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9
1
SOCIAL BEHAVIOR IN SOME
MARINE AND TERRESTRIAL
CARNIVORES
BURNEY J. LE BOEUF
Crown College
University of California, Santa Cruz
1
2
3
4
5
1
Introduction
Morphological Comparisons
Prey, Food Habits, and Feeding Behavior
Social Behavior and Reproduction
Summary
251
255
257
263
274
INTRODUCTION
Once upon a time, around 37 million years ago, when primitive fissiped
carnivores were evolving from their miascid predecessors, some of them
forsook their way of life on land and went to sea to find food. This move
must have worked out well, because many of the descendants of these
1
pioneers-the seals, sea lions, and walruses-are still making a living in
the sea today. Of course, descendants of the carnivores that remained on
land fared well, too. Because land and sea are such different environ­
ments, we might expect the two animal groups to have diverged in
many ways since their separation. Indeed, it should be interesting to
compare representatives of the two groups because of what they have in
common, an ancestor, and because of the obvious differences in the
environments in which they live. In this chapter, I point out some differ251
252
Social Behavior In Some Carnivores
ences and similarities between seals and their terrestrial conterparts with
respect to gross morphology, feeding and predatory behavior, reproduc­
tive behavior, and various aspects of social life. As is necessary when
comparing two large groups of animals, I document points with exam­
ples from
a
few selected species. My selection is biased toward those
species that have been studied most systematically, those that are of
greatest theoretical interest, and those studies with which I am familiar.
The similarities between seals and land carnivores were obvious to
laymen and early scientists (Fig. 9.1). Fifteenth and 16th century sailors
described the seals they saw as marine carnivores and called them "sea
lions," "sea bears," or "Iobos de mar" (sea wolves). Linnaeus and sev­
eral later taxonomists classified seals, sea lions, and walruses in the
order Carnivora. However, as early as 1811, there was an attempt to
place these animals in a separate order (Illiger, 1811). At present, many
investigators class these marine mammals in their own separate order,
the Pinnipedia (e.g., King, 1964). However, classification of these ani­
mals is far from settled. Some maintain that the new order is inapprop­
riate and that pinnipeds should be considered merely a suborder of the
Carnivora (McKenna, 1969). Others argue that the pinnipeds can be
classified into two living families (walruses are placed in the same family
as sea lions) under the superfamily, Canoidea, in the order Carnivora
(Mitchell and Tedford, 1973).
Before I make specific comparisons, it may be useful to present a
sketch of pinniped evolutionary history. Their history, although poorly
documented, may provide clues to differences we might expect between
them and terrestrial carnivores.
The earliest pinnipeds probably entered the sea in one of the following
areas: the Arctic Basin, the northwest coast of Nor.th America, or the
Tethyan-Mediterranean area (Matthew, 1939; Davies, 1958a; McLaren,
1960; Repenning, 1970). It is not clear whether seals and sea lions (includ. ing the walrus) were already differentiated at this time. The oldest pin­
niped remains; ar,e from the Miocene era, a time when they were already
well adapted for aquatic life and when seals and sea lions were already
distinguishable (Downs, 1956; Mitchell and Tedford, 1973). One opinion
holds that all pinnipeds derived from canoid or dog-bear stock (e.g.,
Matthew, 1939; Simpson, 1945; Davis, 1958a; Sarich, 1969). An opposing
view is that sea lions and the walrus derived from ursine stock, and seals
derived from lutrine stock (e.g., Mivart, 1885; McLaren, 1960; King, 1964;
Repenning, 1970). This question is far from settled. Nevertheless, there
is agreement that all pinnipeds are more closely related phylogenetically
to members of the superfamily Canoidea of the order Carnivora--:-the
,...
Gi
UI
a:
::;)
CJ
ii:
253
254
Social Behavior In Some Carnivores
dogs, raccoons, bears, weasels, and the Iike--than to the feloid carni­
vores.
After they entered the water, the distribution and diversity of pin. nipeds were influenced by geomorphic and climatic barriers, distance
between land falls, ocean currents, and water temperature (Davies,
1958a; King, 1964; Hendey, 1972). Davies (1958b) hypothesizes that pin­
nipeds are, and always have been, tied to a cold-water environment. He
argues very persuasively that the distribution and differentiation of
present-day northern pinnipeds reflects periods of expansion and con­
traction of sea and glacial ice that occurred during the Pleistocene era.
Scheffer (1958, p. 37) points out that "as local shore lines and islands
rose and fell and glacial barriers came and went, pinnipeds moved back
and forth in order to maintain favorable breeding grounds along the
edge of the sea." On land, many contemporary carnivores were being
forced into extinction by periodic fires, floods, droughts, and dust
storms.
Scheffer suggests that the early pinnipeds moved out along the shores
of continents, from island to island and later to polar ice fields. The rate
of evolution was faster at the frontiers of the advancing lines where
immigration was unidirectional. He speculates further:
The primitive populations were small, composed of family groups, not very
sociable. The members did not wander far, had no well-developed homing
instinct, were not migratory, and did not need to rendezvous in special places in
order to find mates. Polygyny had not developed in any of the pinniped stocks.
The animals were perhaps smaller than most recent pinnipeds (Cope's rule), had
less fat, lived in more temperate waters, and perhaps made crude dens in beach
grasses and among boulders. The advancing pinnipeds met, from sea birds and
cetaceans, little competition for food. They met no competition for breeding
room, nor do they often today. Pinnipeds early lost the habit of feeding on beach
organisms such as crabs, mussels, periwinkles, and blennies. [Scheffer, 1958, p.
36-371
As each generic stock became isolated, it was transformed in response
to the local physical and biotic environment and adapted to its peculiar
niche. Among species frequenting isolated bays, gulfs, islands, inland
seas, or lakes, there has been, and still is, rapid evolution.
In the last few centuries, the effect of commercial sealing upon island
and continental pinniped populations has been catastrophic. Millions of
seals have been slaughtered and entire breeding po pula tions wiped out.
It is doubtful that some species will ever recover genetically from under­
going such a severe population "bottleneck" (Le Boeuf, 1977). In one
recently exploited species, absolutely no polymorphic variation was
Morphological Comparisons
found in
255
21 blood proteins from 159 se<lls representing five rookeries
1974)! In this regard, some land carnivore popu­
(Bonnell and Selander,
lations have suffered a similar f<lte at the hands of man-for example,
the wolf. The increasing range of human populations is changing the
distribution of both land carnivores and pinnipeds.
Although pinnipeds are marine, all species have retained an attach­
ment to land. In this respect, they differ from the Cetacea, descendants
of a primitive ungulate ancestor, who have become completely aquatic.
All pinnipeds 'give birth on land or ice, and some species spend the
majority of their time resting out of the water.
Pinnipeds have exploited the marine niche for food. A few carnivores,
such as the sea otter,'
Enhydra /utris, and the polar bear, Thalarctos
maritimus, also obtain food from the sea, but they have not undergone
the same degree of morphological transformation to aquatic living as the
pinnipeds. The anatomical and behavioral adaptations of the pinnipeds
to life in the sea clearly distinguishes them from the land carnivores.
Since these adaptations influence their behavior on land, it is worth
reviewing some of the major ones.
2
MORPHOLOGICAL COMPARI�ONS
Many morphological differences between pinnipeds and land carnivores
are simply a reflection of the different environments in which they live.
The average body size of pinnipeds is greater than that of ca!nivores.
The smallest pinniped, the ringed seal,
Pusa hispida, weighs 90 kg. The
Mirounga leonina, weighs
about 3,629 kg and is 650 cm long (Scheffer, 1958). Several seal species are
larger than the largest land carnivore, the grizzly bear, Ursus arctos,
which weighs 771 kg (Erdbrink, 1953). Many land carnivores, such a� the
foxes of Africa (Bekoff, 1975), weigh less than 4 kg. The Fennec fox,
Fennecus zerda, weighs less than 1 kg! The greater body size of pinnipeds
largest pinniped, the southern elephant seal,
evolved mainly in response to the marine environment. Large body size
is b-etter for heat retention in the told sea. Moreover, since the water
medium gives more support than air, large size could mote easily evolve
in the sea than on land.
The pinniped body is adapted for swimming and diving. Body shape
is streamlined to reduce drag. The external ears are reduced or absent,
external genitalia and mammary teats are drawn into the body, limbs are
enclosed within the body and extremities are flattened-the tail is short
and the head is flattened, and the eyes are situated well forward. The
neck, in particular, is thick and muscular and considerably more flexible
256
Social Behavior In Some Carnivores
than in most large carnivores. The skin is adapted to a water environ­
ment (Montagna and Harrison,
1957), and the hair is flattened. Pin­
nipeds never groom the pelage with the mouth or tongue (Scheffer,
1958). The terrestrial carnivore body is a picture of contrast. The canid
body, especially the large one, is designed for long-distance running
(see Romer,
1966; Mech, 1970; Fox, 1975) .
Subcutaneous fat forms a substantial part of all pinniped bodies. Skin
and blubber make up more than 25% of the weight of the Weddell seal,
LeptonycllOtes weddell; (Bruce, 1951) and almost 50% of the body weight of
the southern elephant seal (Laws, 1953). Blubber provides reserve
energy during fasts and lactation, thermal insulation, buoyancy, and the
padding necessary for a streamlined profile. The proportion of body fat
to total weight in land carnivores is apparently much smaller, even in
dormant or hibernating bears, skunks, and raccoons.
Although the pinniped brain has not been studied in great detail, it is
evidently large, like that of terrestrial carnivores, but more spherical and
more highly convoluted (King,
1964). The sea lion brain resembles that
of bears, while the seal brain looks more like that of cats and dogs.
Compared to a dog's brain, the cerebellum is large, probably because of
the increased coordination demanded in swimming. The auditory nerve
is large and the olfactory lobes are reduced. Pinniped eyes (the walrus
excepted) are larger than those of land carnivores, and they function
well at low levels of illumination. Seal vision is excellent both in water
and on land (Schusterman,
1972).
Pinnipeds have fewer and more uniform teeth than most land carni­
vores (except for the walrus, a special case once again). All species have
the pronounced upper and lower canines but lack the carnassial cusps
characteristic of land carnivores. The postcanines in most species are
rudimentary pegs functioning to hold prey that is swallowed whole.
Variation in pinniped teeth reflects the type of flesh the animal feeds on,
and, as in many mammals, it is a useful taxonomic indicator. Generally,
the teeth, mouth, jaw, and associated structures of pinnipeds are de­
signed for grasping, tearing, and swallowing prey whole or in chunks,
rather than for chewing, shearing, or crushing large bones. In contrast
with land carnivores, the deciduous teeth of pinnipeds disappear before
or soon after birth.
Pinnipeds have made numerous physiological adjustments to aquatic
life that set them apart from land carnivores. These adaptations in respi­
ration, circulation, renal physiology, and the like are important and
interesting, but it would be too much of a digression to cover these
topics here. Ridgeway
(1972) provides an excellent review of marine
mammal adaptation to the aquatic environment.
257
Prey, Food Habits, and Feeding Behavior
3
PREY, FOOD HABITS, AND FEEDING BEHAVIOR
Pinnipeds deviated from land carnivores in form and function because
of adaptations that took place as a result of seeking food in the sea. It is
therefore most important to compare these two animal groups from the
point of view of hunting and feeding habits, and in relation to their
respective prey.
Both pinnipeds and land carnivores are meat eaters, but only the
marine mammals eat flesh exclusively. Seals and sea lions feed mainly on
crustacea, molluscs, fish, penguins, and other sea birds. Although diet
varies with the species, the majority of pinnipeds feed on a variety of
'
prey. The Alaska fur seal, Callorizilllls ursillus, is the best studied pin­
niped from the point of view of food habits. Its catholic diet is represen­
tative of the wide range of fishes and squid eaten by the Otariids (the
earred s,eals or sea lions and fur seals, as opposed to the Phocids, the
earless, true seals). Alaska fur seals feed on more than
30 kinds of
27 species of fish, 1 species of octopus, and 5
species of squid (Fiscus, Niggol, and Wilke, 1961). The most common
marine organisms: at least
q
prey of fur seals are northern anchovy, Ellgraulis lIIordax, s uid, and
Pacific herring (C/lipea izamlgus). Various rockfish, Pacific hake,
cius Prodllctlls, Pacific saury,
Merluc­
Cololabis saira, salmon, OncorizYllclluS spp. ,
and American shad, Alosa sapidissimia, are eaten in lesser quantities.
Hermit crabs, amphipods, and various diving sea birds (e. g. , Rhinocer­
ous Auklet, Ceror/lillen lIIollocerata, Red-throated Loon, Cavia stellata, and
Beal Petre\' Oceal/odrollla leucorizoa beali) are eaten occasionally, but they
seem 'to play only a minor role in the seals' diet (Niggol, Fiscus and
Wilke,
1959) .
Fur seals, like most other pinnipeds, are opportunistic feeders. What
is eaten depends on the seasonal distribution and the abundance of
prey. Since fur seals commonly feed on sc � ooling fishes, stomach con­
tents of several animals collected in the same area often contain only one
type of, fish. The Steller sea lion, Eumetopias jubatq (Spaulding, 1964), a�d
the California sea lion,
Zalophus cali/ornianus (Fiscus and Baines, 1966),
eat similar fishes and squid, especially where the feeding range of the
'
three species overlaps. Steller males in the Bering Sea and off Alaskan
:
islands supplement their diet with Alaska fur seal pups (Roger Gentry,
personal communication).
Most true seals, such as the harbor seal, Phoca
seal,
vitulina, and the grey
Halichoerus grypus, are mainly fish eaters, but they occ�sionally feed
on an assortment of crustacea, octopus, eels, and molh,lscs (Spaulding,
1964; Kenyon, 1965; Anderson et al . , 1974) . The northern elephant seal,
Mirouna angustirostris, is a deep diving seal that feeds primarily on squid,
258
Social Behavior In Some Carnivores
skates, rays, small sharks, and ratfish, Hydrolages
co lli er (Huey, 1930;
1970). The small ringed seal, Pusa Hispida, which
inhabits the circumpolar Arctic coasts, feeds on up to 72 different species
Morejohn and Baltz,
of small pelagic amphipods, euphausians, and other crustacea, as well
as on small fish (King,
Only two of the
(King,
1964).
32 different pinniped species in King's classification
1964) are rather specialized feeders that exploit one type of prey
Lobodon carcinophagus, feeds
species almost exclusively. The Crabeater,
on krill, small shrimp-like animals that it catches in quantity and strains
from the water through its multicusped cheek teeth, much as a mysticete
whale sieves krill through its baleen. The walrus,
Odobenus rosmarus,
Mya, Saxicava, and
Cardium, for which it forages in shallow coastal waters less than 40
feeds primarily on three genera of bivalve molluscs,
fathoms deep. Bivalves on the sea bottom are examined and sorted by
the lips and whiskers; feet and fleshy parts are torn off and swallowed
whole or sucked out. But when molluscs are scarce, the walrus may eat
young ringed seals, bearded seals,
Erignathus barbatus, and even young
walruses. It is the only pinniped that apparently feeds on Cetacea. Occa­
sionally a narwhal,
Monodon monoceros, or a beluga, Delphinapterus leucas
is eaten. However, it is not known whether walruses acturally kill them
or feed on corpses (King,
1964).
The diversity of foods eaten by canids and ursids is even greater than
that of pinnipeds. According to Mech
(1970), "probably every kind of
backboned animal that lives in the range of the wolf has been eaten by
the wolf." Wolves,
Canis lupus, eat mice, mink, muskrats, squirrels,
rabbits, various birds, fish, lizards, snakes, grasshoppers, earthworms,
and berries. But predation on small animals plays only a minor role in
the wolf's diet. The main prey of the wolf, like those of the African wild
dog,
Lymoll pictlls, are large animals. The wolf's primary prey in the
United States and Canada are: white-tailed deer, mule deer, moose,
caribou, elk, Dall sheep, bighorn sheep, and beaver. Wild dogs kill
mostly Thomson's gazelles, juvenile wildebeests and Grant's gazelles
(Estes and Goddard,
1967), and, occasionally, warthogs and zebras (van
1971). African foxes feed
Lawick-Goodall and van Lawick-Goodall,
mostly on rodents and small reptiles, birds's eggs, insects and vegetable
matter; jackals eat similar foods in addition to small mammals and carr­
ion (Bekoff,
in Fox
1975). Additional information on other canids can be found
(1975).
Of all the land carnivores, bears have the most diverse tastes. The
grizzly, before man virtually annihilated it, was an omnivorous oppor­
tunist
par excellence . The California grizzly (Storer and Tevis, 1955) ate
almost anything and everything that was available. This long list in-
259
Prey, Food Habits, and Feedi ng Behavior
cluded: meat, fresh or putrid from whales, water birds, fish, elk, deer,
antelope, gophers, lizards, frogs, and domestic livestock; a wider variety
of plant materials than herbivorous animals ate, some of which were
various berries, clovers, nuts, wheat, corn, potatoes, tree bark, and vari­
ous bulbs; and assorted foods like honey, ants and their larvae, yellow­
jacket nests, and mushrooms. Polar bears, Tlwlarctos maritimus, have a
much mQre restricted diet. Although they consume some vegetation and
carrion during the summer (Russell,
1975), they feed primarily on the
1975).
ringed seal during most of the year (Stirling and McEwan,
The remarkable thing about the feeding habits of the large canids is
that the prey is often larger than the predator. In this respect, the large
canids differ from all of the pinnipeds. Wild dogs, whose average weight
does not exceed
18 kg, bring down impala and reedbucks that are double
1967) and zebras that are even
larger. Wolves weighing 36 to 45 kg kill moose and bison that may weigh
over 500 kg. These canids overcome large prey by hllnting in packs that
or triple their size (Estes and Goddard,
range from a few animals up to a score or more of. them. One dog usually
selects a quarry from a retreating herd of g"zelles, and the other dogs
follow it. The quarry is coursed for
1 to 3 km until it becomes exhausted.
The lead dog catches up to it and grabs it or bowls it over. Once overta­
ken, the prey is fallen on by all the following dogs, and it is dismem­
bered and eaten with great dispatch. Wolves bring down deer or moose
in a similar way (Mech,
1970). The first wolf to catch up to the fleeing
prey attempts to grab hold of the rump, flanks, neck, or nose. Once the
prey is down, others attack from every side. Although the technique
may vary with prey species, the strategy of the individual quarry, and
the composition of the hunting pack, it is clear that these pack hunters
get help from each other in getting their food. Lone wolves or even small
packs are at a disadvantage in bringing down large prey. In black­
backed jackals, Canis mesome/as,. Wyman (1967) observed that two hun­
ters were more than four times as successful as one in bringing down
gazelle fawns.
,
Like land carnivores, the size of prey eaten by pinnipeds varies
greatly, but, except for the dubious possibility that walruses kill small
whales, pinnipeds always kill and eat animals that are much smaller
than themselves. Prey size determines how Alaska fur seals, Steller sea
lions, and harbor seals eat their food. Small fishes less than
30 cm long,
such as anchovy, herring, saury, or squid, are consumed whole under
water (Niggol, Fiscus, and Wilke,
1959; Spaulding, 1964). Except for very
small food items like lanternfish, the p�ey is swallowed head first.
Larger prey measuring more than
30 cm long, such as hake, rockfish, or
salmon, is brought to the surface, grasped by the head, shaken vio-
260
Social Behavior In Some Carnivores
lently, and reduced to chunks that are swallowed piecemeal. As fur seals
get older and larger, large fish up to
proportion of their diet (Spaulding,
92 cm make up an increasingly high
1964).
Some pinnipeds feed in groups, but, unlike the case with canids,
grouping does not enable them to exploit larger animals. Rather, group­
ing seems to make it easier for them to exploit large aggregations of
small prey. Fiscus and Baines
(1966) saw Steller sea lions leaving their
hauling grounds in the vicinity of Unimak Pass, Alaska, in compact
groups of several hundred to several thousand animals. They swam out
to feeding areas, where they dispersed into smaller groups of less than
50
animals containing both sexes and mixed sizes of animals. Massings of
this type fed on large schools of fish or squid. Casual observations
suggest that groups of sea lions feed more effiCiently on schools of fish
because cooperation among the hunters enables them to herd and con­
trol the movements of the school. When large fish schools are absent,
sea lions feed singly or in small groups of two to five animals. Similar
behavior has been observed in the California sea lion (Fiscus and Baines,
1966; Michael Bonnell, personal observation).
Before Steller sea lion females leave the rookery and their pups to go
to sea and feed, they may engage in activities that resemble, and that
may have an analogous function to, the prehunt greeting ceremony of
wild dogs (Estes and Goddard,
Lawick-Goodall,
1967; van Lawick-Goodall and van
1971). Several females may gather at water's edge,
where they mill about restlessly in close contact with each other. They
may vocalize repeatedly and engage in what appears to be low-intensity
aggressive behavior before swimming off as a group to feed. In both
species, these activities may reinforce group cohesion and unity as well
as synchronize the time of departure.
Similarly, canids and pinnipeds may travel several kilometers to get a
meal. Van Lawick-Goodall and van Lawick-Goodall
(1971) report that
wild dogs did not start their first chase until they were 8 km from where
they started. The chase itself covered a distance of
pursue moose or caribou for up to
(Mech,
5
51/2
km. Wolves may
to 8 km but usually give up sooner
1970). They may range up to 32 km from their den (Kelsall, 1957).
Steller sea lions and California sea lions travel much further from their
rookeries or hauling grounds to feed. The former have been seen feed­
ing as far as
112 to 136 km from land (Kenyon and Rice, 1961) and the
62 km from land (Fiscus and Baines, 1966; Le Boeuf,
unpublished data). Usually, only small groups of less than 30 individu­
latter as far as
als are seen this far from land. Large groups of sea lions
(100 or more) are
seldom seen feeding more than 16 to 24 km from a hauling ground or
rookery. It would be interesting to compare land carnivores and pin-
Prey, Food Habits, and Feedi ng Behavior
261
nipeds on the s u ccess of thei r respecti v e h u n ti ng a n d fi shing expedi­
tion s , but the di fficu l ti es i nherent in observ i n g sea l s feed i n g i n the wa ter
makes this impos sible at presen t . H owever, some indirect evi d ence is
p erti n e n t . Virtu a l l y all fur seals collected early in the day by the U. S .
Fish a nd Wild l i fe S ervice had food i n their stomachs (Niggol , Fiscu s , a n d
Wi lke, 1959).
Al though both p i n n i peds a n d the la rger land carnivores a re noma d i c ,
the former travel grea ter d i s ta n ces i n t h e i r a n nual feed i n g migra tions .
Th e record holder is the Ala ska fur sea l , whi ch migra tes from i ts breed ­
ing gro u n d s i n the Beri n g Sea to i ts wi n ter feeding grou nds along the
shores' of the ea s tern a n d western Paci fiC a s far sou th as San Diego,
California, and H okkai do, Ja pan , a dista nce of some 5000 km . · Wolves
may follow caribou on their a n nual migra tions (Kelsall, 1968).
Wi ld dogs a n d d h ol e s , Ceron a/pinus, prefer to h u n t in early morning
or early afternoon or ev e n i n g , a l though hu nts occur on moonlight nights
a s well (Estes a n d God d ar d , 1967; v,a n La w i c k - G o oda l l a n d v a n
Lawick-G ooda ll, 1971; Dav i dar, 1975). Wolves a nd bears may h u n t by
day or nigh t . Most of th e smal f er foxes are nocturna l feeders (Bekoff,
1975). Fur seals a n d sea l i o n s are pri mari l y n ig h t a nd early morning
feeders. S tomach con te n ts of animals col lected a t vari ous hours a fter
su nri se show decrea sing a m o u n ts of food. However, in some a rea s
wh ere large schools of fi sh are pres e n t , daytime feedi ng also occurs .
Typically, the midday hours are s p e n t resti ng, and feeding a c ti v i ties
gradually resume in late afte�noon (Niggol , Fiscu s , and Wilke, 1959;
Spaulding, 1964). Th e harbor seal is a daytime feeder ( S p a u l ding, 1964).
Ca nids ea t grea t a m ou n ts of food in a single feeding bou t . A wolf may
gorge 9 kg of mea t i n a single meal; th i s may represent 20% of its body
weigh t . H owever, 'e stima tes of a li ttle more tha n half tha t amou n t per
day per wolf may be more representa tive (Mech , 1970). Estes a nd Go.d­
dard (1967) estima te that wild d ogs average 2.72 kg of mea t per day, or
approxima tely 6.7% of their body weigh t . But these ca nids may have to
'
go wi thou t eating for several days a t a time . Fi ve to 7 day fa s ts have been
recorded i n wolves, and one wolf went for 17 days with o u t food (Mech ,
1970). It is common knowledge that domestic dogs ca n go for several
days w i thou t ea ting, pa rticu l a rl y when s ex u a l activi ty is probable .
G ri zzlies a n d black bears in the more northerly parts of N<;>rth America
may go dorma nt for varyi ng periods of time during the win ter when
food is scarce. I n ge neral, all of ,the land carnivores will feed regularly if
possibl e . Even bears will a ctively feed through ou t the year when food is
available.
Most pinnipeds feed daily except a t certa i n times of the year, u s u ally
the breedi ng season, when they u ndergo long fasts . Th u s, feas ts and
262
Social Behavior In Some Carnivores
fa mine are even m ore extreme in their annual cycle. Wh en they are
feedi ng, fu r sea l s and sea lions i ngest less food relative to body weight
than the ca nids menti oned above. Spaulding (1964) estima tes tha t fur
sea l , sea lion, a nd ha rbor seal food requi rements ra nge from 2 to 11 % of
body weight per day, with 6% being th e average. During th e breeding
season, fu r seal a nd sea lion males fa s t for approxima tely 6 weeks; har­
bor sea l s do not a p pear to fa st duri ng th i s time. The record for long fa sts
i n pinni peds goes to male north ern elepha n t seals. They may go wi th out
food for
3 mon ths (Le Boeu f, a nd Peterson 1969). Throughout this time
they are actively fig h ti ng and a ttem p ti ng to copula te, and they may lose
over 450 kg. Females of this species fas t for an a verage of 34 days during
the peri od in whi ch they give bi rth a nd nurse th eir pups daily for
28
days (Le Boeu f, W h i ting, a n d G a ntt, 1972). A female's total body weight
may be redu ced by almost 50% during this fast. During the nonbreeding
season, males and females fa st aga i n for approxima tely 30 days while
un dergoing the a n nual mol t.
Most ca nids a n d ursids scavenge for food at some time. Perhaps bears
exploi t carrion more tha n any other l a n d ca rnivore. Carri on was a major
part of the Ca l i fornia grizzly's diet more than 100 years ago, before man
killed th e grizzly and i ts prey. It is said tha t these bears kept th e Califor­
nia coa s t clea n of pinniped and whale carcasses tha t repea tedly washed
ash ore. There are n u m erou s reports of 12 to 15 bears feedi ng a t the same
time on a s ingle whale ca rca ss (S torer a n d Tev i s , 1955). Many of the
larger land carnivores typically desert the rema i n s of a kill and ca che i t
a w a y a n d return to ea t from i t la ter. Polar bears a r e a n excep tion to this
rule. They do not cache ringed seal carcasses th a t they kill, despite the
fa ct tha t they may only feed on the blubber a n d leave the meat behind
( S ti rl i n g and McEwa n, 1975). In marked contra s t to these feeding habits
of la nd carnivores , pinnipeds do not cache food, nor d o they s cavenge,
although the walru s may be an excep tion to this rule. Pinnipeds con­
s u m e most of their prey en tire and discard only th e heads of large fish,
particularly rockfish.
One drawback that pinnipeds experience i n their marine habitat to
which the larger land carnivores are rela tively i mmune is predation. In
carving their niche in the sea, pinnipeds beca m e exposed to large pre­
da tors who were also adapted to aquati c l i fe a n d more formidable than
themselves. A t sea, the main predators on pinnipeds are large sharks,
particu larly the great white shark,
whale,
Carcharodon carcharias, and the killer
Orcinus orca. All p i n niped speci e s living i n the Pacific are preyed
on by these animal s . The degree of preda tion i s u nknown. When pin­
nipeds return to land or ice to give birth and nurse their young, they are
vulnerable to preda tion by large land carnivores such a s polar bears and
263
Socia l Behavior and Reproduction
grizzly bea rs, perhaps wolves, a n d, of course, man. S m a l l carnivores,
such a s foxes, and skuas, gulls, and ha wks, may prey on their you ng.
Being a d a p ted prima rily for locomotion i n wa ter, they are no ma tch
defendi ng themselves against la rge land preda tors. Thu s, it woul d ap­
pear that preda tor pressure wa s importa n t i n ca using pinnipeds to breed
in remote sanctu a ries free from these preda tors; tha t is, offshore islands
or rocks, sa ndbars a n d ice floes, a n d offshore breeding a reas are limi ted .
This is parti cularly tru e of the food-rich wa ters of the temperate north
Padfic, the north A tla n ti c, and the circu mpolar A n ta rcti c .
4
SOCIAL BEHAVIOR AND REPRODUCTION
Given this set of circum s ta nces, it is li kely that pin nipeds were forced to
share limited breeding areas, a n d this led to the high degree of sodality
a n d the l a rge soda I gatherings that we fi n d i n pinnipeds toda y . In terms
of the sheer number of a nimals tha t congrega te together, the pinnipeds
are much more social th an any land ca rnivore. As many a s
2 million
Alaska fur seals ca n be fou nd every summer on two tiny Pribilof Islands
in the Beri ng Sea . Three thou s a n d northern elepha n t seals may be found
packed together tigh tly on one beach on Isla d e Guadalupe, Mexico,
during the peak of the breeding s e a s o n (Fig .
9. 2). Two thousand
California sea lions may be found sleeping together in close contact
1/2
km inland on S a n Miguel Isla nd, California, during the nonbreeding
season ( Fig.
9. 3). The largest wolf or wi ld dog packs are miniscule in
comparison.
Pi nnipeds are a mong the most p olygynous mammal s . Apparently,
this mode of life developed v ery early i n their his tory, and i t was closely
tied to their amphibious habi ts (Bartholom ew, 1970). When females
bega n clustering i n time and spa ce, the condi tions became ideal for
promoting male- male competi ti on, polygyny, and sex ual dimorphism
(see Trivers,
1972). Males must have competed to mate with a s many
females a s possible. Those who kept other males away from areas con­
'
taining females sired more p up s . In their s hort breedi ng seasons on
tra ditional rookeries, fur seals and sea lions devel oped a s ocial s tructure
9.4 and 9.5). Males
chara cterized by territoriality among males (Fig .
withou t territories d o not copula te, or they copulate only rarely; those
who secure the most well-placed territories-territories containing the
most estrou s females--do most of the breeding (Bartholomew,
G entry,
1953;
1970). Another syste m that developed was for males to domi­
na te other males a n d thus gain access to more estrous f emales wherever
they were situate d . This was the s tra tegy adopteq by northern elephant
!
FIGURE 9.2 Approximately 2000 northern elephant seal males. females. and newborn pups gathered on Pilot Rock Beach. Isla de Guadalupe.
Mexico on 5 February 1973. a week after the peak of the breeding season. An observer is located in the extreme foreground.
8i
-; ', .
'I"
.
. ,:",�,
"i.�.,
..
,,' _
..
,,-;JV
'
.
,:
�.
.'
�
J
'�'&'= ...
�.
'I
•
� ...,
.. . .
� ,-.r.. ..�
:<l�- �:��?j.:.::. :. :}���:
..
.�
�. : :
:
.",',>..:••
-' .� > .:::
"
...... .,.
""-....: . ;1",
�.;
.'
..
��":<:., ..." .
A large aggregation of California sea lions about 1/2 km inland from the water's edge on San Miguel Island, California, during the
..0;
.
.
""""" ,
startled and rose up on their foreflippers.
nonbreeding season. Normally the sea lions would be sleeping in close contact with each other, but shortly before the picture was taken they were
FIGURE 9.3
•
< �.�:>r�/":·�, ,
,t,� ,'1 ", �,)"; . .,:�:. ':.;�1,>�'
:: 70.'2; ; - ,
- '�':
.,;!_.
I
.-<
,
.
.
..
Both California sea lion males and Alaska fur seal males are territorial during the breeding season, and on San Miguel
r'
r'";'
...
-.�=?:- :.:�. ��:--
�':"'�;;fYmr.� �,. ;..-
are sleeping subadult male elephant seals.
fluctuations in several variables. The sea lions are all other animals except those circled and those indicated by arrows; the latter
foreground) are well defined spatially, and males actively herd females. California sea lions' territories are plastic and change with
Island they breed at the same time, and there is often interspecific competition over territories. The fur seal territories (circled in the
RGURE 9.4
...
�
\-4
-:--...-
.-.-.......:.:....-....
- �-�
�
-
Social Behavior and Reproduction
FIGURE 9.5
267
Two Steller sea lion males engaged in a mutual threat at the boundary between
their respective territories.
seal males. They fight for social status in a dominance hierarchy. The
highest ranking male or males--depending on the number of females
present-locate themselves nearest the females and keep all others
away. One or a few of the highest-ranking males do most of the breed­
ing in a harem; most males in a colony do not breed at all. One male may
dominate a harem for several years and inseminate more than 200
females (Le Boeuf,
1974).
Grey seal society is similar to that of elephant
seals.
Thus, in most of the well-studied pinnipeds, males are polygynous
and are either territorial or exhibit a dominance hierarchy during the
breeding season. This is a reflection of the manner in which the largest
and strongest males go about monopolizing a large number of females.
Since size and physical aggression convey such a great reproductive
advantage to males, these traits have become increasingly elaborated in
time. Sexual dimorphism is greater in some seals and sea lions than in
other mammals. For example, the Alaska fur seal male is five times
larger than the female (Fig.
9.6).
Social Behavior In Some Carnivores
268
FIGURE 9.6
Northern fur seal harem on Kitovi Rookery, Pribilof Islands, Alaska, during the
breeding season. The male in the foreground has eight females in his territory; the male
immediately behind him has none. Note the extreme sexual dimorphism. Photographed by R.
S. Peterson in June, 1963.
Canids are "social" i n quite a differen t way from pinniped s . Soci a l
orga niza tion is chara cterized by seasonal or permanent p a i r b o n d s i n
foxes ( S torm,
1965; MacPherson, 1969) a n d jackals ( e. g. , v a n Lawick­
1971; Eisenberg and Lockhard, 1972)
G oodall and van La wick- G oodall,
a n d by coopera tive h u n ting a n d group l i fe in the wolf, d hole, a n d wild
dog (Kleiman and Eisenberg,
1973; Mech , 1975). Black bears are essen­
tially solitary, but female-cub a ssoci a tions are long lasti ng, a n d strong
and female cubs i n h erit their mother's feedi n g territories (Rogers,
1974).
Wolf packs are orga nized i n to separate male a n d female social hierar­
chies . Al though the female h ierarch y is more a mbiguous, it is clearly
m ore pronounced than the size-related dom i nance observed i n female
elephant seals. Hu n ti ng dog pa cks have similar h ierarchies, particularly
269
Social Behavior and Reproduction
during the breeding sea s o n , b u t it is the fem a l e h ierarch y tha t is most
evident (van Lawick - G ooda l l a n d v a n Lawick- G ooda l l , 1971). I n wolves
and wild dogs , the domina n t breeding fem a l e may prev ent s ubordin a te
females from breeding or may drive them ou t of th e pack a n d kill their
p u p s . Domina nt fe m a l es may also enlist the help of males , and they may
prevent other a d u l ts fro m feeding the s u bordina te fem a l e or her you n g .
A l though m a n y ca nids a n d ursids a re territoria l i n t h e sense tha t they
defend a feeding or h ome ra nge against other individ u a l s or gro u p s of
�
individ u a l s , this behavior is q u ite different from the defense of rigid
individ u a l territories by fu r seal males a n d S teller sea lion males agains t
conspecific males d u ring t h e breeding s ea s o n .
Wh en i t comes t o repro d u ctiv e behav ior a n d parenta l c a r e , pinnipeds
a nd terrestria l carnivores differ in m a n y ways . Most ca nids and u rsids ,
like pinnipeds, h a v e o n e es tru s per year ( A s d e l l , 1964; Kleim a n , 1968).
H owever, male-female sex u a l in tera cti ons in the land ca rniv ores ca n be
prolonged a nd complex . Ca nids exhibit extensive preco p u l a tory court­
ship, frequent olfactory exa mina tion of the partners, a n d frequ en t u rine
a nd fa ecal ma rking (Kleima n , 1968) . In some domes tic ca nids, females
activ ely solicit m a l e s ( Le Boeu £, 1967) . Ca nid copula tion is characterized
by a ge nita l lock or tie . A fter i n tromission is ach ieved, the pair rem a ins
j oined together by the genita l ia a nd fa ce in opposite directions for sev­
eral m i n u tes or a s long a s a n hour. Females may be in es trus for several
days a n d may cop u l a te severa l times with d ifferent males . Usually, only
th e alpha female in a wolf pack breeds (Kleima n a n d Eisenberg , 1973;
Rabb, Wool py, a n d G insburg, 1967; Zimen, 1975, 1976) .
I n contrast, th ere is l ittle or no courts h ip in pin niped sexual beh a v ior
( Le Boe u f, 1972) . Male north ern elepha n t sea l s pounce on fema les a n d
a tte m p t t o force cop u l a tion a t any t i m e d u ring t h e breeding sea s o n ,
regardless of t h e fema le's reprodu c tive condition ( Cox a n d Le Boeu£,
1977) . Sea lions a n d fur sea l s may inves tiga te the fema l e ' s genita lia be­
fore mou nting, b u t there is little parallel to the direct solicita tion of males
by females seen in the domestic dog ( B each and Le Boeuf, 1967; Beach ,
1976) . Male seals do not mark with urine or feces , a n d they do not exhibit
a copula tory lock . Cop u l a tion d u ra tion is equally long in both a nimal
grou ps, l a s ting
3 to 7 min in eleph a n t sea ls a n d 15 to 20 min i n S teller sea
lions . Some ph ocid females a re in estrus for several days; most otariid
females (in which the males are territoria l) cop u l a te only once d u ring the
estrou s period ( Le Boeu£, 1972) . Cop u l a tion is
m o re
ca n em
excep t in
elep h a n t sea l s , wh ere the male mou n ts from th e rear a n d to one side,
perhaps a necessity because of the large bulk of th e animal s . Elep h a n t
s e a l males, like male wolves , prevent o r in terr u p t each oth er's cop u la ­
tions; female sea l s , u nlike w o l f bitch es, do not prevent other females
270
Social Behavior In Some Carnivores
from ma ting, but they threa ten, attack, and on occa sion ki l l p u p s of
neigh bori ng females (Le Boe u f a n d Briggs,
Canids have
a
1977) .
short gesta tion period o f about 2 months a n d a l itter
size tha t ra nges from 2 to
15, with 5 being the mode (Asdell, 1964;
1968) . The gesta ti on peri od of bears is 6 to 7% month s , a n d the
li tter size is usually 2 to 4 ( A sdell, 1964) . Wild canid p u p s are born in
dens or bu rrows. Wea ning from moth er's milk is at abou t 40 days in the
Kleima n ,
domestic dog. B u t d u ring the tra nsition to solid food in most canids, the
a l tricial young are provisioned by regurgi ta tion feedi n g of partially mas­
ticated food . Thi s is done by the pare n t male as well a s by the mother in
the smaller pair-bonding ca nids. I n wolves a n d wild dogs, a l l adults in
the pack, male a n d fema le, regurgitate to young pups a s well a s to the
nursing mother not i nvolved in the h u n t. It is not clear how long this
type of provisioni ng of the young goes o n , b u t apparen tly it lasts u n til
the you ng are able to obta i n food on their own (see Mech ,
reviews in Fox,
1970; several
1975) .
Females of all pi nniped species give birth to a single pup o n ce a year.
They do not eat the placenta as the carnivores d o . With i n a few days or
weeks a fter parturi tion, copula tion takes pla ce. G esta tion, i n cluding de­
layed implanta ti o n , lasts
1 year. Thu s , females give birth a t about the
same time each year and u sually in the same place (except for sea l s tha t
breed on i ce). Most species give birth i n the open, whether it is on sand,
rock or i ce; the ri nged seal is a n exce p ti o n and gives birth i n subnivean
lairs in the pressure ri dges of sea ice ( S tirling and McEwan ,
1975). New­
borns of all species a re p h y sically well developed a t birth. Pups are
nursed for a minimum of
3 weeks (southern elephant seal, Law s , 1956)
1970; walrus, Brookes,
to more tha n a year (Steller sea lion, Gentry ,
1954). The northern eleph a n t seal female is a mammalia n phenomenon
i n that she doesn't feed during the e n tire 4 week n ursing period! Wea n ­
i n g is abrupt i n s pecies l i k e the elepha n t s e a l a n d gradual i n others like
the Pacifi c sea lions. Apparently, mothers feed their pups excl usively by
nursing; they have not been observ ed carrying food to the young or
regurgita ting to them. Females of several species tha t breed colonially
wil l n o t nurse pups other tha n their own. Northern elephan t seal
females bite a n d som etimes kill orp h a ned pups that attempt to suckle
them. Adoption of orphaned pups by females who have lost their own
pups may occur in eleph a n t seals, grey s ea l s , a n d , rarely , i n S teller sea
lions. U n like the ca nids, male pinnipeds that breed in colonies do not
help in rearing or feeding the young. S o far as I know, no male seal
helps the femal e in any way . Quite the contrary, males are i n a dvertently
responsible for a sizeable portion of pup mortality i n elepha n t sea l s a n d
Social Behavior and Reproduction
271
grey seals (Le Boeuf, Wh i ti ng , a n d G a ntt, 1972; Le Boeuf a n d Briggs,
1977; Coulson a nd H ickling, 1964).
Ev idently, there are fu ndamenta l di fferences i n the social and rep ro­
ductive behavior o f these two rela ted group s of animals . Two basic
differences having to do with the origin of sociality and the rate of
reprodu cti ve behavior sta n d ou t .
Social behavi or s e e m s to h a v e q u i te a d i fferen t origin i n th ese two
j
a nimal gro u p s; t h a t is, they m u s t have been exposed to qui te different
sel ection p ressu res . In the carnivores, there d eveloped a prem i u m on
grou ping together, coopera ti ng in the h u n t, and feeding other pack
members, because these behav iors increa sed the efficiency of pred a ti o n
(Wilson, 1975). Being s ocia l enha nced su rviva l through the process o f
kin selection (Maynard S m i t h , 1964; Hamilton, 1964). On the other hand,
pinniped sociality seems to have developed fi rst because of a paucity o f
breeding s i tes free of preda tors; la ter, t h i s trend becam e elaborated
through the opera tion of sexual selection . It is unique to pin nipeds that
feed ing a n d breeding have a lways been spatially separa ted and have
occurred in di ffere n t habitats (Bartholomew, 1970). Unlike the typical
terrestrial carnivore with a n extensive home ra nge i n w h ich mating also
ta kes place, p i n n i peds opted to tra vel grea t dista nces to breed on tradi­
tional sa nctuaries . Since such sanctuaries were limited, females were
forced to c l u m p together; this situation predisposed males to maximize
their reproductive success by a ttem p ti n g to inseminate as many females
as possible ( see Trivers, 1972). S u ccessful males were belligerent and big .
Unbridled male-male competition led q u ickly to extreme polygyny, sex­
ual dimorp h i s m , lack o f parental beha vior on the part of the male, sex
differences in beha vior, and conti n ued gathering of a large number of
animals for bree d i n g . Once this system was started, the inertia of sexual
selection kept it moving in the sam «;; di rection . For example, females that
move from the main aggregation o f fema les are apt to be fertilized by a
male who is u n succes s ful in competing with other males-that is, a
" marginal" male . I f he is subordinate because of genetic factors, his
progeny will be less succes sful in the next genera tion . Thus, any ten­
dency o f females to become less gregarious would be selected against
and would be self- limiting (McLaren, 1967). G regariousness in females
should be positively reinforced a s long a s competition among males-­
whether for territories or for socia l status-results in the exclusion of
some males (Bartholomew, 1970).
The second fu nda mental di fference between pinnipeds a n d terres trial
carnivores has to do with reproductive turnover. I n general, natural
selection has fa vored two contrasting responses to the problems of
272
Socia l Behavior In Some Carnivores
popu l a tion re p l a cement a n d the col onization of new h a bita ts . According
to M a cArth ur and W i l son (1 967) , in e n v i ro n m e n ts where there i s li ttle
crow d i n g a n d food is abu n d a n t, r - se l ection will fa vor a n imals tha t u ti l ize
the most food and rea r the l a rger fa m i l ies-th a t is, those with a high
popu l a ti o n g rowth ra te . O n the other h a n d , when there is crowding a n d
i n tense com peti tion for food , K - selection w i l l fa vor a ni m a l s t h a t can
rep l a ce themselves w i th the lowest possible i n ta ke of food . The fi rst is a
stra tegy of p rod uctivity, a n d the seco n d is one of e ffici ency of reproduc­
tion .
In genera l , p i n n i peds a p pear to have been K - selected and terrestrial
ca rni vores r - selected-Pi a n ka ' s (1970) a d m o n i t i o n that all orga nisms
represent a com p rom i se on t h i s conti n u u m is certa i n l y a pplicable here .
One of t h e most i n teresti n g correl a tes of r - a nd K - selection is the pair of
com p l exes that M a rtin (1 975) ca l l s , respectively, the a l tricial a n d preco­
cial com p l ex . Pi n n i peds con form to m a n y of the a ttribu tes of the p reco­
cia l complex . O n l y one p u p is born per year (one every 2 years in the
walrus, a ccord i n g to Brooks, 1 954) a n d the y o u n g exhibit adva nced
physical d evelo p m e n t at birth . A l a s ka fu r sea l s ca n locomote and voc­
alize within a m o m e n t o f birth , and they a re a s adva nced a t birth as
d om e s tic dogs a b o u t
3 w e e k s o l d ( B a rt h o l o m e w , 1 959) . Northern
elepha nt seal pups open their eyes a n d may exhibit the characteristic
sa nd - fli pping movements of the fore fl i p pers before com pletely expelled
from the birth ca n a l . Harbor seal pups a re able to swim a n d dive at birth
(Bishop, 1967) . Lacta tion i s of long dura tion, lasting more than a year in
some sea lions a n d the walru s . Even i n sea l s having a relatively short
n u rsing period, energy i n vestment in the offspring may be enormou s .
Elepha n t seal m i l k is a p p roxi m a tely
55% fa t, a n d a l though p u p s are
wea ned a t 4 weeks of age, they a re left with a tremendous fa t s tore (Fig.
9 . 7) . These pups weigh more a t wea n i n g than they do a year later! Other
a ttribu tes of the precoci a l com p l ex are la te sexu a l maturi ty and a long
l i fe . Most pinniped females o v u l a te for the first ti m e a t about 2 to 5 years
of age; m a les reach sexu a l maturi ty at about
3 to 6 years of age (e . g . ,
harbor seals, Bigg, 1969; Ala ska fur sea l s , B artholomew a n d Hoe!, 1953;
anonymous, 1 975; northern eleph a n t seals, Le Boeu£, u npublished data ) .
I n the colonial species, males d o not complete s u ccessfully for females
u n til much later i n l i fe, u n ti l approximately 10+ years of age in the
northern eleph a n t seal (Le Boeuf, 1974) . Pinnipeds may live a long time .
Harbor sea ls live for 20 to 29 years (B iggs, 1969) . Alaska fur seals for up
to 26 years (Niggo!, Fiscu s, and Wilke, 1959) , and the life span o f the
northern eleph a n t seal is typical l y abou t 1 4 years (Le Boeuf a n d Briggs,
unpublished data ) . Like that in pri mates a nd other long- lived mammals,
Social Behavior and Reproduction
FIGURE 9.7
273
A suckling northern elep hant seal pup appears almost as large as its mother
shortly before it is weaned. At this time, it is fou r or five times larger than it was at birth, and
heavier than it will be a year later. Its sole nurturance has come from mother's milk. The mother
has provided this enormous fat store to her offspring during a four-week period in which she
did not feed.
the trend i n pinni ped repro d u ction h a s been toward q u a l i ty ra ther than
quantity.
Terres tri a l carni vores, especially the s m a l l e r ca n i d s , exhibit the a l tri­
cial complex . Most can ids give birth in a shelter to a la rge li tter of p u p s
t h a t a re poorly developed a n d i nca pable of coordinated behavior except
suckli n g . G esta tion i s short a n d lactation brief co mpared to most p i n ­
niped s . S e x u a l ma tu rity occu rs a t a n early a g e , a t t h e end o f the second
year in wild wolves (Mech , 1970) , and a t the end of the fI rst year i s some
captive wolves ( Medj o and Mech , 1976) a n d i n the A rctic fox (Chese­
more, 1975) . L i fe spans a re s h ort compared to those o f pinnipeds . Al­
though wolves have lived 10 to 16 years i n ca ptivity (You n g , 1944), 10 is
considered very old (Mech, 1970) . Seven is considered old age i n l arge
dogs (Fuller and D u B u i s , 1962) . The l i fe spa n of faxes is considerably
less, 6 years of age i n the gray fox, Urocyon littoralis (Laughlin, 1973).
Apparently, ca n i d s and ursids h a v e been selected for q u a n ti ty i n repro­
duction . The paradoxically low fecu n d i ty rate i n wolf and wild dog
packs, wh ere only one female may give birth , may be relate d to low
availabi l i ty of food . It i s i n teresting to note that i f only one female gives
birth i n a pack, i t is usually the most domina n t one. One might expect
'
the fecun d i ty rate to go up when food was abunda n t . Whether this
274
Social Behavior In Some Carnivores
happens
or
not, the point is tha t the potential for a h i g h reprod uctive
ra te i s there .
U n l i ke l a n d ca rn i vores, pi n n i peds m ll s t d e a l with problem s t h a t arise
from maki ng a living to two quite di fferent habitats. The high metabolic
rate a nd th i ck layer of b l u bber that a re so adapti ve in cold water crea te
d i ffi c u l ties in thermo reg u l a ti o n on l a n d , pa rticu l a rly when the ambient
temperature i s h i g h , or a fte r s ll sta i n e d ac tivi ty has taken place . N u mer­
ous behavioral adj u stments are importa n t in preventing overheating
( W h i te a nd Odel l , 1 971 ; G en try, 1 973; Odell, 1 974; Whittow, 1 974 ) . In
a d d i tion, one negative conseq uence o f the la rge social gatherings on
land i s that the a ni ma l s a re more v u l n erable to disease a n d parasite
tra n s mi ssion . The colon i a l breeding sea l i on s contain a wide variety of
i n ternal a n d externa l pa rasi tes ( D a i l ey and B rownell, 1 972) . For exa mple,
seven s pecies o f trematod es, eigh t s peci es o f n e m a todes, one cestode,
and variou s speci es of Aca n th ocep h a l a , Aca ri n a , a n d Anoplura have
been identi fied i n the Ca li fornia sea l i on . L u n gworm i n festa tion is par­
ticu larly heavy i n this speci es and may be a n i m porta n t ca u se of j u venile
morta l i ty . La n d c a rn ivores do not seem to harbor the variety or the same
high levels of para s i te i n festation as do these ma rine mamma l s . H ow­
ever, this comparison needs to be i n vestiga ted m ore systema tica l l y .
5
S UMMARY
I n s u m mary, these two groups of a ni m a l s have obvious simila ri ties ,
some of which ca n be a ttribu ted to h a v i n g a common ancestor . Pin­
nipeds bear the stamp of their ca nine, u rsine, a n d lu trine rela tives,
especi a l l y i n th e stru ctu re of the head and the pronou nced ca nine teeth .
Most species in both orders are flesh ea ters; they ea t grea t qua n ti ties a t a
ti me, a n d they ea t a wi de variety of prey . However, subsistence in
different h abitats a n d exp o s u re to differenti a l selection for a t least
20
million years have produ ced some even more obvious a n d interesting
differences in social behavi'o r. The larger canids evidently were selected
to join social group s , in which there is a high degree of coop eration,
because this allowed them to exploi t la rge prey . Canids tha t prey on
a nimals smaller t h a n themselves are far less s ocial , and kin- n e twork
welfare is red u ced . Pi nnipeds are social for a differen t reason . The most
gregari ous of them congrega te i n immense gath eri ngs tha t may include
thousa nds of i ndividuals . These aggrega tions are l oca ted o n offshore
areas t h a t are free from preda tors . Si nce these l oca tions are restri cted in
number a nd size, it appears tha t p i n nipeds were forced to group to­
gether in order to avoid preda tors . Once this s tep was taken, sexual
275
References
selecti on p u t a premi u m on conti n u ed soci a l l i v i ng; for exa mple, males
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mo re offspring th a n those who pair- bonded w i th
s ingle
females a way
from th e grou p . The few p i n n i ped s p ecies tha t are soli tary live in a
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envi ron ment , wh i l e p i n n i p ed s a ppea r to have been selected to produce
fewer p rogeny of h igh q u a l i ty , a stra tegy that is most characteri stic of
s table envi ron ments .
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