Functional
Ecology 2006
20, 105–113
Factors influencing the prediction of metabolic rate in a
reptile
Blackwell Publishing Ltd
T. D. CLARK,*† P. J. BUTLER‡ and P. B. FRAPPELL*
*Adaptational and Evolutionary Respiratory Physiology Laboratory, Department of Zoology, La Trobe University,
Melbourne, Victoria 3086, Australia, and ‡School of Biosciences, University of Birmingham, Birmingham
B15 2TT, UK
Summary
1. Measurements of the rate of oxygen consumption (◊o2) in the field are usually
impractical, so several studies of endotherms have utilized heart rate ( fH) as a correlate
of ◊o2 because of the tight relationship that often exists between the two variables.
There have been several reports, however, where the relationship between fH and ◊o2
changes or disassociates under different physiological or psychological circumstances.
This may be further confounded in ectothermic vertebrates, which experience relatively
large fluctuations in body temperature (Tb).
2. The aim of the present study was to characterize in Rosenberg’s Goanna (Varanus
rosenbergi) the relationship that exists between Tb, fH and ◊o2 at rest and at different levels
of exercise, during periods of heating and cooling, and following ingestion of a meal.
3. The combinations of Tb and fH were accurate at predicting ◊o2 of animals at different
levels of exercise and recovery, and during the postprandial period.
4. Predictions of ◊o2 became less reliable during periods of relatively rapid heating
when fH and blood flow increase for thermoregulatory purposes with no associated
increase in ◊o2. To counter this, fH was excluded from the prediction equation when the
rate of heating exceeded 20% of the predicted mass-dependent maximum attainable
rate, and ◊o2 was predicted using Tb alone.
5. The resultant ◊o2 prediction equation was used to estimate ◊o2 of seven animals
that were allowed to thermoregulate behaviourally, and the mean predicted ◊o2
(◊o2pred) was not significantly different from the mean measured ◊o2 (◊o2meas) for fasting
or postprandial lizards.
Key-words: Body temperature, ectotherm, energy expenditure, heart rate, lizard, rate of oxygen consumption
Functional Ecology (2006) 20, 105–113
doi: 10.1111/j.1365-2435.2006.01066.x
Introduction
© 2006 The Authors
Journal compilation
© 2006 British
Ecological Society
Energy turnover and, in particular, how it is allocated
to specific activities, is of central importance to the
understanding of the physiological, behavioural and
evolutionary ecology of organisms (McNamara &
Houston 1996). Measurements of energy turnover
through the determination of oxygen consumption rates
(Vo2) are usually impractical in the natural environment
and, as a consequence, two main approaches have
been developed to estimate the energetics of animals
in the field (see Butler et al. 2004 for review).
One of these approaches is to utilize the association
that exists between Vo2 and heart rate ( fH). Heart rate
may be readily telemetered or logged in wild animals,
†Author to whom correspondence should be addressed.
E-mail: timothy.clark@adelaide.edu.au
and shares a relationship with Vo2 as described by the
Fick equation for convection of oxygen through the
cardiovascular system:
Vo2 = fH × VS × (CaO2 − C◊O2),
eqn 1
where V S is cardiac stroke volume (the product of fH
and VS is cardiac output) and ( CaO2 − C◊O2) is the
difference in oxygen content between arterial (CaO2)
and mixed venous (C◊O2) blood. If oxygen pulse (VS ×
(CaO2 − C◊O2)) remains constant, or varies in a systematic manner with Vo2, then it should be possible to
predict Vo2 based on measurements of fH.
For this to work, it is necessary to calibrate the relationship between fH and Vo2, preferably using animals
undergoing activities of interest and in similar environmental conditions to those in the wild. For example,
Barnacle Geese and Bar-Headed Geese display a
105
106
T. D. Clark et al.
© 2006 The Authors
Journal compilation
© 2006 British
Ecological Society,
Functional Ecology,
20, 105–113
markedly different fH/Vo2 relationship walking on a
treadmill compared with flying in a wind tunnel (Ward
et al. 2002). Furthermore, it has been shown in King
Penguins that the relationship between fH and Vo2
resulting from treadmill exercise is different from that when
thermoregulating at low environmental temperatures
(Froget et al. 2002). The cardio-metabolic consequences of digestion confuse the relationship between
fH and Vo2 in Steller Sea Lions, where postprandial
increases in metabolism (i.e. specific dynamic action,
SDA) are not associated with concomitant increases
in fH (McPhee et al. 2003); implying that increases in
VS and /or (CaO2 − C◊O2) are more important than an
increase in fH for circulatory oxygen transport during
the digestive period. Despite possible changes
occurring in the relationship between fH and Vo2 with
physiological or psychological state, several recent
studies of endotherms have demonstrated that fH is an
accurate correlate of the rate of energy expenditure
(e.g. Bevan et al. 1995; Green et al. 2001; Ward et al.
2002).
Predictions of Vo2 based on fH may be confounded
in ectothermic vertebrates because of large daily and
seasonal fluctuations in body temperature (Tb). In this
context, it has been reported for some species of fish
that the effect of an increase in temperature is a right
shift in the linear regression that describes the relationship between fH and Vo2 during exercise (e.g. Claireaux
et al. 1995; see Clark et al. 2005a and references
within).
Few studies exist that determine the combined
effects of activity and temperature on fH and Vo2 in
reptiles (Bennett 1972; Wilson 1974; Butler et al. 2002;
Clark et al. 2005b). A study of marine iguanas exercising on a treadmill indicated that the fH/Vo2 regression
line at a T b of 36 °C was significantly right-shifted
in comparison with that at a Tb of 27 °C and, consequently, the subsequent Vo2 prediction equation incorporated a Q10 function (Butler et al. 2002). This earlier
study was limited to only two temperatures, and it is
not known how accurately the equation predicts Vo2 at
temperatures other than those studied. A further complication in predicting Vo2 from fH in reptiles could
result from the thermally related hysteresis in fH that is
known to exist in some species during heating and
cooling (Bartholomew & Tucker 1963; Smith, Robertson
& Davies 1978; Grigg & Seebacher 1999), where fH and
blood flow are modified for thermoregulatory purposes without an accompanying change in Vo2 (see
Clark, Butler & Frappell 2005c).
Consequently, using the Fick equation to predict
field Vo2, particularly of ectotherms, requires a model
that incorporates other variables in addition to fH. The
present study characterizes in a reptile, Rosenberg’s
Goanna (Varanus rosenbergi), the relationship that
exists between Tb, fH and Vo2 at rest and at different
levels of exercise, during periods of heating and cooling, and following ingestion of a meal. Thus, it was our
aim to test the hypothesis that a combination of these
biological factors could be utilized accurately to predict Vo2 of animals under a controlled situation that
simulated natural conditions. Rosenberg’s Goanna was
chosen as it lives at a relatively high latitude and is
therefore exposed to a broad range of T b values in
the natural environment (10–38 °C; see Christian &
Weavers 1994; Rismiller & McKelvey 2000).
Materials and methods
animals
Lizards were obtained at the beginning of March from
Kangaroo Island, South Australia, and kept in a
temperature-controlled holding facility at La Trobe
University at approximately 28 °C for a maximum of
25 days before use in experiments. They were maintained
on a 12:12 h light : dark photoperiod with access to a
heat lamp during light hours. Animals had unlimited
access to water and were fed twice weekly, though,
where appropriate, they were fasted prior to experimentation (see below). All animals were returned to
the holding facility following experimentation.
rate of oxygen consumption
The rate of oxygen consumption was determined by
placing a light-weight (approximately 4·5 g), transparent
and loose-fitting mask over the head of the animal.
The mask was fitted with an outlet tube through which
air was drawn at a rate appropriate for the activity (1–
8 l min−1), monitored by a calibrated mass flow meter
(Sierra, model 810C Monterey, CA). A subsample of
the air leaving the pump was passed through columns
containing a drying agent (Drierite, Hammond, Xenia,
Ohio, USA) and a carbon dioxide absorbent (Dragersorb,
Lübeck, Germany) and analysed for the fractional
content of oxygen by a gas analyser (model S-3A/I,
Applied Electrochemistry, Pittsburgh, PA, USA). The
rate of oxygen consumption was calculated from
airflow through the mask and the difference between
incurrent and excurrent fractional concentrations of
dry, carbon dioxide-free air (see appendix in Frappell
et al. 1992). All values of Vo2 are at standard temperature, pressure and dry (STPD) and expressed per kg.
heart rate and body temperature
Heart rate was obtained by attaching self-adhesive Ag/
AgCl electrocardiogram (ECG) electrode pads and
ECG leads to the dorsal surface of the animal, positioned such that they triangulated the heart. The ECG
leads were connected to an amplifier (BIO amp, ADInstruments, Sydney, Australia). Body temperature was
obtained by inserting a thermocouple 5–6 cm into the
cloaca. All outputs were collected at 100 Hz (Powerlab
800, ADInstruments, Sydney, Australia) and displayed
on a computer using Chart software (ADInstruments,
Sydney, Australia).
107
Predicting
metabolic rate
of a reptile
proto col
The relationship between Tb, fH and Vo2
Six adult animals (three males, three females; determined by morphological examination) of mean body
mass (Mb) 1·55 ± 0·15 kg were fasted for at least 3 days
then individually placed in a constant temperature
room at the desired experimental temperature (range
14 –36 °C) for at least 6 h prior to experimentation.
Although Tb was determined immediately before and
after experimentation, Tb values stated in the present
report are an average of the two. Animals were then
instrumented as described above and left to rest for
at least 40 min on a variable-speed treadmill in the
controlled temperature room until low, stable values
of fH and Vo2 were observed. They were then run on
the treadmill at the maximum speed they could
maintain for 5–10 min. The treadmill was ramped to
maximum speed typically within 1 min; maximum
speed varied with individual goannas and Tb (range
0·3–2·9 km h−1). When a lizard would no longer run
while being enticed with gentle tapping on the hind
legs, the treadmill was stopped and recordings continued for approximately 60 min during the recovery
phase. All animals were studied on seperate occasions
at six or more Tb values between 14 and 36 °C. Values
(∼1 min average of data points) were obtained from
each animal 3– 4 min prior to running, during the
final 2 min of exercise, and at three stages during
the recovery period while on the treadmill. Data
from these experiments were used to establish an
equation from which Vo2 could be predicted from fH
and Tb.
Validating the prediction equation
© 2006 The Authors
Journal compilation
© 2006 British
Ecological Society,
Functional Ecology,
20, 105–113
active animals were used to validate the equation
established for predicting Vo2.
Raw data collected from V. rosenbergi used in a previous study (see Clark et al. 2005c) were used to validate
the equation established for predicting Vo2. In brief,
seven animals were used both when fasted (>3 days
fasted; fasted Mb 1·35 ± 0·09 kg), and then 25 h after
ingestion of a meal (meal size 10 ± 1% of fasted Mb;
causes a significant increase in fH and Vo2 approximately 25 h after ingestion, see Clark et al. 2005c).
They were individually instrumented in a manner similar to that described above, and placed in a constant
temperature room (floor area 1·5 × 2·0 m) at 14 °C
until Tb (monitored continuously) fell below 19 °C. At
this point, a heat lamp positioned in one corner of the
room was switched on. The lizards were free to move
(all leads and tubes were suspended from a central
point above the animal) and, on the majority of occasions,
lizards positioned themselves under the lamp within
an hour, otherwise they were moved by the experimenter. After heating, lizards were free to explore the
room for 1–5 h, after which time the heat lamp was
switched off and the lizards were allowed to cool again
until Tb fell below 19 °C. All data from resting and
data analysis and statistics
Repeated measures multiple linear regression was used
to determine the relationship between Tb, fH and Vo2.
For the validation studies, data were averaged into 5-s
blocks. Least-squares regressions were used where
appropriate (see Results, Figs 4 and 5) to determine
the relationships between variables. Repeated measures anova was used to compare between measured
and predicted values for animals freely allowed to thermoregulate (Table 1). Significance was considered at
P < 0·05. Data are presented as mean ± SEM unless
otherwise indicated. N = number of animals, n = number
of data points.
Results
the relationship between
T b, F h
and
VO2
The relationship at different Tb values between fH and
◊o2 changed in a predictable fashion during periods of
treadmill exercise and recovery (Fig. 1). The rate of
oxygen consumption, as a function of fH and Tb, was
best described by the following equation:
log(Vo2) = 1·738 log( fH) − 0·028Tb − 1·472,
eqn 2
where r2 = 0·80, P < 0·001, N = 6, n = 290.
the effect of feeding
The increase in Vo2 that occurs following consumption
of a meal (∼10% of fasted Mb) has previously been
characterized for V. rosenbergi (see Clark et al.
2005c) and, as it is primarily governed by fH and Tb (Tb
increases owing to the heat increment of feeding),
equation 2 reliably predicts changes in Vo2 during the
postprandial period (Fig. 2).
the effect of heating and cooling
When given the opportunity to thermoregulate behaviourally, the majority of the lizards basked under
the heat lamp to obtain a Tb of 32–35 °C (e.g. Fig. 3a).
Once the desired Tb was reached, the lizards either
moved away and cooled or shuttled in and out from
underneath the heat lamp to maintain their preferred
Tb until the lamp was switched off.
During these periods, equation 2 tended to overestimate Vo 2 for all individuals, particularly during
periods of relatively rapid radiant heating when the
increase in fH was not matched by an increase in Vo2
(Fig. 3b). The maximum rate at which a lizard can heat
(∆Tb /∆t; °C min−1) is negatively related to body mass
for fasting and postprandial animals:
log(∆Tb /∆t) = − 0·823 log(Mb) + 0·012,
eqn 3
108
T. D. Clark et al.
Table 1. Measured and predicted values of the mean and total rate of oxygen consumption (Vo2) for V. rosenbergi when
thermoregulating behaviourally
Measured Vo2
∆ (%)
Predicted Vo2
Total duration
(min)
Mean
(ml min−1 kg−1)
Total
(ml kg−1)
Mean
(ml min−1 kg−1)
Total
(ml kg−1)
Fasted
1
2
3
4
5
6
7
Mean
Absolute ∆
370
391
357
443
375
395
380
387
3·88
3·22
2·61
2·63
2·95
2·07
2·03
2·77
1421
1249
778
1125
1094
814
766
1035
3·93
3·24
2·49
2·78
3·14
2·18
2·14
2·84*
Postprandial
1
2
3
4
5
6
7
Mean
Absolute ∆
100
372
624
635
234
153
383
357
4·89
2·69
1·46
1·73
3·05
2·19
1·68
2·53
444
994
931
1076
714
335
624
731
4·84
2·75
1·37
1·77
2·87
2·32
1·54
2·49*
Animal
Mean
Total
1437
1257
777
1180
1162
856
807
1068
1·44
0·61
− 4·87
5·52
6·16
4·89
5·11
2·69
4·09
1·13
0·61
− 0·12
4·70
5·82
4·89
5·11
3·16
3·20
440
1016
870
1121
672
354
584
723
− 1·00
2·17
− 7·05
2·57
− 6·32
5·42
− 9·11
− 1·90
4·81
− 1·00
2·17
− 7·01
4·01
− 6·32
5·42
− 6·84
− 1·37
4·68
Predicted values were calculated using equation 5. ∆ is the difference between the predicted and measured Vo2 expressed as a
percentage.
*No significant difference between mean values of measured Vo2 and predicted Vo2 within a group (P > 0·1, repeated measures
anova).
Data for Animal 1 in the fasted state are illustrated in Fig. 3.
Fig. 1. The relationship between body temperature (Tb),
heart rate ( fH) and the rate of oxygen consumption (Vo2) for
V. rosenbergi at different levels of activity. Included are the
relationships between fH and Vo2 that occur during exercise
and recovery at 15 °C, 22 °C, 29 °C and 36 °C (dashed lines),
calculated using equation 2. N = 6, n = 290.
© 2006 The Authors
Journal compilation
© 2006 British
Ecological Society,
Functional Ecology,
20, 105–113
where r2 = 0·28, P < 0·05, N = 7, n = 14. It was iteratively determined that equation 2 became inaccurate
when the rate of heating exceeded 20% of the maximum mass-dependent attainable rate (this typically
occurred during 70% of the initial heating period, and
for approximately 15% of the entire experiment). As
Fig. 2. The measured change in the rate of oxygen consumption
(Vo2) of V. rosenbergi at an ambient temperature of 30 °C
following consumption of a meal (∼10% of fasted body mass;
closed circles, N = 7, modified from Clark et al. 2005c).
Displayed in open squares is the trend in postprandial Vo2
predicted using equation 2. There was no significant difference
between measured and predicted Vo2 at any given time
(P > 0·05).
such, when an animal heated faster than 20% of the
predicted maximum attainable rate for its body mass
(calculated from equation 3), it was assumed that fH and
Vo2 had uncoupled for the purpose of thermoregulation.
During these periods, fH was excluded from the prediction equation and Vo2 was recalculated using only Tb
for resting animals when undergoing rapid heat exchange
109
Predicting
metabolic rate
of a reptile
Fig. 3. Traces for a fasted individual animal of (a) body temperature (Tb) and heart rate ( fH), and (b) and (c) the measured rate
of oxygen consumption (Vo2meas) while thermoregulating behaviourally in a 14 °C room with access to a heat lamp (available for
the duration indicated by the black bar at top of figure). Data have been binned into 5-min blocks. Also illustrated are the
predicted values of Vo2 (Vo2pred; dashed lines) calculated using (b) equation 2 and (c) equation 5.
result of SDA (Fig. 4). A common regression was adopted
for the purpose of the Vo2 prediction equation:
log(Vo2) = 0·022Tb − 0·132,
eqn 4
where r2 = 0·43, P < 0·001, N = 7, n = 68.
predicting
Fig. 4. Semi-log plot of the rate of oxygen consumption (Vo2)
as a function of body temperature (Tb) during heating and
cooling for fasting (open circles, N = 7) and postprandial (closed
circles, N = 7) V. rosenbergi (modified from Clark et al. 2005c).
There was no difference for either group between heating and
cooling. Dashed lines describe the regression for fasting
[log(Vo2) = 0·023Tb − 0·267; r2 = 0·94] and postprandial
[log(Vo2) = 0·021Tb + 0·002; r 2 = 0·93] animals. ancova revealed
no difference in slope between the groups (P > 0·3). The solid line
is the common regression (given as equation 4 in main text).
© 2006 The Authors
Journal compilation
© 2006 British
Ecological Society,
Functional Ecology,
20, 105–113
(see equation 4; Fig. 4). This relationship between Tb
and Vo2 is the same during heating and cooling within
fasting and within postprandial lizards, but postprandial lizards have a higher Vo2 than fasting lizards as a
VO2
To summate, equation 2 reliably predicts Vo2 except
during periods of relatively rapid heating when an
uncoupling of fH and Vo2 results in an overestimate of
Vo2 (see Fig. 3b). This overestimation is corrected by
incorporating equation 4 when Tb increases faster than
20% of the predicted maximum attainable rate (see
Fig. 3c). The resultant equation can therefore be formalized as:
log(Vo2) = C1 × log( fH) − C2 × Tb − C3,
eqn 5
where C1 = 1·738, C2 = 0·028 and C3 = 1·472 when the
mass-dependent rate of heating is less than 20% of
the predicted maximum attainable value for the given
body mass, and C1 = 0·000, C 2 = −0·022 and C 3 =
0·132 when the rate of heating exceeds 20% of the
maximum attainable value.
Subsequently, equation 5 was used to predict Vo2 of
the seven individuals that were used in the validation
study. Mean measured Vo2 (Vo2meas) of these animals
ally during exercise and proportionally across temperature such that a strong relationship between fH and Vo2
is maintained (Wilson 1974; Gleeson et al. 1980; Grubb
et al. 1983; Jones et al. 1989; Butler et al. 1992; Clark
et al. 2005a,b).
110
T. D. Clark et al.
digestion
Fig. 5. Predicted rate of oxygen consumption (Vo2pred; calculated
using equation 5) as a function of the measured rate of oxygen
consumption (Vo2meas; N = 7 both for fasting and postprandial
animals). Each point is the average value for 1 h. The solid
line is the regression that describes the relationship both for
fasting and postprandial animals (Vo2pred = 1·11Vo2meas − 0·16;
r 2 = 0·88, n = 79), with 95% confidence limits (dotted lines).
ancova revealed that the regression was not different from
the line of equality (dashed line; P > 0·2).
was not significantly different from mean predicted
Vo2 (Vo2pred) either for fasting or postprandial animals
(Table 1). The data were binned into 1-h blocks to be
analysed at a greater resolution and, in this case, equation 5 could still reliably be used to predict Vo2 (Fig. 5).
Discussion
exercise
© 2006 The Authors
Journal compilation
© 2006 British
Ecological Society,
Functional Ecology,
20, 105–113
As far as the circulatory system of varanids is concerned,
the increase in Vo2 during maximum exercise is typically
achieved by increases in all of fH, VS and (CaO2 − C◊O2)
(Gleeson, Mitchell & Bennett 1980; Frappell, Schultz
& Christian 2002a,b; Clark et al. 2005b), where the
increase in fH usually accounts for the majority of the
increase in cardiac output (Bennett 1972; Gleeson et al.
1980; Frappell et al. 2002b). In the present study of V.
rosenbergi, a tight relationship between fH and Vo2 was
maintained at all levels of treadmill exercise over the
entire range of temperatures studied, and it is likely
that the observed range in these variables encompasses
the majority of what would be experienced in the field.
For a given activity over a range of intensities, the
relationship between fH and Vo2 at a given Tb is linear
or curvilinear for most animals studied, including birds
(e.g. Grubb, Jorgensen & Conner 1983; Bevan et al.
1994; Green et al. 2001; Ward et al. 2002), mammals
(e.g. Fedak, Pullen & Kanwisher 1988; Williams,
Kooyman & Croll 1991; Williams, Friedl & Haun 1993),
reptiles (e.g. Butler et al. 2002; present study) and some
fish (e.g. Armstrong 1986; Lucas 1994; Webber,
Boutilier & Kerr 1998; Clark et al. 2005a). With the
possible exception of some species of fish (see Clark
et al. 2005a and references within), it appears for most
vertebrates that VS and (CaO2 − C◊O2) change systematic-
The act of processing, digesting and absorbing food is
typically associated with an increase in the rate of
oxygen consumption (i.e. SDA) which, in most vertebrates, is largely accompanied by an increase in fH
(Kelbaek et al. 1987; Dumsday 1990; Wang, Burggren
& Nobrega 1995; Hicks, Wang & Bennett 2000; Secor,
Hicks & Bennett 2000; Wang et al. 2000, 2001; Clark
et al. 2005c). However, in accordance with that occurring during exercise, the overall increase in systemic
blood flow during digestion may not match the factorial increase in Vo2 (e.g. Hicks et al. 2000; Secor et al.
2000), thus an associated increase in (CaO2 − C◊O2 ) may
also occur. It appears for V. rosenbergi that the pattern
of change in circulatory variables is such that the relationship between fH and Vo2 determined for exercising
animals (equation 2) is maintained during digestion
(Fig. 2).
The combined effects of digestion and exercise have
been shown in some animals to elicit a response in Vo2
that is higher than either process could elicit on its
own, yet the response is typically less than the sum of
both processes combined (i.e. a partial ‘additivity’ of
the responses; Segal & Gutin 1983; Secor et al. 2000;
Bennett & Hicks 2001). From the data currently available,
it appears that fH may be at least partly responsible
for the cumulative effect in Vo2 that occurs during
simultaneous digestion and exercise (e.g. Secor et al.
2000). Given that equation 5 did not significantly
underestimate Vo2 of active postprandial V. rosenbergi
in the present study (Table 1 and Fig. 5), this ‘additivity’
of the responses should not be problematic when predicting energy expenditure of free-ranging, ectothermic vertebrates.
thermoregulation
Of all vertebrates, reptiles may undergo the largest and
most rapid daily fluctuations in Tb, thus contributing
to the difficulty of predicting field metabolic rate
of such animals. To account for the disassociation
between fH and Vo2 that occurs in V. rosenbergi during
rapid heat exchange (see Introduction), the prediction
of Vo2 in the present study required that fH be excluded
from the prediction equation during periods of relatively rapid heating (>20% of maximum attainable
rate). Clearly, operative temperatures in the natural
environment will vary from those to which the animals
were exposed in the present study, thus heat exchange
will occur at different rates. It may be necessary in
the wild therefore to monitor thermal conditions at
ground level (e.g. radiant and ground temperatures), in
111
Predicting
metabolic rate
of a reptile
combination with maximum attainable rates of heat
exchange (determined from Tb trace on data loggers or
transmitters; see below), to calculate at which point fH
is to be excluded from the Vo2 prediction equation.
Correcting for this should not be difficult given that
variations in operative temperature should influence
only the intercept, but not the slope, of the relationship
between Mb and the rate of heating (see equation 3).
As it is unlikely that the digestive state of an animal
in the natural environment would be known, the incorporation of equation 4 into the prediction equation
during relatively rapid heating assumes that all animals
have a small postprandial increment in Vo2 during
this period. Because such periods of rapid heating
typically occur rather infrequently during the day for
reptiles in the natural environment (see Christian &
Weavers 1994; Rismiller & McKelvey 2000; Seebacher
& Grigg 2001), it is unlikely that this assumption
would produce a substantial error in a prediction of
daily energy expenditure irrespective of the digestive
state of the animal. In this context, if equation 4 is
replaced either with the linear regression equation
describing the relationship between Vo2 and Tb during
the fasting period, or with that describing the relationship during the postprandial period (see Fig. 4), the
resultant mean Vo2pred values are not significantly
different from the mean Vo2meas values (P > 0·1 in both
cases).
predicting field metabolic rate
© 2006 The Authors
Journal compilation
© 2006 British
Ecological Society,
Functional Ecology,
20, 105–113
With the advent and continued miniaturization of
electronic data loggers/transmitters of fH and Tb (e.g.
Woakes, Butler & Bevan 1995), it is becoming increasingly possible to predict and monitor the energetics of
animals in the natural environment (see Butler et al.
2004 and references within). The principal advantage
of using this approach is that it can provide estimates
of Vo2 at a far greater temporal resolution than many
other methods (e.g. doubly labelled water, DLW); the
resolution being limited only by the calibration procedure and the method of recording fH. The present
study provides the most comprehensive evidence to
date that it should be possible, based primarily on fH
and Tb, to predict the energy expenditure of free-ranging
ectotherms that undergo daily fluctuations in Tb.
The suggested approach (i.e. equation 5) is accurate
at predicting the mean Vo2 of a group of animals,
though the mean values hide the individual algebraic
errors which ranged from −4·87% to +6·16% in fasting
animals, and from −9·11% to +5·42% in postprandial
animals (Table 1). Similar interindividual error ranges
are typical when using a group prediction equation to
estimate Vo2 of individual animals (e.g. Nolet et al. 1992;
Bevan et al. 1994, 1995; Boyd et al. 1995; Hawkins
et al. 2000; Froget et al. 2001; McPhee et al. 2003). It
is generally accepted therefore that, although this
method is capable of predicting Vo2 for specific
activities in the field, it should be used only to estimate
mean Vo2 of a group of animals rather than estimating
it for an individual animal (see Butler et al. 2004),
unless specific animals are individually calibrated such
that the prediction of energy expenditure of each
animal is based on its own calibration equation. In
either case, the error associated with the prediction of
Vo2 must be taken into account (see Green et al. 2001;
Butler et al. 2004).
Having established that there is a significant relationship between Vo2 and the variables of the prediction
equation (e.g. fH and Tb), it is important to remember
that the relationship may not remain constant for
individuals of a different physiological state. For
example, it has been noted for King Penguins that the
fH/Vo2 relationship changes during the fasting period
when animals spend long periods ashore (Froget et al.
2001; Fahlman et al. 2004). There are data for lizards
that suggest seasonal changes in Vo2 (Christian & Conley
1994; Christian, Bedford & Schultz 1999; de Souza
et al. 2004), though it has not yet been ascertained
whether fH follows a similar seasonal pattern. Nevertheless, if our approach is to be used to predict field
Vo2 of V. rosenbergi throughout the entire year, it will
be necessary to clarify the relationship between Tb, fH
and Vo2 on wild-caught animals at different times of
the year, and potentially incorporate a seasonal component into the resultant equation.
The present study details the principles involved
when predicting metabolic rate of a thermoregulating
ectotherm, and the findings suggest that it is possible
to use primarily fH and Tb to predict the energetics of
free-ranging ectothermic vertebrates in the natural
environment. The interest in using fH to estimate field
Vo2 of birds and mammals has increased in recent
years, and the principles outlined in the present study
should encourage similar studies on energetics of freeranging ectothermic vertebrates.
Acknowledgements
Animal collection and experiments were performed
with the approval of the Animal Ethics Committee of
La Trobe University (code: 99/42 L). Brian Green is
thanked for the collection of animals. Eva Suric and
Tobie Cousipetcos are commended for animal husbandry. Craig White is thanked for statistical advice.
TDC was the recipient of an Australian Postgraduate
Award scholarship. PJB is a La Trobe University Visiting Distinguished Professor to the Adaptational and
Evolutionary Respiratory Physiology Laboratory.
References
Armstrong, J.D. (1986) Heart rate as an indicator of activity,
metabolic rate, food intake and digestion in pike, Esox
lucius. Journal of Fish Biology 29, 207 –221.
Bartholomew, G.A. & Tucker, V.A. (1963) Control of changes
in body temperature, metabolism, and circulation by the
agamid lizard, Amphibolurus barbatus. Physiological Zoology
36, 199 –218.
112
T. D. Clark et al.
© 2006 The Authors
Journal compilation
© 2006 British
Ecological Society,
Functional Ecology,
20, 105–113
Bennett, A.F. (1972) The effect of activity on oxygen consumption, oxygen debt, and heart rate in the lizards Varanus
gouldii and Sauromalus hispidus. Journal of Comparative
Physiology 79, 259 – 280.
Bennett, A.F. & Hicks, J.W. (2001) Postprandial exercise:
prioritization or additivity of the metabolic responses?
Journal of Experimental Biology 204, 2127 –2132.
Bevan, R.M., Woakes, A.J., Butler, P.J. & Boyd, I.L. (1994)
The use of heart rate to estimate oxygen consumption of
free-ranging black-browed albatrosses Diomedea melanophrys. Journal of Experimental Biology 193, 119 –137.
Bevan, R.M., Woakes, A.J., Butler, P.J. & Croxall, J.P. (1995)
Heart rate and oxygen consumption of exercising gentoo
penguins. Physiological Zoology 68, 855 – 877.
Boyd, I.L., Woakes, A.J., Butler, P.J., Davis, R.W. &
Williams, T.M. (1995) Validation of heart rate and doubly
labelled water as measures of metabolic rate during swimming
in California sea lions. Functional Ecology 9, 151–160.
Butler, P.J., Woakes, A.J., Boyd, I.L. & Kanatous, S. (1992)
Relationship between heart rate and oxygen consumption
during steady-state swimming in California sea lions.
Journal of Experimental Biology 170, 35 – 42.
Butler, P.J., Frappell, P.B., Wang, T. & Wikelski, M. (2002)
The relationship between heart rate and rate of oxygen
consumption in Galapagos marine iguanas (Amblyrhynchus
cristatus) at two different temperatures. Journal of Experimental Biology 205, 1917 –1924.
Butler, P.J., Green, J.A., Boyd, I.L. & Speakman, J.R. (2004)
Measuring metabolic rate in the field: the pros and cons of the
doubly labelled water and heart rate methods. Functional
Ecology 18, 168 –183.
Christian, K.A. & Conley, K.E. (1994) Activity and resting
metabolism of varanid lizards compared with ‘typical’ lizards.
Australian Journal of Zoology 42, 185–193.
Christian, K.A. & Weavers, B. (1994) Analysis of the activity
and energetics of the lizard Varanus rosenbergi. Copeia 2,
289 – 295.
Christian, K.A., Bedford, G.S. & Schultz, T.J. (1999) Energetic consequences of metabolic depression in tropical and
temperate-zone lizards. Australian Journal of Zoology 47,
133 –141.
Claireaux, G., Webber, D.M., Kerr, S.R. & Boutilier, R.G.
(1995) Physiology and behaviour of free-swimming
Atlantic cod (Gadus morhua) facing fluctuating temperature
conditions. Journal of Experimental Biology 198, 49 – 60.
Clark, T.D., Ryan, T., Ingram, B.A., Woakes, A.J., Butler, P.J.
& Frappell, P.B. (2005a) Factorial aerobic scope is independent of temperature and primarily modulated by heart
rate in exercising Murray cod (Maccullochella peelii peelii).
Physiological and Biochemical Zoology 78, 347 –355.
Clark, T.D., Wang, T., Butler, P.J. & Frappell, P.B. (2005b)
Factorial scope of cardio-metabolic variables remains
constant with changes in body temperature in the varanid
lizard, Varanus rosenbergi. American Journal of Physiology:
Regulatory, Integrative and Comparative Physiology 288,
R992 –R997.
Clark, T.D., Butler, P.J. & Frappell, P.B. (2005c) Digestive
state influences the heart rate hysteresis and rates of heat
exchange in the varanid lizard, Varanus rosenbergi. Journal
of Experimental Biology 208, 2269 –2276.
Dumsday, B. (1990) Resting heart rate of the toad Bufo marinus:
a long-term study of individual differences and environmental influences. Physiological Zoology 63, 420 – 431.
Fahlman, A., Handrich, Y., Woakes, A.J., Bost, C.A.,
Holder, R., Duchamp, C. & Butler, P.J. (2004) Effect of
fasting on the VO 2–f H relationship in king penguins,
Aptenodytes patagonicus. American Journal of Physiology:
Regulatory, Integrative and Comparative Physiology 287,
R870 –R877.
Fedak, M.A., Pullen, M.R. & Kanwisher, J. (1988) Circulatory responses of seals to periodic breathing: heart rate and
breathing during exercise and diving in the laboratory and
open sea. Canadian Journal of Zoology 66, 53 – 60.
Frappell, P.B., Lanthier, C., Baudinette, R.V. & Mortola, J.P.
(1992) Metabolism and ventilation in acute hypoxia: a
comparative analysis in small mammalian species. American
Journal of Physiology 262, R1040 –R1046.
Frappell, P.B., Schultz, T.J. & Christian, K.A. (2002a) The
respiratory system in varanid lizards: determinants of O2
transfer. Comparative Biochemistry and Physiology 133,
239 – 258.
Frappell, P.B., Schultz, T.J. & Christian, K.A. (2002b)
Oxygen transfer during aerobic exercise in a varanid lizard
Varanus mertensi is limited by the circulation. Journal of
Experimental Biology 205, 2725 –2736.
Froget, G., Butler, P.J., Handrich, Y. & Woakes, A.J. (2001)
Heart rate as an indicator of oxygen consumption: influence of body condition in the king penguin. Journal of
Experimental Biology 204, 2133 – 2144.
Froget, G., Handrich, Y., Le Maho, Y., Rouanet, J.L.,
Woakes, A.J. & Butler, P.J. (2002) The heart rate/oxygen
consumption relationship during cold exposure of the king
penguin: a comparison with that during exercise. Journal of
Experimental Biology 205, 2511 –2517.
Gleeson, T.T., Mitchell, G.S. & Bennett, A.F. (1980) Cardiovascular responses to graded activity in the lizards Varanus
and Iguana. American Journal of Physiology 239, R174–
R179.
Green, J.A., Butler, P.J., Woakes, A.J., Boyd, I.L. & Holder, R.L.
(2001) Heart rate and rate of oxygen consumption of exercising macaroni penguins. Journal of Experimental Biology
204, 673 – 684.
Grigg, G.C. & Seebacher, F. (1999) Field test of a paradigm:
hysteresis of heart rate in thermoregulation by a free-ranging
lizard (Pogona barbata). Proceedings of the Royal Society
of London B 266, 1291–1297.
Grubb, B., Jorgensen, D.D. & Conner, M. (1983) Cardiovascular changes in the exercising emu. Journal of Experimental
Biology 104, 193 –201.
Hawkins, P.A.J., Butler, P.J., Woakes, A.J. & Speakman, J.R.
(2000) Estimation of the rate of oxygen consumption of the
common eider duck (Somateria mollissima), with some
measurements of heart rate during voluntary dives. Journal
of Experimental Biology 203, 2819 – 2832.
Hicks, J.W., Wang, T. & Bennett, A.F. (2000) Patterns of
cardiovascular and ventilatory response to elevated metabolic states in the lizard Varanus exanthematicus. Journal
of Experimental Biology 203, 2437– 2445.
Jones, J.H., Longworth, K.E., Lindholm, A., Conley, K.E.,
Karas, R.H., Kayar, S.R. & Taylor, C.R. (1989) Oxygen
transport during exercise in large mammals. I. Adaptive
variation in oxygen demand. Journal of Applied Physiology
67, 862 – 870.
Kelbaek, H., Munck, O., Christensen, N.J. & Godtfredsen, J.
(1987) Autonomic nervous control of postprandial hemodynamic changes at rest and upright exercise. Journal of
Applied Physiology 63, 1862 –1865.
Lucas, M.C. (1994) Heart rate as an indicator of metabolic
rate and activity in adult Atlantic salmon, Salmo-salar.
Journal of Fish Biology 44, 889 – 903.
McNamara, J.M. & Houston, A.I. (1996) State-dependent
life histories. Nature 380, 215 – 221.
McPhee, J.M., Rosen, D.A.S., Andrews, R.D. & Trites, A.W.
(2003) Predicting metabolic rate from heart rate in juvenile
Steller sea lions Eumetopias jubatus. Journal of Experimental
Biology 206, 1941–1951.
Nolet, B.A., Butler, P.J., Masman, D. & Woakes, A.J. (1992)
Estimation of daily energy expenditure from heart rate and
doubly labelled water in exercising geese. Physiological
Zoology 65, 1188 –1216.
Rismiller, P.D. & McKelvey, M.W. (2000) Spontaneous
arousal in reptiles? Body temperature ecology of Rosenberg’s
113
Predicting
metabolic rate
of a reptile
© 2006 The Authors
Journal compilation
© 2006 British
Ecological Society,
Functional Ecology,
20, 105–113
goanna, Varanus rosenbergi. Life in the Cold: 11th International Hibernation Symposium (eds G. Heldmaier & M.
Klingenspor), pp. 57–64. Springer-Verlag, Berlin.
Secor, S.M., Hicks, J.W. & Bennett, A.F. (2000) Ventilatory
and cardiovascular responses of a python (Python molurus)
to exercise and digestion. Journal of Experimental Biology
203, 2447 –2454.
Seebacher, F. & Grigg, G.C. (2001) Changes in heart rate are
important for thermoregulation in the varanid lizard Varanus
varius. Journal of Comparative Physiology B 171, 395–400.
Segal, K.R. & Gutin, B. (1983) Thermal effects of food and
exercise in lean and obese women. Metabolism 32, 581–589.
Smith, E.N., Robertson, S. & Davies, D.G. (1978) Cutaneous
blood flow during heating and cooling in the American
alligator. American Journal of Physiology 235, R160–R167.
de Souza, S.C.R., de Carvalho, J.E., Abe, A.S., Bicudo, J.E.P.W.
& Biancocini, M.S.C. (2004) Seasonal metabolic depression, substrate utilisation and changes in scaling patterns
during the first year cycle of tegu lizards (Tupinambis
merianae). Journal of Experimental Biology 207, 307 –318.
Wang, T., Burggren, W.W. & Nobrega, E. (1995) Metabolic,
ventilatory, and acid–base responses associated with specific
dynamic action in the toad Bufo marinus. Physiological
Zoology 68, 192 –205.
Wang, T., Axelsson, M., Jensen, J. & Conlon, J.M. (2000)
Cardiovascular actions of python bradykinin and substance
P in the anesthetized python, Python regius. American
Journal of Physiology 279, R531–R538.
Wang, T., Taylor, E.W., Andrade, D. & Abe, A.S. (2001)
Autonomic control of heart rate during forced activity and
digestion in the snake Boa constrictor. Journal of Experimental Biology 204, 3553 – 3560.
Ward, S., Bishop, C.M., Woakes, A.J. & Butler, P.J. (2002)
Heart rate and the rate of oxygen consumption of flying and
walking barnacle geese (Branta leucopsis) and bar-headed
geese (Anser indicus). Journal of Experimental Biology 205,
3347 –3356.
Webber, D.M., Boutilier, R.G. & Kerr, S.R. (1998) Cardiac
output as a predictor of metabolic rate in cod Gadus morhua.
Journal of Experimental Biology 201, 2779 –2789.
Williams, T.M., Kooyman, G.L. & Croll, D.A. (1991) The
effect of submergence on heart rate and oxygen consumption
of swimming seals and sea lions. Journal of Comparative
Physiology 160, 637 – 644.
Williams, T.M., Friedl, W.A. & Haun, J.E. (1993) The physiology of bottlenose dolphins (Tursiops truncatus) – heart
rate, metabolic rate and plasma lactate concentration during
exercise. Journal of Experimental Biology 179, 31–46.
Wilson, K.J. (1974) The relationship of oxygen supply for
activity to body temperature in four species of lizards.
Copeia 1974, 920 – 934.
Woakes, A.J., Butler, P.J. & Bevan, R.M. (1995) Implantable
data logging system for heart rate and body temperature –
its application to the estimation of field metabolic rates in
Antarctic predators. Medical and Biological Engineering
and Computing 33, 145 –151.
Received 28 February 2005; revised 15 August 2005; accepted
6 September 2005
Editor: Steven L. Chown