aggression males seem to wait longer to initiate reconciliation. Males
might only seek long-term benefits of reconciliation such as relaxed relationships
(Koyama, 2001), while females might also take into account potential immediate benefit
of reconciliation.
Close ranking chimpanzees initiated shorter reconciliations sooner after the conflicts
than distantly ranked opponents, and they usually fight harder (with more intensity) than
distantly ranked ones (chapter 539; Wittig & Boesch, 2003b). Similarly female capuchins
(Cebus capucinus) were more stressed by the presence of close ranking individuals than
by far ranking ones (Manson & Perry, 2000). Thus conflicts between close ranking
37
38
see Table 4.1
see 5.3.1.1
102
Chapter 7: How are relationships repaired ?
opponents compared to distantly ranked opponents appeared to be more disturbing. Rank
neighbours might not be able to accept subordination as easily as distantly ranked
subordinates. In rats, for example, daily rhythmicity of heart rate and body temperature
of subordinates were more disturbed the more individuals resisted the aggression of
dominants (Meerlo et al., 1999). It seems that Taï chimpanzee opponents shortened the
latency of reconciliation when relationships were strongly disturbed.
7.4.4 Repairing the damage
Taï chimpanzees reacted with more complex reconciliation after more intense
aggression. This reaction suggests that the disturbance to the opponents’ relationship
increased with the intensity of the preceding conflict. This would contradict the integrated
hypothesis (Aureli, 1997), which proposes that damage to relationships is only dependent
on the benefit of relationships and not on the intensity of conflicts. Three studies showed
that behavioural stress indicators did not increase with intensity of conflicts (Macaca
fascicularis: Aureli, 1997; Macaca fuscata: Kutsukake & Castles, 2001; Papio anubis:
Castles & Whiten, 1998b). The two macaque species rather had increased stress
indicators after conflicts with opponents with high affiliation rates or relatedness.
Longtailed macaques (Macaca fascicularis), nonetheless required more reconciliation after
more severe aggression, since they showed a greater proportion of reconciled conflicts
after severer aggression (Koyama, 2001).
This discrepancy could have several explanations. Behavioural indictors of stress
might not be the most reliable measures of stress, as for example there are studies that
detected either stable or even decreasing rates of self-directing behaviours after
aggression (e.g. Cebus capuchinus: Manson & Perry, 2000; Pan troglodytes: Arnold &
Whiten, 2001). More direct measures of stress are needed to understand the interactions
of conflict intensity, relationship quality and stress (e.g. heart rate: Aureli et al., 1999;
hormones: Sapolsky, 1986; Creel, 2001). An alternative explanation is that categories of
conflict intensities used in other studies may have been too broad to detect a correlation.
In the present study the two extreme values out of five categories of conflict intensity
were responsible for the significant influence on the complexity of reconciliation. Another
explanation is that stress may not increase with the disturbance of the relationship, but
rather with lost benefit from disturbed cooperations. If so there would be no discrepancy
39
see Table 5.4a
103
Chapter 7: How are relationships repaired ?
between the integrated hypothesis and our results. A last possibility is that dominance
style may affect stress levels. The two macaque species have despotic dominance styles
(Thierry et al., 2000b), which imply a high degree of asymmetry in conflicts (Thierry,
2000). The mere presence of dominants may be enough for stress to increase (Castles et
al., 1999) and may be as stressful as open aggression (Sapolsky, 1993; Abbott et al.,
2003). In contrast chimpanzees have a more egalitarian dominance style allowing
subordinate individuals to win some conflicts (chapter 540; Wittig & Boesch, 2003b). It is
conceivable that victims in despotic groups perceive mild aggression (e.g. threats) as
being as stressful as intense aggression, while victims in egalitarian groups are less
stressed by threats than by contact aggression. Nonetheless in Taï chimpanzees an
increased level of disturbance of relationships seems to require an increased complexity of
reconciliation to repair the relationships of former opponents.
I conclude that conflicts in Taï chimpanzees disturbed the relationship between
opponents and that reconciliation repaired the relationship. Reconciliation cleared up the
disturbance with a more complex pattern for more intense conflicts. However, high value
relationships were reconciled for all levels of disturbance. Therefore, I suggest that the
function of reconciliation is to reduce disturbance in all relationships. Previous authors
have suggested reconciliation occurs as a repair mechanism in high value relationships
only, either through stress reduction or cooperation repair (Aureli, 1997; de Waal &
Aureli, 1997). Our data has shown that disturbance resulting from strong fighting is also
repaired by reconciliation independent from relationship quality of opponents. Repair of
low value relationships has not previously been predicted, but its occurrence in primates
could be beneficial in societies where out-competed partners may react with revenge and
contra-intervention (de Waal & Luttrell, 1988; Drukker et al., 1991; Aureli et al., 1992;
Silk, 1992). Since it might be of equal benefit to have a friend as it is costly to have an
enemy, social living animals should prevent losing friends and making enemies. Thus
reconciliation maintains the beneficial relationship of high value partners, but it also
seems to prevent low benefit partners turning into enemies.
Based on the results of the variation within reconciliation of Taï chimpanzees I
suggest a functional framework of reconciliation across species as follows: Reconciliation
is likely to repair all relationships independent of the relationship value for the conflict
partners. Since any disturbance is costly for high value partners, these partners should
40
see Figure 5.1
104
Chapter 7: How are relationships repaired ?
reconcile many conflicts. In contrast low value partners only suffer costs from strong
disturbances and therefore they should reconcile some but less conflicts than high value
partners. Such a pattern is evident for almost all studies on reconciliation in primates
(Cords & Aureli, 2000; Aureli et al., 2002), although reasons for it have rarely been
discussed. Obviously we need to know more about long-term consequences of
unreconciled conflicts on the relationship quality of conflict partners.
105
Chapter 8:
Open questions and possible answers - a general
discussion
Figure 8.0 Male Marius is resting on a liana hang-matt, while he is waiting for females to
finish off feeding on a fruit tree close by (Photo by R. Wittig).
Chapter 8: Open questions and possible answers – a general discussion
8.1 Contribution of this study to our knowledge in conflict management
Taï chimpanzees used conflict management in situations of conflicts of interest
before, during and after aggressive interactions. Conflict management in Taï chimpanzees
was dependent upon the dominance relationships among conflict partners, which were
based on contest and ranked in a linear hierarchy (chapter 441; Wittig & Boesch, 2003a,c).
However the effect of dominance relationships was strongest during pre- and peri-conflict
management (chapter 542; Wittig & Boesch, 2003b), with rank playing only a minor role in
the choice of post-conflict interactions (chapter 643; Wittig & Boesch, 2003d). During preconflict management Taï chimpanzees considered their likelihood of winning and the
benefit of resources when deciding whether to avoid or to initiate aggression (chapter 544;
Wittig & Boesch, 2003b). Furthermore the quality of the relationship among the
competitors as well as the potential costs of aggression influenced the decision-making
during the pre-conflict management. Peri-conflict management mainly varied according to
differences in the intensity of aggression, as Taï chimpanzees adjusted their conflict
intensity to their likelihood of winning (chapter 545; Wittig & Boesch, 2003b). Conflict
partners avoided hard fighting when it was not needed and they accepted more risk when
they were likely to access the resource. Interactions showing the most variability were
found during post-conflict management of Taï chimpanzees. Taï chimpanzees initiated one
out of five possible different post-conflict interactions (PCIs), according to the potential
advantages and disadvantages of each PCI (chapter 646; Wittig & Boesch, 2003d).
Reconciliation, one of the possible interactions of post-conflict management, resolved
conflicts of interest among Taï chimpanzees (chapter 747; Wittig & Boesch, in review).
Conflict partners were more likely to initiate reconciliation when the relationship was of a
high benefit to them, and they carefully invested more in reconciliation when the
relationship was more disturbed.
Although the subject of conflict resolution in non-human primates is rather new, the
topic is extensively discussed (Aureli & de Waal, 2000b, chapter 1, Appendix A). However,
we are far away from understanding all functions and mechanisms of conflict
41
42
43
44
45
46
see
see
see
see
see
see
4.3.1 and 4.3.3
Figure 5.5
Figure 6.4
5.3.1.2
5.3.2
Figure 6.4
107
Chapter 8: Open questions and possible answers – a general discussion
management among animals (e.g. Aureli et al., 2002). The contribution of the present
study to the field of conflict management is that it investigates decision-making before
and during aggressive interactions, tests for the first time the Relational Model with data
of one species and makes significant extensions to the model. It provides information on
the economics of post-conflict management by describing the choice of post-conflict
interactions in terms of an evaluation process. Besides answering the question of when
relationship repair is applied, this study shows what is needed for the reparation, and it
proves the shaping power of contest competition on dominance relationships.
Furthermore several open questions were formulated by de Waal & Aureli (2000). I
discuss below what my results can contribute to a better understanding of the remaining
open questions. I compare the results from wild chimpanzees with our knowledge of
conflict management in humans, in non-human primates and in non-primate animals. This
approach may help to provide a better understanding of the evolution of conflict
management and the forces behind its development.
8.2 Avoidance and prevention of conflicts
There is only fragmentary knowledge about the mechanisms that help individuals to
avoid or prevent conflicts (de Waal & Aureli, 2000). In Taï chimpanzees a low likelihood of
winning was usually found to prevent chimpanzees from escalating conflicts of interest
into aggression (chapter 548; Wittig & Boesch, 2003b). This result is not surprising since
dominance relationships are supposed to enhance the net-benefit of competitive
situations for group-living animals by regulating the access to resources without or by
means of low-cost fighting (Walters & Seyfarth, 1987; Drews, 1993). Many other primate
species have conflict avoidance mechanisms, which are also based on dominance
relationships (Preuschoft & van Schaik, 2000).
Investigation of human negotiation behaviour in conflicts of interest revealed the
use of five different conflict handling behaviours (Kelly et al., 1970; Ruble & Thomas,
1976): avoiding, accommodating, compromising, competing and collaborating. These
different conflict managing styles represent a two-dimensional model of conflict
behaviour, that describes each style as more or less assertive and more or less
47
48
see 7.3.2
see Figure 5.1
108
Chapter 8: Open questions and possible answers – a general discussion
cooperative. Thus, for example, the competing (also called forcing) managing style is
highly assertive but not cooperative, while the avoiding style is neither assertive nor
cooperative. Similar to other primates, dominance relationships play an important role
when human conflict partners decide about the style they use to resolve their conflict of
interest (Rahim, 1986; Kozan, 1991; Thomas, 1992). Jordanian managers mostly avoid
conflicts with partners of the same dominance level, while they are more likely to
compete forcefully with subordinate partners and collaborate with partners superior to
them (Kozan, 1991). Cross-cultural comparisons revealed that although the frequencies of
the managing styles are marginally different between different cultures (e.g. USA: Rahim,
1986; Jordan: Kozan, 1991), dominance relationships between conflict partners seem to
be the main factor determining whether to avoid confrontation or to use the forcing
management style to achieve a goal.
Dominance provides a wide range of benefits in primates and non-primate animals.
Dominant individuals can exclude subordinates from certain feeding sites or resources
(e.g. spotted hyena (Crocuta crocuta): Tilson & Hamilton III, 1984; chimpanzee (Pan
troglodytes): chapter 449; Wittig & Boesch, 2003a), dominant males are the preferred
mating partners of females (e.g. American cockroach (Periplaneta americana): Breed &
Rasmussen, 1980; anemonefish (Amphiprion akallopisos): Fricke, 1979; fallow deer
(Dama dama): McElligott & Hayden, 2000; savanna baboon (Papio cynocephalus):
Bercovitch, 1995; Weingrill et al., 2000), or only dominant individuals reproduce in a
group
(e.g.
clownfish
(Amphiprion
percula):
Buston,
2003;
naked
mole-rat
(Heterocephalus glaber): Reeve, 1992; wolf (Canis lupus): Bibikow, 1990; several species
of marmosets and tamarins (Callithrichinae): Garber, 1997). Most of the time those
benefits are achieved without aggression, which qualifies dominance relationships as a
conflict avoidance mechanism.
The dominance relationships in fish are usually triggered by body size. In groupliving clownfish (Amphiprion percula), individuals are ranked in a linear hierarchy
according to body size and breeding is exclusive to the dominant pair (Fricke, 1979;
Buston, 2003). When one group member disappears from the group, the individuals that
are subordinate relative to the disappeared member enter a growth period while the size
of the relatively dominant members remains constant (Buston, 2003). Thus, clownfish
queue in the dominance hierarchy and wait for their time to reach the dominant position
109
Chapter 8: Open questions and possible answers – a general discussion
before reproducing. This means that although a subordinate clownfish never challenges
the position of a dominant, he is guaranteed of reaching the dominant breeding position,
as long as he does not die before those dominant to him. This queuing mechanism
resolves an evolutionary conflict of interest and qualifies therefore as a mechanism which
prevents conflicts before they are created.
There are also many other strategies to avoid aggressive interactions. For example,
individuals can increase tolerance levels towards each other (e.g. primates: Judge, 2000;
humans: Fry, 2000; other mammals: Rasa, 1979). Furthermore territory defence by vocal
(e.g. birds: Krebs, 1977; frogs: Wells, 1977), olfactory (e.g. isopoda: Linsenmair, 1972;
mammals: Rasa, 1973) or behavioural (e.g. sharks: Johnson & Nelson, 1973) signals
restrict the use of a certain area to the owner (one individual or a group), without
needing to constantly fight against competitors from outside. Pre-conflict management
thus seems to be a wide spread strategy throughout all animal classes (e.g. isopoda,
insecta, amphibia, mammalia), in both intra- and inter-group situations and in both social
and solitary living animals.
8.3 Conflict regulation in a wider social context
The involvement of third parties in conflicts raises the question of how conflicts are
regulated in a wider social context (de Waal & Aureli, 2000). In Taï chimpanzees
consolation seemed to substitute for reconciliation when approaching the former
opponent was too risky (chapter 650; Wittig & Boesch, 2003d). As mentioned in chapter 6,
one reason for individuals to offer consolation could be that they are either friends of the
conflict partner or potential targets of redirected aggression. Another possibility may be
that all community members, in chimpanzees, share the responsibility to maintain the
benefits of community living (e.g. defence against neighbours and predators).
Consequence of unrepaired disturbance among community members could be cheating or
defecting behaviour, such as not indicating danger to others (e.g. Seyfarth et al., 1980) or
not aiding in defence against neighbours (e.g. Nishida et al., 1985). Accordingly each
chimpanzee would have to understand the type of conflict-induced disturbance to the
relationships between other community members and the consequences of this
49
50
see Table 4.1
see 6.4.5
110
Chapter 8: Open questions and possible answers – a general discussion
disturbance to the benefits of community living for everybody. Maintaining the benefits of
group living for each member might have been one selective pressure for the evolution of
consolation in mammal societies.
In contrast to offered consolation, solicited consolation in Taï chimpanzees did not
qualify as post-conflict management (chapter 651; Wittig & Boesch, 2003d). Chimpanzees
seem to be the only species, except for humans, that offer consolation (Watts et al.,
2000). De Waal & Aureli (1996) concluded that the reason for the difference in
consolation between macaques and chimpanzees may be the cognitive constraints of
macaques. However, since macaques use their knowledge of triadic relationships when
they redirect aggression towards kin of former opponents (Macaca fascicularis: Aureli &
van Schaik, 1991a; Macaca fuscata: Aureli et al., 1992), their cognitive ability seems to
cope with the recognition of triadic relationships. Therefore cognitive constraints in
macaques seem to be restrictive only in terms of affiliative behaviours. Emotions, such as
compassion and empathy, could be absent in the emotional repertoire of macaques while
they may be present in chimpanzees. Empathy and compassion might be conditional for
someone offering consolation (de Waal & Aureli, 1996). Whether or not the absence of
such emotions is the reason for the absence of offered consolation in monkeys and
whether or not this absence is constrained by cognitive abilities needs further research
and discussion.
8.4 Effectiveness of the mechanisms of conflict management
When terms like reconciliation or consolation were introduced into the research of
animal behaviour (de Waal & van Roosmalen, 1979), the functions of these behaviours
were unknown, even though the terms imply certain functions. The effects of some postconflict interactions are still not completely understood and one has to be careful applying
such terms to a behaviour (de Waal & Aureli, 2000). However Taï chimpanzees have
provided good evidence for the conciliatory function of reconciliation (chapter 752; Wittig
& Boesch, in review). Reconciliation in Taï chimpanzees was so effective that former
opponents were able to interact affiliatively up to eight times quicker after reconciliation
than without. Under the condition, for example, where a female needs food to be shared
51
52
see 6.3.1
see Figures 7.2 & 7.3
111
Chapter 8: Open questions and possible answers – a general discussion
to access it, reconciliation seems to have a major effect on her food budget. This result,
together with the tolerance experiments on long-tailed macaques (Macaca fascicularis:
Cords, 1992; Cords & Thurnheer, 1993), proves the effectiveness and the function of
reconciliation in primates.
Evidence for the effectiveness of management during a conflict (peri-conflict
management) was shown in the distribution of conflict intensity, since fights were
significantly less intense when the opponents had a large fighting asymmetry (chapter
553; Wittig & Boesch, 2003b). Taï chimpanzees risked more contact aggression when
conflict partners did not have a large rank difference. In other species, potential conflict
partners are also assessing asymmetries in fighting abilities to avoid unnecessary and
potentially harmful escalations of aggression (e.g. Red deer (Cervus elaphus): CluttonBrock & Albon, 1979; toads (Bufo bufo): Davies & Halliday, 1979; spiders (Agelenopsis
spec.): Maynard Smith & Riechert, 1984). Peri-conflict management, including the use of
ritualised aggression, seems to be a wide spread and effective mechanism to manage
conflict situations.
Ritualised submission may be an effective tool to terminate conflicts. Since greetings
clearly indicate the submission of the sender towards the receiver in Taï chimpanzees
(chapter 454; Wittig & Boesch, 2003a), they could point out the acceptance of the conflict
outcome by the subordinate conflict partner. Thus submissive greetings after aggression
may prepare the way for opponents to reconcile, as the subordinate might signal he has
accepted his defeat. An effective preparation of reconciliation could be the reason for the
correlation between pant-grunt vocalisations (see Table 3.1) and reconciliation events in
chimpanzee (Budongo: Arnold & Whiten, 2001; Taï: R.M. Wittig & C. Boesch, unpublished
data). Such a vocalisation, when used all the time in combination with conciliatory
behaviours, may over time become effective enough to function as reconciliation itself. A
functional change from a preparation behaviour for reconciliation to reconciliation itself
could be a possible scenario for the evolution of vocal reconciliation (Cheney et al., 1995;
Cheney & Seyfarth, 1997).
53
54
see 5.3.2.1
see 4.3.1 & Figure 4.1
112
Chapter 8: Open questions and possible answers – a general discussion
8.5 Similarity of conflict management in humans and chimpanzees
The comparison between human and chimpanzees forces itself into the discussion,
since chimpanzees are humans’ closest living relatives (Ebersberger et al., 2002;
Kaessmann & Pääbo, 2002). One of the most striking similarities between humans and
other primates has already been mentioned by (de Waal & Aureli, 2000). They argued
that one of the shared principles of conciliatory mechanisms is that they operate mainly
between cooperative partners (more details for humans: Fry, 2000; Yarn, 2000;
chimpanzees: chapters 655 & 756 Wittig & Boesch, 2003d; Wittig & Boesch, in review;
other primates: Cords & Aureli, 2000). However, there may be many more similarities.
Ritualised non-contact aggression plays an important role when chimpanzees want
to prevent conflicts from escalating into harmful fighting (chapter 557; Wittig & Boesch,
2003b). In humans, ritualised non-contact aggression is applied similarly in situations
when individuals want to avoid getting into fights or keeping a fight from becoming
serious (Fry, 2000). Humans have supposedly developed such fighting performances with
non-contact aggression in order to control their harmful aggression. Thus self-control in
humans may be seen as a mechanism to decrease costs of aggression. The same
mechanism seems to be the reasons for chimpanzees to use non-contact aggression
(chapter 558; Wittig & Boesch, 2003b).
The intervention of friendly peacemakers is one of the most important strategies in
humans. Those third party interventions can function as judges (Nader, 1991; Yarn, 2000)
or as mediators (Fry, 2000; Neu, 2000). While judges can enforce the end of the conflict,
mediators have no enforcement possibilities and their intervention can function only when
both sides accept the mediators suggestion. Although chimpanzees sometimes enforce
the end of a conflict (usually when dominant individuals intervene aggressively in an
ongoing conflict but without supporting any side), it is unknown whether or not the
intervening individual judges the conflict. In contrast, mediation in conflicts seem to
happen in chimpanzees. Chimpanzees offer consolation when direct contact among
former opponents is risky (chapter 659; Wittig & Boesch, 2003d). Later consolation may
55
56
57
58
59
see
see
see
see
see
6.3.5.1
Figure 7.3
5.3.2.1
5.4.3 & 5.4.5
6.3.5.2
113
Chapter 8: Open questions and possible answers – a general discussion
lead to peace-making, when the mediator brings former opponents in proximity so that
they can reconcile (humans: Pruitt et al., 1993; primates: de Waal, 1996b, Appendix D).
Human mediators, like chimpanzee mediators, seem to intervene in situations when
former opponents cannot meet again without further aggression. In humans, conflicts are
mediated when they would lead to serious violence (e.g. Zapotec Indians: Fry, 2000) or
they include serious violence already (e.g. in wars and political conflicts: Neu, 2000). Thus
both humans and chimpanzees may employ the mechanism of mediation and seem to
manage their conflicts in a similar way on a dyadic level (de Waal & Aureli, 2000). Testing
how comparable the motivation is of humans and chimpanzees behind these apparently
similar mechanisms, can be seen as the challenge for the future.
8.6 Prospects
Despite providing many answers, this study also offers several new questions. For
example, would high benefit partners really withhold future beneficial cooperation
because of disturbed relationships? One of the future research efforts should try to
answer the question of what happens to relationships when they are not reconciled after
a disturbing conflict. This includes not only the long-term consequences of nonreconciliation, but also of reconciliation, which Koyama (2001) has started to tackled in a
study on Japanese macaques (Macaca fuscata).
Another important question is whether or not offered consolation is really an
adequate substitute for reconciliation. Since there is the potential exchange of consolation
for reconciliation, research may need to focus on the functional similarities of both and
the relationship of the individual that offers consolation to both conflict partners.
Consolation offered by friends (or kin) of the consoled conflict partner may have another
function than consolation offered by friends (or kin) of the former opponent.
Furthermore it would be of interest to quantify the costs of aggression due to
conflict duration. When concentrating on energetic costs physiological measurements may
be needed to see whether or not there is a cost difference between short and long
conflicts. A similar approach is required for measuring stress. It seems that behavioural
stress indicators are not precise enough to reflect the complete pattern. However
114
Chapter 8: Open questions and possible answers – a general discussion
physiological measurements started to be introduced in the research of stress production
and stress reduction (e.g. Aureli et al., 1999).
One of the remaining open questions is how good comparisons are between
humans and other primates. A comparative study of conflict behaviour between humans
and apes, applying the same methods, seems to be overdue. First steps in this direction
have been done by several scientists focusing on conflict management in pre-school
children with controlled methods (Butovskaya et al., 2000). However the inclusion of
juveniles in studies on conflict management seems to be critical, since correct application
of decision-making requires experience. Therefore future research, and not only in
humans, should concentrate on adults. The present study is one of the rare case of
conflict management among adults and provides thus a strong data-set for comparisons
between adult humans and chimpanzees.
What would social living animals do without conflict management? This rhetorical
question may have now an easy answer. Social living animals would probably not live in
social units, since the costs of competition due to social living would presumably prevail,
for most of the group members, over the benefits of social living. Although conflict
management is not an invention of social living animals (see: 8.2 last paragraph), the
large diversity in social living animals would suggest a boost in evolving behaviours that
manage conflicts. Thus conflict management possibly went through a behavioural
radiation each time passing from solitary to social living. These patterns of conflict
management seemingly make social living more advantageous than disadvantageous and
therefore may allow animals to live social lives.
115
Summary
9.1 Summary
Besides many advantages, social living also holds several disadvantages. Social
partners compete for the same resources or seek contrary goals. When facing such
conflicts of interest, competitors go through a decision-making process of whether or not
to fight over a resource. However aggressive interactions, which I will refer to here as
conflicts, incur costs, which can be separated into costs of aggression (increased risk of
injury, higher energy usage) and social costs. Social costs are created by the
consequences for the social life, such as the disturbance of cooperative relationships or
stress. Conflict management should diminish the costs of conflicts by avoiding escalation
to aggression, regulating the intensity of the escalation or dealing with the social
consequences (e.g. relationship disturbance or social stress) of the conflict. Thus conflict
management can be used before (pre-conflict management), during (peri-conflict
management) and after the conflict (post-conflict management). The underlying
hypothesis for optimal conflict management is that the benefits prevail over the costs,
meanwhile the net-benefit is maximised.
I investigated the conflict management of wild chimpanzees (Pan troglodytes verus)
in the Taï National Park, Côte d’Ivoire, West Africa. Of the 1071 conflicts observed during
full-day focal animal follows of adults (4 males, 11 females), I analysed 876 dyadic
conflicts among adult chimpanzees of both sex. Multivariate analysis was carried out to
detect the variables that influence the decision-making process, while dyadic statistics
were usually conducted for mono-factorial testing.
Dominance relationships can regulate access to resources and thus help to avoid
aggression (pre-conflict management). Although linear hierarchies are commonly found
among male chimpanzees, they are believed to be absent among females. However, I
detected a formal linear dominance hierarchy among the Taï females based on greeting
behaviour directed from the subordinate to the dominant female. Females faced contest
competition over food, which increased when either the food was monopolisable or the
number of competitors increased. Dominant females usually possessed the food after the
conflict. Winning contests over food, but not age, was related to the dominance rank.
9: Summary
Affiliative relationships among the females did not help to explain the absence of
greetings in some dyads. However post hoc comparison among chimpanzee populations
made differences in food competition, predation risk and observation time apparent,
which may explain the difference in dominance relationships.
I also examined the decision-making process of whether or not to initiate
aggression and how strong to fight. An extended version of the Relational Model (de
Waal, 1996a) was developed to describe the dynamics of the decision-making process in
Taï chimpanzees, such that the net-benefit determines the occurrence of conflicts. Both
sexes fought more frequently for the resources that were most important to them: food
for females and social contexts for males. Individuals used two different strategies
according to their likelihood of winning the aggressive interaction, which was determined
by the dominance relationship of the conflict partners. Dominant initiators had longer and
more intense aggressive interactions, but they limited their social disadvantages by
fighting non-cooperative partners. Subordinate initiators had shorter and less intense
aggressive interactions, but risked more social costs, which they could reduce afterwards
by reconciliation. Both strategies included a positive overall net-benefit. The extended
Relational Model fits the complexity of wild chimpanzee conflicts and allows for more
flexibility in the decision-making process compared to the original model.
Post-conflict management in social living animals can reduce costs that remain after
aggressive interactions by means of a variety of interactions implemented after
aggression (e.g. reconciliation, consolation, redirected aggression). Each post-conflict
interaction (PCI) provides different advantages and disadvantages, although the functions
may sometimes overlap. Individuals can therefore choose a PCI to achieve the most
favourable outcome within a given conflict situation. I investigated which conflictcondition led to which type of PCI and related the choice of PCI to its advantages and
disadvantages. Taï chimpanzees used reconciliation to resolve conflicts among high value
partners and when approaching the former opponent was unlikely to entail further
aggression. Consolation seemed to substitute for reconciliation, when opponents were low
value partners or approaching the former opponent was too risky, such as when further
aggression was likely. Taï chimpanzees renewed aggression after undecided conflicts and
when losers were unexpected. They used redirected aggression after long conflicts,
possibly because friendly PCIs were likely to fail. However, Taï chimpanzees continued
with business as usual when conflicts were very short, and they avoided further
117
9: Summary
interactions when the accessibility of the resource was unlimited. Taï chimpanzees
appeared to follow a clear-cut evaluation process as they seemed to weigh advantages
against disadvantages for the appropriate choice of PCI.
However reconciliation appears to be the only PCI that is able to repair the
relationships of former opponents after being disturbed by aggressive interactions.
Despite a consensus about the benefits of reconciliation, it remains unclear how former
opponents achieve these benefits. Variation within reconciliation is evident in many
species, but understanding what causes the variation has been mostly neglected until
now. Therefore I investigated how Taï chimpanzees reconciled. This study provides
evidence for the repair function of reconciliation, since aggression disturbed tolerance
levels among former opponents and reconciliation restored tolerance to normal levels
again. Partners with highly beneficial relationships reconciled more often compared with
partners of low mutual benefit. Latency and duration of reconciliation varied in
combination, such that short reconciliations were initiated soon after the conflict, while
long reconciliations were initiated later. Latency increased with the risk of further
aggression, while duration decreased when costs were incurred from interruption of
beneficial activities. In contrast, the complexity of reconciliation varied according to the
intensity of the preceding conflict, such that reconciliation was more complex after more
intense conflicts. My results suggest that relationships between opponents are
increasingly disturbed with increasing conflict intensity and reconciliation repairs all
relationships independent of their relationship value. I propose that the function of
reconciliation is to reduce the disturbance created by aggression, but that reconciliation
occurs more frequently the more beneficial it is for former opponents.
Taï chimpanzees engaged in conflict management before, during and after the
conflict. The decision-making process of Taï chimpanzees is based on economic rules in
terms of costs and benefits. Conflict management provides Taï chimpanzees with a tool to
minimise the disadvantages of group-living.
9.2 Zusammenfassung
Das Leben in Gruppen beinhaltet neben vielen Vorteilen auch zahlreiche Nachteile.
Gruppenmitglieder konkurrieren über dieselben begrenzten Ressourcen oder verfolgen
118
9: Summary
unterschiedliche Ziele. Während eines Interessenkonfliktes durchläuft jeder Konkurrent
einen Entscheidungsprozeß, in dessen Zentrum die Frage steht, ob es sich lohnt für eine
bestimmte Ressource zu kämpfen. Dabei muß einbezogen werden, daß aggressive
Auseinandersetzungen Kosten verursachen. Diese Kosten können zum einen in
Aggressionskosten, z.B. ein erhöhtes Verletzungsrisiko oder hohen Energieverbrauch, und
zum anderen in Sozialkosten, z.B. die Störung kooperativer Beziehungen oder
Streßreaktionen,
aufgeteilt
werden.
Die
unter
dem
Begriff
Konfliktmanagement
zusammengefaßten Verhaltensweisen helfen Konfliktkosten zu verringern. So können in
Interessenkonflikten
Konsequenzen
Aggression vermieden,
verringert
werden.
deren
Stärke gedämpft
Konfliktmanagement
kann
vor
und
soziale
(pre-conflict
management), während (peri-conflict management) und nach (post-conflict management)
dem Auftreten von Aggression eingesetzt werden. Die Hypothese, die einem optimalen
Konfliktmanagement zu Grunde liegt, ist daß der Nutzen eines Konfliktes seine Kosten
übersteigen muß, wobei der Profit (Nutzen – Kosten) aus dem Konflikt maximiert wird.
Vereinfachend nenne ich aggressive Auseinandersetzungen von nun an Konflikte.
Das Konfliktmanagement von freilebenden Schimpansen (Pan troglodytes verus)
wurde im Taï National Park, Côte d’Ivoire (Westafrika), untersucht. Von 1071
beobachteten Konflikten, die ich während ganztägiger Beobachtungen an 4 männlichen
und 11 weiblichen Fokustieren gesammelt habe, wurden 876 zwischen erwachsenen
Schimpansen beider Geschlechter analysiert. Multivariate Analysemethoden wurden
angewandt, um die entscheidenden Faktoren des Entscheidungsprozesses bei Konflikten
aufzuspüren, während überwiegend Paarstatistik für einfaktorielle Analysen verwandt
wurde.
Dominanzbeziehungen können den Zugang zu Ressourcen regulieren und damit den
Ausbruch von Aggression verhindern (pre-conflict management). Frühere Studien haben
gezeigt, daß häufig lineare Rangordnungen unter männlichen, nicht aber unter weiblichen
Schimpansen bestehen. Die vorliegende Arbeit konnte hingegen eine lineare Rangordnung
auch unter den Weibchen der Taï Schimpansen nachweisen, welche auf Grußlauten der
untergeordneten Weibchen gerichtet an die Dominanten beruhte. Im Nahrungskontext
waren die Taï Weibchen untereinander direkter Konkurrenz (contest competition)
ausgesetzt.
Dieser
Wettstreit
wurde
intensiver,
sobald
eine
Nahrungsquelle
monopolisierbar war oder die Anzahl von Konkurrentinnen anstieg. Der Rang in der
Hierarchie unter den Weibchen war abhängig vom Gewinn der Wettstreite aber
119
9: Summary
unabhängig vom Alter. Warum zwischen einigen Weibchen keine Begrüßungen
beobachtet wurden, konnte nicht mit dem Fehlen sozio-positiver Beziehungen erklärt
werden. Ein Vergleich zwischen Populationen von Schimpansen zeigte Unterschiede in der
Nahrungskonkurrenz, dem Raubdruck und der Beobachtungszeit. Diese Faktoren könnten
der Grund für die unterschiedlichen Dominanzbeziehungen unter den Weibchen sein.
Anschließend untersuchte ich Variablen, die darüber entscheiden, ob und wie
intensiv Individuen kämpfen. Dazu erweiterte ich das Relational Model (de Waal, 1996a),
um die gesamte Dynamik des Entscheidungsprozesses bei Taï Schimpansen beschreiben
zu können. Das erweiterte Relational Model basiert auf der Annahme, daß der zu
erwartende Profit den Ausbruch von Aggression bestimmt. Schimpansen beider
Geschlechter kämpften häufiger um Ressourcen, die von besonderer Bedeutung für sie
waren: Nahrung für Weibchen und sozialer Rang für Männchen. Schimpansen benutzten
zwei Strategien, die auf ihre Wahrscheinlichkeit diesen Kampf zu gewinnen zurückgeführt
wurden.
Die
Wahrscheinlichkeit
einen
Kampf
zu
gewinnen
wurde
durch
die
Dominanzbeziehung der Gegner bestimmt. Dominante Angreifer initiierten längere und
intensivere Kämpfe, aber bemühten sich Sozialkosten zu begrenzen, indem sie selten
Kooperationspartner angriffen. Untergeordnete Angreifer kämpften kürzer und weniger
intensiv,
riskierten
jedoch
höhere
Sozialkosten,
die
sie
anschließend
durch
Versöhnungsmechanismen wieder zu verringern versuchten. Beide Strategien resultierten
in einem positiven Profit für den Angreifer. Mit dem erweiterten Relational Model kann die
gesamte Komplexität von Konflikten zwischen Taï Schimpansen beschrieben werden. Es
erlaubt eine größere Flexibilität im Vergleich zur ursprünglichen Version des Models.
Das Post-conflict Management sozial lebender Tiere kann dazu eingesetzt werden,
Kosten zu reduzieren, die am Ende des Konfliktes bestehen. Dazu werden eine Vielzahl
von Verhaltensweisen angewandt, so z.B. Versöhnung (reconciliation), Trost (consolation)
oder Weiterleitung von Aggression (redirected aggression). Jede dieser Interaktionen, die
erst nach dem Konflikt initiiert werden (PCI = post-conflict interaction), bietet
unterschiedliche Vor- und Nachteile, die gelegentlich überlappen. Um den bestmöglichen
Vorteil aus einer Konfliktsituation zu ziehen, können Individuen unter verschiedenen PCIs
wählen. Die vorliegende Arbeit untersuchte, welche Konfliktsituation bei Taï Schimpansen
zu welchen PCIs führten, und überprüfte, ob die Vor- und Nachteilen der ausgewählten
PCI mit den Bedürfnissen zur Kostenbegrenzung der Konfliktpartner übereinstimmte.
Ehemalige Gegner versöhnten sich nach Konflikten, wenn ihre Beziehung wertvoll für sie
120
9: Summary
war, und wenn eine Annäherung aneinander nur unwahrscheinlich zu erneuter Aggression
geführt hätte. Das Trösten durch Dritte schien manchmal die Versöhnung zu ersetzen.
Trost wurde von Dritten angeboten, wenn zwischen ehemaligen Gegnern keine wertvolle
Beziehung bestand oder eine Annäherung der Gegner vermutlich wieder zu Aggression
geführt hätte. Taï Schimpansen nahmen einen Konflikt wieder auf, wenn die vorherige
Auseinandersetzung unentschieden war oder einen unerwarteten Verlierer aufwies. Nach
lang anhaltenden Konflikten, oder wenn es wahrscheinlich ausging, daß friedliche PCIs
fehlschlagen würden, leiteten Taï Schimpansen die Aggression häufig an Unbeteiligte
weiter. Im Gegensatz dazu verhielten sich Taï Schimpansen nach kurzen Konflikten so,
weiter als wäre nichts geschehen, und verweigerten jede Art von Interaktion (keine PCI),
wenn die betreffende Ressource nicht an Ort oder Zeit gebunden war. Taï Schimpansen
schienen Vor- und Nachteile klar gegeneinander abzuwägen, um die geeignetste PCI
(Strategie) auszuwählen.
Insgesamt scheint Versöhnung die einzige PCI zu sein, mit der es möglich ist, die
aggressionsbedingte Störung einer Beziehung zu beseitigen, d.h. eine Beziehung zu
reparieren. Obwohl der Nutzen von Versöhnungen allgemein anerkannt ist, war
annähernd keine Kenntnis darüber vorhanden, wie ehemalige Gegner eine solche
Reparatur durchführen. Frühere Studien gaben Anhaltspunkte über unterschiedliche
Längen, Latenzen und Verhaltensweisen von Versöhnungen innerhalb einer Art. Die
Gründe für die Variabilität im Versöhnungsverhalten waren jedoch weitgehend unbekannt.
Aus diesem Grund untersuchte ich besonders das Versöhnungsverhalten der Taï
Schimpansen. Die Daten bestätigten, daß die Versöhnung eine Beziehung reparieren
kann. Aggression störte die Toleranz zwischen den Gegnern, Versöhnung normalisierte
diese wieder. Ehemalige Gegner mit wertvollen Beziehungen versöhnten sich häufiger als
Partner mit weniger wertvollen Beziehungen. Die Latenz und Dauer der Versöhnung
verändern sich in Abhängigkeit voneinander, da kurze Versöhnungen schnell nach einem
Konflikt erfolgten. Lange Versöhnungen hingegen dauerten auch lange, bis sie zustande
kamen. Hinzu kam, daß Taï Schimpansen eine lange Latenz wählten, wenn ein erneutes
Aufflammen der Aggression wahrscheinlich erschien, aber nur kurze Zeit in die
Versöhnung investierten, wenn die Zeit anderweitig vorteilhafter genutzt werden konnte.
Dahingegen war die Komplexität der Versöhnung abhängig von der Stärke des Konfliktes.
Je härter zuvor der Kampf geführt wurde, desto komplexer war die Versöhnung. Diese
Ergebnisse deuten darauf hin, daß die Beziehung zwischen Konfliktpartnern um so stärker
121
9: Summary
gestört wird, je heftiger der Konflikt ist, und daß alle Beziehungen (auch die weniger
wertvollen) repariert werden können.
Taï Schimpansen benutzten Konflikt Management vor, während und nach
aggressiven Auseinandersetzungen. Der Entscheidungsprozeß der Taï Schimpansen
basiert auf ökonomische Regeln, die auf einer Kosten – Nutzen Abschätzung beruhen.
Konfliktmanagement stellt den Taï Schimpansen ein Werkzeug zur Verfügung, das die
Nachteile, die ein Leben in der Gruppe mit sich führt, auf ein erträgliches Maß reduziert,
und die Vorteile des Gruppenlebens überwiegen läßt.
122
Acknowledgements
I am grateful to the ‘Ministère de la Recherche Scientifique’, the ‘Ministère de
l’Agriculture et des Ressource Animales’ of Côte d’Ivoire, the director of the Taï National
Park and the ‘Projet Autonome pour la Conservation du Parc National de Taï’ for
permission to conduct this study.
I am very grateful to my supervisor Christophe Boesch, who accepted me as a PhD
student. He helped me to develop this PhD project and to separate the wheat from the
chaff in my ideas. Furthermore I am grateful to the Swiss National Foundation and the
Max-Planck Society for their financial support.
Many thanks are due to the ‘Centre Suisse de la Recherche Scientifique’ (CSRS) in
Abidjan, especially to their directors and families, Jakob and Maria Zinsstag and Olivier
and Simone Girardin, for personal and logistical support in Côte d’Ivoire. To Odile Soublé,
who took care of me during an period of illness. To Henry, Homer, Omarou and Jürg and
all other staff-members and scientists of the CSRS for their welcoming friendliness.
I am very grateful to all staff-members of the ‘Projet Chimpanzé Taï’, especially
Kpazahi Honora Néné and Nohon Gregoire Kohon, for their company and assistance in the
forest. Furthermore to Patricia, Parfaite, Vivianne and Félicité for taking care of the camp,
and to many students for their delightful company, especially Becky, Dean, Ilka, Lionel,
Nick, Tobias and Yasmin.
Additionally many thanks to the staff-members and scientists of the ‘Projet Singes
Taï’ and at the ‘Station du Centre de Recherche en Ecologie’. All of them contributed in
one way or another to make my stay successful in Côte d’Ivoire. Here, I want to thank
especially Germain, Mandy, Klaus, Pablo und Pierre for the good logistical cooperation and
technical support.
Many thanks are due to Ousman and Henry, for the restful holidays at Farafina
plage, and to the tailor of the “KM 17” for his nice work.
10: Acknowledgements
As data are useless without proper analyses, I am deeply indebted to Gunter Weiss,
who made me familiar with multivariate analyses, and to Daniel Stahl, who advised me on
any other statistical topic.
Many thanks are due to Filippo Aureli, Josep Call, Nadja Corp, Tobias Deschner,
Frans de Waal, Diane Doran, Julia Fischer, Ilka Herbinger, Julia Lehmann, Elaine Madsen,
Toshisada Nishida, Ulrich Reichard, Martha Robbins, Bernhard Thierry and Linda Vigilant
for stimulating discussion on various parts of the thesis or different papers.
Also to Rocco Buchholz, Astrid Eckstein, Leila Kunstmann, Claudia Nebel and Silke
Streiber for administrative support in Leipzig. To the people from the MPI EVA, which are
not mentioned here, but have contributed in one way or the other to my work or simply
to an enjoyable time in Leipzig. To my parents for their moral support.
Finally, I am very grateful to my wife Cathy, who I met in Côte d’Ivoire, for the
wonderful time in the forest and for her never-ending support during our joint time in
Leipzig – especially her positive influence on my English was most helpful indeed.
124
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Appendices
Appendix A. Studies on reconciliation. Overview of within- and between-species variation
of three variables of reconciliation in primates and other mammals.
Species
Prosimii
Eulemur fulvus 1
Lemur catta 2
Plathyrrhini
Callithrix jacchus 3
Cebus apella 4
Cebus capucinus 5
Saimiri sciureus 6
Catarrhini
Cercocebus torquatus 7
Cercopithecus aethiopis 8
Colobus guereza 9
Erythrocebus patas 10
Macaca arctoides 11,12
Macaca fascicularis 13,14
Macaca fuscata 15,16
Macaca maurus 17
Macaca mulatta 12,18,19
Macaca nemestrina 20,21
Macaca nigra 22
Macaca silenus 23
Macaca sylvanus 24
Macaca tokeana 19
Papio anubis 25
Papio papio 26
Papio ursinus 27
Rhinopithecus roxellanae 28
Semnopithecus entellus 29
Theropithecus gelada 30
Trachypithecus obscura 31
Hominidae
Gorilla gorilla 32
Pan paniscus 33
Pan troglodytes 34,35,36
Homo sapiens 37
other Mammalia
Carpa hircus 38
Crocuta crocuta 39
Tursiops truncatus
Type of
BehaviourÊ
Variation of
LatencyË
Variation of
Initiator¸
?
gr
yes
yes
A<V
A=V
gr, pl
co, gr, pl
hb, mo
?
yes
?
yes
yes
?
?
A>V
?
co, em, gr
?
em, gr
gr
?
?
gr
gr
em, gr, ls
mo
co, gr, mo, pl
co, gc, gr, mo
?
co, gr, mo
co, mo
gr
gv
co, em, gr, hh
em
gr, ls
co, em, gr, mo
?
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
no/yes
yes
yes
yes
yes
?
yes
yes
yes
yes
A<V
?
A<V
A>V
A<V
A<V
A>V, A=V
A<V
A>V
A=V
A=V
A=V
?
A=V
A>V
A=V
A>V
A<V
A<V
A<V
A>V, A<V
em, to
gc
co, gc, ki
ap, em, so
yes
?
yes
?
?
A>V*
A=V, A<V
A>V, A=V
ki
gc , to
br, co
?
yes
yes
A>V
A<V
?
$
40
12: Appendices
Note Appendix A: Ê behaviour typically used in reconciliation, ap: appologies, br: body
rubbing, co: contact (e.g. sitting, laying), em: embrace, gc: genital contacts, gr:
grooming, gv: grunt vocalisation, hb: hold-bottom, hh: hold-hand, ki: kiss or mouthmouth contact, ls: lip-smack, mo: mounting, pl: play, so: share or offer object, to: touch.
Ë variable delay of reconciliation observed within the same study (yes / no).
¸ rough proportion of reconciliation initiated by aggressor (A) or victim of aggression (V),
A<V: A initiates less than 40% of reconciliation, A=V: A initiates between 40% and 60%
of reconciliation, A>V: A initiates more than 60% of reconciliation.
*contact aggression; $genital contact is greeting.
1.
(Kappeler, 1993)
2.
(Rolland & Roeder, 2000)
3.
(Westlund et al., 2000)
4.
(Verbeek & de Waal, 1997)
5.
(Leca et al., 2002)
6.
(Pereira et al., 2000)
7.
(Gust & Gordon, 1993)
8.
(Cheney & Seyfarth, 1989)
9.
(Björnsdotter et al., 2000)
10.
(York & Rowell, 1988)
11.
(Perez-Ruiz & Mondragon-Ceballos, 1994)
12.
(de Waal & Ren, 1988)
13.
(Aureli et al., 1989)
14.
(Aureli, 1992)
15.
(Aureli et al., 1993)
16.
(Kutsukake & Castles, 2001)
17.
(Matsumura, 1996)
18.
(de Waal & Yoshihara, 1983)
19.
(Demaria & Thierry, 2001)
20.
(Judge, 1991)
21.
(Castles et al., 1996)
22.
(Petit & Thierry, 1994a)
23.
(Abegg et al., 1996)
24.
(Aureli et al., 1994)
25.
(Castles & Whiten, 1998a)
26.
(Petit & Thierry, 1994b)
27.
(Silk et al., 1996)
28.
(Ren et al., 1991)
29.
(Sommer et al., 2002)
30.
(Swedell, 1997)
31.
(Arnold & Barton, 2001)
32.
(Watts, 1995a)
33.
(de Waal, 1987)
34.
(de Waal & van Roosmalen, 1979)
35.
(Arnold & Whiten, 2001)
36.
(Wittig & Boesch, 2003d)
37.
(Butovskaya et al., 2000)
38.
(Schino, 1998)
39.
(Wahaj et al., 2001)
40.
(Samuels & Flaherty, 2000)
150
12: Appendices
Appendix B. Observation file of November 1, 1997. The focal animal was female Castor.
The first column (TIME) contains the starting time of the behaviour in seconds after the
observation start. The second column (ACT) displays the name of the actor/initiator of the
behaviour. The middle column (BEH) shows the behaviour. The forth column (REA)
contains the name of the reactor/receiver of the behaviour, while the last column (MOD)
shows sometimes modifications of the behaviour. On the far right side, explanations of
the behaviours are given sometimes, referring to codes in the table by matching colours.
TIME
1
36
205
285
314
357
425
454
586
925
1048
1085
1086
1179
1324
1457
1530
1848
1878
1888
1922
1928
1935
1940
1957
1966
2054
2064
2072
2180
2188
2502
2528
2553
2774
2825
2924
2940
3335
3362
3455
3511
3581
3589
3659
ACT
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
MYS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
MAC
NAR
?
CAS
NIN
CAS
MAC
FEM
CAS
PER
CAS
CAS
CAS
CAS
CAS
CAS
CAS
END
CAS
CAS
CAS
CAS
CAS
CAS
CAS
BEH
DEP
DEP
MAN
DEP
MAN
REP
DEP
REP
MAN
MAN
DEP
HAV
APP
REP
DEP
MAN
DEP
REP
DEP
DEP
VOC
VOC
VOC
DEP
VOC
DEP
VOC
VOC
MAN
VOC
MAN
DEP
DEP
DEP
DEP
REP
DEP
HAV
DEP
DEP
REP
DEP
REP
DEP
REP
REA
ARB
SOL
L33
ARB
L33
MOD
NNN
NNN
ARB
T33
T33
SOL
N11
MYS
SOL
F33
SOL
SOL
ARB
ALL
ALL
ALL
ARB
ALL
ARB
ALL
ALL
F33
ALL
F33
ARB
SOL
SOL
ARB
ARB
?
ARB
SOL
SOL
SOL
151
K02
K03
K03
FIX
K05
DSE
PHO
PHO
TAM
DSE
PHO
NNN
TAM
NNN
?
12: Appendices
3706
4061
4082
4234
4410
4445
4453
4479
4508
4688
4752
4875
4971
6740
6750
6782
6895
6962
7013
7102
7299
7330
7461
7727
7818
7830
7835
7839
7890
7895
7911
7913
7955
7960
7968
7970
8550
8658
8695
8809
8810
8822
8823
8824
8915
9100
9274
9318
9322
9454
9502
9503
9609
10331
10387
10510
10548
10748
10797
10929
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
END
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
VEN
CAS
CAS
CAS
VEN
CAS
VEN
CAS
VEN
CAS
VEN
CAS
CAS
CAS
CAS
CAS
MAC
CAS
MAC
CAS
CAS
CAS
CAS
END
CAS
CAS
FOS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
DEP
REP
DEP
REP
DEP
REP
DEP
REP
DEP
REP
DEP
DEP
HAV
DEP
WAI
DEP
REP
DEP
MAN
DEP
REP
DEP
MAN
MAN
SUP
SCR
TBA
MAN
SUP
CHA
FLI
MAN
APP
SBR
LEA
MAN
DEP
REP
DEP
LEA
ATT
CRO
LEA
REP
DEP
REP
GRO
GRO
REP
DEP
HAV
APP
DEP
DEP
REP
DEP
DEP
REP
DEP
REP
SOL
SOL
SOL
SOL
SOL
SOL
ARB
?
ARB
CAC
SOL
SOL
T33
SOL
ARB
F33
F33
CAS
VEN
VEN
F33
CAS
VEN
CAS
F33
CAS
VEN
CAS
F33
ARB
SOL
MAC
CAS
MAC
CAS
?
K05
K10
K14
FSI
FSI
K14
FSI
RSC
RSC
K14
FIX
SCR
?
K14
OES
SCR
Conflict
Macho - Castor
DIR
?
Castor solicits
consolation of
Cacao and
receives it
SOL
CAC
CAC
SOL
I11
FOS
SOL
ARB
ARB
SOL
SOL
152
Castor, being in
oestrous, leaves
Macho
K03
FIX
12: Appendices
10941
10944
10947
10948
11058
11080
11265
11266
11268
11269
11445
11485
11521
12483
15082
15136
15137
15138
15175
15244
15249
15295
15326
15482
15582
15764
15814
15944
15956
16124
16148
16268
16283
16337
16348
16396
16675
16689
16735
16857
16917
16919
17011
17029
17043
17097
17106
17145
17176
17251
17455
17557
17586
17691
17971
18037
18159
20613
20736
20910
CAS
CAS
CAS
CAS
CAS
CAS
MAC
CAS
MAC
CAS
CAS
CAS
CAS
END
CAS
NIN
NIN
CAS
CAS
NIN
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
NIN
CAS
CAS
CAS
NIN
NIN
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
END
CAS
CAS
CAS
APP
KIS
LEA
DEP
REP
DEP
APP
PGR
LEA
REP
DEP
REP
MAN
HAV
DEP
APP
LEA
DEP
REP
WAI
DEP
MAN
DEP
MAN
DEP
REP
DEP
REP
DEP
REP
DEP
DEP
REP
DEP
MAN
DEP
REP
MAN
DEP
REP
APP
DEP
REP
CPR
GRO
GRO
MAN
REP
DEP
REP
MAN
DEP
DEP
MAN
MAN
MAN
HAV
DEP
REP
GRO
MAC
MAC
MAC
SOL
SOL
CAS
MAC
CAS
PGR
FRE
?
?
?
ARB
F33
?
ARB
CAS
CAS
SOL
CAS
SOL
L11
SOL
F33
SOL
K06
?
?
?
K02
NNN
SOL
SOL
SOL
ARB
ARB
T33
SOL
K04
L33
SOL
NNN
CAS
SOL
ROC
NIN
CAS
END
L33
OES
DIR
?
NNN
SOL
I11
SOL
ARB
F33
F33
F33
?
ARB
K02
CAC
DIR
153
K04
K07
K08
?
Castor has a genital
swelling and
Nino grooms her
12: Appendices
21131
21135
21169
21250
21464
21482
21652
21665
21827
21871
22099
22130
22319
22358
22359
22360
22363
22366
22368
22503
22844
22960
23038
23107
23211
23260
23261
23262
23272
23273
23287
23368
23369
23375
23382
23413
23418
23419
23422
23430
23431
23432
23446
23580
23588
23666
23721
23750
23766
24278
24300
24618
24653
24672
24720
24809
24901
26126
27207
27258
END
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAC
CAS
CAS
CAS
FOS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
MAC
CAS
MAC
MAC
CAS
CAS
MAC
CAS
MAC
CAS
MAC
CAS
MAC
CAS
CAS
CAS
MAC
CAS
MAC
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
END
CAS
CAS
GRO
DEP
REP
DEP
WAI
DEP
REP
DEP
REP
DEP
MAN
DEP
MAN
SCR
SPP
CHR
ATT
FLI
REP
CON
REP
DEP
REP
DEP
REP
APP
PGR
LEA
HAV
APP
REP
HAV
MEN
SCO
REP
BSW
CRO
ROC
CRO
APP
PRS
COP
REP
HAV
MEN
REP
JEU
REP
DEP
REP
DEP
REP
DEP
DEP
REP
DEP
MAN
HAV
DEP
DEP
CAC
SOL
?
SOL
CAC
SOL
SOL
SOL
L33
SOL
N33
FED
CAC
FOS
FOS
CAS
NNN
K08
LOS
SCR
CAC
SOL
SOL
CAS
MAC
CAS
N11
MAC
?
?
K03
HRE
N11
MAC
CAS
OES
CAS
MAC
CAS
MAC
MAC
MAC
CAS
SCR
?
SCR
?
OES
K02
N11
MAC
K03
K03
CAC
SOL
SOL
SOL
ARB
ARB
F33
?
ARB
SOL
154
K02
?
Conflict context:
sex
Conflict between
Macho and Castor
Reconciliation
between Macho
and Castor
12: Appendices
27378
27389
27482
27612
29209
29252
29273
29422
29447
29522
29570
29648
29755
29956
30068
31031
31068
31112
31226
31349
31424
31567
31577
31598
31651
31848
31906
32444
32498
32596
32611
32768
32843
33317
33985
34138
34204
34505
34565
34566
34570
34572
34573
34574
34655
34656
34659
34665
34666
34724
34725
34729
34730
34999
35139
37173
37255
37280
37387
37388
CAS
CAS
CAS
END
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
END
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
NIN
CAS
SIR
CAS
SIR
SIR
CAS
CAS
CAS
SIR
CAS
CAS
CAS
CAS
CAS
CAS
LOU
CAS
DEP
REP
DEP
HAV
DEP
REP
DEP
REP
DEP
REP
DEP
REP
DEP
DEP
MAN
DEP
DEP
REP
DEP
REP
DEP
REP
DEP
DEP
REP
DEP
HAV
DEP
DEP
REP
DEP
REP
DEP
MAN
DEP
MAN
DEP
REP
APP
VOC
KIS
TOU
LEA
DEP
APP
FIM
TGE
LEA
DEP
APP
KIS
VOC
DEP
DEP
MAN
REP
DEP
DEP
HAV
APP
ARB
ARB
?
ARB
?
Focal animal
lost:
no observation
SOL
SOL
SOL
SOL
ARB
L33
ARB
SOL
K02
SOL
SOL
SOL
SOL
ARB
?
ARB
SOL
?
SOL
SOL
T33
SOL
I11
SOL
MAC
MAC
MAC
NIN
CAS
SOL
CAS
SIR
CAS
CAS
SOL
SIR
SIR
CAS
SOL
ARB
F33
ARB
SOL
F33
LOU
155
K03
NNN
?
GRU
FRE
?
?
PAN
?
?
?
FRE
PAN
K10
K12
FIX
Castor was eating fruits
(non-monopolisable) at
the same time with10
competitors
12: Appendices
37490
37549
37592
37604
37674
37697
37769
37902
37909
37984
38049
38070
38128
38133
38140
38671
38843
39001
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
CAS
MAC
CAS
?
CAS
{end}
REP
DEP
JEU
DEP
REP
DEP
REP
JEU
DEP
REP
DEP
MAN
DEP
VOC
REP
NID
NID
SOL
CAC
SOL
SOL
CAC
SOL
SOL
L33
ARB
CAS
NNN
PAN
End of observation
after 39001 s
156
12: Appendices
Appendix C. Dyadic association index (DAI) of males and adult females in the North
community of Taï chimpanzees. DAIs for the period before the death of Brutus (10.1996
– 2.1997) are shown above the diagonal in italic numbers, while DAIs without Brutus are
shown under the diagonal. Individuals are separated by sex (males in italic letters) and in
alphabetical order.
DAI Brutus Macho Marius
Brutus
0.35
0.42
Macho
0.48
Marius
0.68
Nino
0.45
0.41
Belle
0.34
0.29
Castor
0.24
0.20
Dilly
0.18
0.18
Fossey
0.19
0.18
Goma
0.18
0.17
Loukoum
0.32
0.27
Mystère
0.24
0.23
Narcisse
0.20
0.18
Perla
0.24
0.22
Ricci
0.27
0.28
Venus
0.24
0.26
Nino
0.25
0.35
0.31
0.33
0.25
0.21
0.23
0.22
0.30
0.28
0.24
0.24
0.41
0.28
Belle Castor
0.14 0.13
0.13 0.15
0.11 0.13
0.15 0.18
0.23
0.24
0.23 0.31
0.21 0.46
0.22 0.32
0.25 0.25
0.26 0.39
0.25 0.29
0.26 0.31
0.29 0.24
0.23 0.28
Dilly
0.17
0.13
0.15
0.15
0.21
0.30
0.27
0.48
0.25
0.35
0.27
0.38
0.21
0.30
Fossey Goma Loukoum Mystère Narcisse Perla
0.11 0.14
0.17
0.19
0.15
0.19
0.11 0.14
0.32
0.19
0.15
0.15
0.12 0.13
0.16
0.19
0.14
0.16
0.15 0.18
0.25
0.19
0.16
0.17
0.22 0.19
0.11
0.26
0.19
0.24
0.52 0.34
0.16
0.41
0.17
0.29
0.31 0.47
0.11
0.30
0.16
0.29
0.35
0.11
0.36
0.15
0.25
0.35
0.15
0.36
0.21
0.29
0.23
0.25
0.16
0.13
0.14
0.30
0.37
0.26
0.19
0.31
0.25
0.31
0.24
0.31
0.21
0.27
0.36
0.29
0.36
0.30
0.25
0.21
0.25
0.26
0.28
0.23
0.27
0.31
0.38
0.30
0.29
0.35
157
Ricci
0.25
0.24
0.27
0.38
0.22
0.18
0.18
0.19
0.17
0.15
0.23
0.21
0.21
0.25
Venus
0.18
0.17
0.25
0.29
0.15
0.16
0.22
0.17
0.28
0.23
0.17
0.15
0.16
0.19
12: Appendices
Appendix D. Peace making in Taï chimpanzees. I did this observations and wrote it down
in terms of a story by reason of the retirement of my old professor Prof. Dr. Dr. Hubert
Hendrichs from University of Bielefeld, Germany.
„Frieden stiften“ bei Schimpansen
Im November 1996 befanden sich drei erwachsene Männer in der von mir
beobachteten Schimpansengesellschaft: Macho, der a-Mann, der alte Brutus, an der bPosition, und der junge Herausforderer Marius. Marius hatte schon die letzten Wochen
versucht Machos Regeln immer wieder zu durchbrechen. Brutus, hatte sich in dieser Zeit
meistens von den beiden fern gehalten. Jetzt waren sie wieder alle zusammen. Die
Spannung zwischen Macho und Marius explodierte und führte zu einer minutenlangen
Auseinandersetzung, die Macho letztendlich gewann. Marius war die daraus resultierende
Anspannung anzumerken. Er schlich immer wieder um Macho herum und versuchte
zaghaft Kontakt zu ihm aufzunehmen, aber Macho blieb resistent. Er groomte Brutus und
wies Marius Annäherungsversuche zurück. Brutus wurde sichtlich nervöser. So ging es
über eine halbe Stunde.
Schließlich stand Brutus auf, mittlerweile wurde er nicht mehr von Macho gegroomt
sondern lag neben ihm, und begann mit Macho zu spielen. Macho faßte Brutus Knöchel
und Brutus lief los. Sie bildeten eine Formation mit Brutus an der Spitze und Macho
folgend, immer noch mit beiden Händen die Knöchel von Brutus umfassen. Sie spielten
„Eisenbahn“. Brutus führte Macho immer wieder um einen einzelnen Baum herum –
minutenlang. Plötzlich veränderte Brutus die Richtung und bezog einen zweiten Baum in
den Parcours mit ein, nämlich den Baum, neben dem Marius saß und den beiden
zuschaute. Brutus schritt nun eine Acht um die beiden Bäume herum, immer noch mit
Macho im Schlepptau. In der dritten Runde griff Brutus nach Marius Knöcheln, als sie bei
ihm vorbei kamen. Nun hatte sich eine „Eisenbahn“ aus drei Schimpansenmännern
gebildet, die mit „Lachen“ dem achter Parcours um die beiden Bäume folgte. Angeführt
von Marius, der von Brutus an den Knöcheln gehalten wurde, der wiederum von Macho
an den Knöcheln gehalten wurde, liefen sie Minute für Minute im Kreis.
Doch auf einmal zog sich Brutus aus dem Spiel heraus. „Automatisch“ nahm Macho
die Knöchel von Marius und die beiden setzten das Spiel alleine fort. Brutus saß am Rand
des Parcours und sah den beiden zu, wie sie ihre „Versöhnung“ begingen, die von ihm
initiiert worden war. Es sollte die einzige Situation bleiben, in der Brutus mir gezeigt hat,
wie Schimpansen Frieden stiften. Er starb im März 1997, vermutlich an Altersschwäche.
158
Curriculum vitae
Roman Martin Wittig
Born 23/07/1968 in Karlsruhe / Germany
Scientific education
1989 – 1996
Studies in biology at the University of Bielefeld
1996
Diploma degree in biology
Topic of Diploma thesis: “Changes of behavioural patterns
during the integration of strangers in a group of captive
chimpanzees”. Supervision by Prof. Dr. Dr. Hubert Hendrichs.
1996 – 1999
Observations of wild chimpanzees in the Taï National Park,
Côte d’Ivoire. Scientific member and project manager (for 1.5
years) of the “Projet chimpanzé Taï”. Project leader: Prof. Dr.
Christophe Boesch.
since 1999
PhD thesis at the Max-Planck Instuitute for Evolutionary
Anthropology in Leipzig, under the supervision of Prof. Dr.
Christophe Boesch.
Leipzig, September 25, 2003
Roman Wittig
Declaration of independence
Herewith I declare that I have conceived and written this dissertation without any
inadmissible help and/or material that has not been explicitly indicated. All sources
of information that were used have been indicated. This disseratation has not
been submitted elsewhere, neither inside nor outside this country. I have not
previously attempted to complete this or any other PhD thesis.
Leipzig, September 25, 2003
Roman Wittig
160