The relationship between heart rate and rate of oxygen consumption in Galapagos marine iguanas (Amblyrhynchus cristatus) at two different temperatures
1 School of Biosciences, The University of Birmingham, Birmingham, B15 2TT,
UK
2 Department of Zoology, La Trobe University, Melbourne, Victoria 3086,
Australia
3 Department of Ecology and Evolutionary Biology, Princeton University,
Princeton, NJ 08544-1003, USA
Present address: Department of Zoophysiology, University of Aarhus,
Universitetsparken, Aarhus 8000C, Denmark
* Author for correspondence (e-mail: p.j.butler{at}bham.ac.uk )
Accepted 9 April 2002
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Summary |
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Key words: heart rate, rate of oxygen consumption, exercise, Galapagos marine iguana, Amblyrhynchus cristatus
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Introduction |
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![]() | (1) |
Heart rate has not previously been used to predict FMR of reptiles. In
these animals, the relationship between
O2 and
fH may be complicated by variations in metabolic rate associated with
changes in ambient temperature and the possible variation in shunting between
the left and right sides of the heart. Thus, the aim of the present study was
to determine the relationship between
O2 and
fH in a reptile at two different temperatures that represent the
extremes of its average daily range during summer
(Fig. 1). Using implantable
heart rate data loggers (HRDL; Woakes et
al., 1995
), this relationship will be used in ongoing field
studies employing HRDLs to determine FMR and the energetic costs of specific
behaviours (see Fig. 4).
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The reptile chosen was the Galapagos marine iguana, Amblyrhynchus
cristatus. This is the only lizard that dives beneath the sea to feed and
whose food is primarily marine algae. There appears to be a size limit to
these animals that is related to the availability of food, with smaller
animals out-competing larger ones when food availability declines
(Wikelski et al., 1997).
During particularly lean (El Niño) years, some animals `shrink'
(reduction in body length as well as in body mass), and those that `shrink'
the most survive the longest (Wikelski and
Thom, 2000
). On top of this is the cost of reproduction. Females
actively choose mates (Wikelski et al.,
2001
) and suffer a cost of reproduction in terms of a lower
probability of survival during the following season
(Laurie, 1989
). Thus, there
are many potential applications for the fH method in this species,
which occupies an unusual niche for a lizard, in order to determine the energy
costs of specific behaviours.
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Materials and methods |
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The sterilised loggers were inserted with one electrode close to the heart and the other laying along the side of the HRDL. The body of the HRDL was fixed in place with two sutures of surgical silk (#2, Dexon, Germany) through the body wall. The logger incorporated a low-power radio frequency transmitter which emitted a short pulse on each QRS wave of the electrocardiogram (ECG). Detection of this signal by a radio receiver indicated when the electrodes were in the correct position. The muscle layer and skin were individually closed together with dissolvable surgical suture (#1, Dexon, Germany) and an antibiotic/antifungal spray (Chloromycetin, Parke Davis, USA) was used on the wound after surgery. The animal was then taken off Isoflurane and woke within 5 min. The iguanas were left for at least 24 h before they were used in an experiment.
To determine whether iguanas suffered from post-surgical stress after the implantation of data loggers, we took a blood sample from the tail vein of four implanted females approximately 24 h after surgery and of six control females that had not undergone surgery. Plasma corticosterone levels were determined using a standard radioactive immuno-assay procedure (Romero and Wikelski, 2000). There was no difference in the levels of corticosterone between the two groups (implanted, 6.3±1.8 ng ml-1; control, 5.4±1.4 ng ml-1; means ± S.D., t-test, P=0.39). Thus, we conclude that iguanas do not suffer post-surgical trauma that would be indicated by an increased corticosterone level compared to that of controls. None of the animals showed signs of infection or discomfort and all data loggers were removed 3 days after implantation. All the animals survived the surgical procedures and four of the SC animals were seen 9 months later at the location where they had been caught and released.
The animals were studied either in the early morning or early afternoon when their body temperatures were at approximately 27 °C or 36 °C, respectively. When required, the use of a refrigerator or an infrared lamp enabled us to maintain the animals at these temperatures. Body temperature was determined by a thermocouple placed 3-4 cm into the cloaca. Once body temperature had been maintained close to the required value for at least an hour, the animal was fitted with a loop aerial on the top of its body. This enabled the transmitted heart beat signal from the implanted data logger to be detected by a radio receiver and the output from the receiver to be sent to a pre-amplifier (Isleworth, Electronics, England, model A101) and the signal appropriately filtered.
A transparent mask constructed from a plastic water bottle was placed over
the head of the animal and held in place with a rubber collar around the neck.
An airtight seal between the collar and the skin of the iguana was achieved
with a layer of quick-setting, non-toxic polyether material (Impregum, ESPE
Dental AG, Germany). The mask was fitted with inlet and outlet tubes through
which air was drawn at a rate of approximately 2.61 min-1 STPD by a
pump (Reciprotor, Denmark, model 506R) on the outlet side. The air flow rate
was set and monitored by a mass flow meter and controller (Sierra, models 840L
and 902C). A subsample of the air leaving the pump was passed through a drying
column (Drierite, Hammond) and analysed for the fractional content of
O2 and CO2 by a gas analyser (ADInstruments, model
ML205). The gas analyser was calibrated with room air and was accurate to 0.01
% for both gases. Outputs from the ECG pre-amplifier and gas analyser were
collected at 1 kHz (Powerlab 800, ADInstruments) and displayed on a computer
using Chart software (ADInstruments) as heart rate and rate of oxygen
consumption. Rate of oxygen consumption was determined from the airflow
through the mask and the difference between incurrent and excurrent fractional
concentrations of dry air following consideration of respiratory quotient
(RQ)-related errors (see Appendix in
Frappell et al., 1992).
After instrumentation, the iguanas were placed on a variable-speed
treadmill (1.2 m long and 0.5 m wide) and allowed to settle for at least 30
min, when fH and
O2 had reached
steady (pre-exercise) values. They were then run at the maximum speed that
they could comfortably maintain for a few minutes (maximum exercise). Although
this included bursts of locomotion, the animals were not run to exhaustion.
When an iguana no longer wanted to run, the treadmill was stopped and
recordings continued for approximately 60 min during the recovery phase (see
Gleeson, 1980
, for recovery
times of
O2
after exhaustive exercise in marine iguanas). Rate of oxygen consumption and
fH data were obtained from each animal during the pre-exercise
period, at maximum exercise (one datum point at each) and at four
approximately equally spaced points during recovery. Data were averaged over
30-60 s. At the Darwin Station, each iguana was run, in random order, at body
temperatures of 27 °C and 36 °C. On board the Quest, the
animals were run only at a body temperature of 36 °C and values of
fH and
O2 were recorded
only during the pre-exercise period and at maximum exercise. All values of
O2 are at
standard temperature and pressure, dry (STPD).
Least-squares regressions were used to determine the relationships between
fH and
O2 for
individuals and for the group data at the two different temperatures for the
SC animals. Regression equations were compared using an analysis of variance
(ANOVA) general linear model (GLM; Zar,
1984
) and, after testing for normality (Kolomogorov-Smirnov test),
a Student's t-test was used to compare the significance of any
difference between the means of two populations. When more than two means were
compared, a repeated-measures ANOVA was used with two grouping factors
(location and level of exercise). Post-hoc modified t-tests
with Bonferroni corrections were used to test for differences between the
various factors. Two means were considered to be significantly different when
P<0.05 and are quoted at the level at which they were found to be
significant. All mean values are given ± S.D.
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Results |
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In SC animals, pre-exercise fH was 94 % higher at a
Tb of 36 °C compared with that at 27 °C, which is
equivalent to a Q10 of 2.1. The comparable pre-exercise values for
sO2 were 55 %
higher, equivalent to a Q10 of 1.6. The maximum fH value
recorded during exercise was 75 % higher at a Tb of 36
°C compared with that at 27 °C, which yields a Q10 of 1.9.
The comparable value for maximum
s
O2 during
exercise was 45 % higher, equivalent to a Q10 of 1.5. Mass-specific
oxygen pulses during the pre-exercise period were not significantly different
at 27 °C and 36 °C (P=0.11) whereas sO2 pulse
during maximum exercise at 27 °C was significantly (21 %) greater than
that at 36 °C (P=0.02). There was no significant difference
between the values obtained from the SC and SF animals at 36 °C during the
pre-exercise period and at maximum exercise for fH,
s
O2 and
sO2 pulse.
Heart rate and
sO2 were well
correlated in each individual SC iguana
(Table 2) and the relationships
were well described by a linear function. Analysis of covariance (ANCOVA) was
used to compare the values of the intercepts (a) and slopes
(b) of the individual regressions within each group (i.e. at 27
°C and at 36 °C). While there was no significant difference between
the slopes, there were significant differences between the intercepts. Thus,
the intercepts were regarded as a random sample from a distribution of
intercept values and a random-effects model was adopted (see
Green et al., 2001
). The group
regression equations derived in the present study are:
![]() | (2) |
![]() | (3) |
|
The intercept is a random factor, so if one of the above equations is to be
used to estimate
sO2 from an
average value of fH measured in the field, the usual method for
estimating the standard deviation (
) of an estimate using regression
equations (see equation 17.28 in Zar,
1984
) has to be modified, as indicated in equation 11 of Green et
al. (2001
):
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Discussion |
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Perhaps because of their propensity for short bursts of locomotion, we did not find it easy to persuade all the animals to walk/run at speeds below the maximum speed they could maintain for a few minutes. Nonetheless, we were successful in achieving this with three animals at both temperatures and there was no significant difference between the slopes of the regression lines of data obtained from animals during the pre-exercise period and when walking/running at different speeds, and data obtained during the recovery period (P=0.67 at 27 °C and 0.07 at 36 °C, Fig. 4). Thus our experimental procedures simulated as closely as possible what is known for marine iguanas exercising in the wild and we are confident that the regression lines given in Fig. 2 represent both exercise at different levels and recovery from maximum exercise.
The sO2 data
that we obtained are similar to those obtained by other workers studying the
Galapagos marine iguana (Bennett et al.,
1975
, who electrically stimulated the animals in order to obtain
activity; Bartholomew and Vleck,
1979
; Gleeson,
1979
,
1980
), with the exception of
pre-exercise s
O2
at 27 °C, where our mean value is approximately twofold greater than the
`resting' values obtained by the above workers. On the other hand, our mean
value for pre-exercise fH at 27 °C is within the range given by
Bartholomew and Lasiewski
(1965
), but our mean value for
pre-exercise fH at 36 °C is approximately 75 % of that reported
by the latter authors while the iguanas were being heated and cooled. As far
as we can determine, there are no values in the literature for fH of
marine iguanas during exercise.
It would appear from the present study that it should be possible to use
fH as an indicator of
sO2 for iguanas
in the field, as between 86 and 91 % of the variation in the latter could be
explained by the fitted regressions from the calibration experiments. The
utility of the fH method for the estimation of
s
O2 in the
marine iguana was further supported by the fact that the mean values of
s
O2 from four
animals from a population different from that involved in producing the
calibration equations were within the 95 % prediction intervals of the
regression. However, the effect of temperature is to vary the intercept of the
relationship between the two variables, rather than to extend a single
regression line (see Fig.
2).
It is clear from the field data that have been obtained so far (M. Wikelski
and A. J. Woakes, unpublished data) that temperature can change by between
6-12 °C or so within 1-3 h, particularly when the animals are foraging so,
during such periods estimations of
sO2 from
fH will have to involve adjustments to equations (2) and (3) based on
the Q10 values determined from the data from the SC animals
(Table 1). A convenient way of
incorporating temperature as an influence on
s
O2 is to
include Q10 in the regression. There are linear relationships
between log
O2
and Tb for resting and active Sauromalus hispidus
(an iguanid) and the marine iguana between 25 and 35°C
(Bennett, 1972
;
Bennett et al., 1975
) and
between logfH and Tb for resting and active
S. hispidus between the same temperatures
(Bennett, 1972
). Thus, it is
assumed that there are also linear relationships between logfH and
Tb for resting and active marine iguanas between 27 and
36°C. Given that there is no difference between the slopes of the pooled
regression equations (Table 2),
the relationship between the two lines can be ascribed to a Q10
effect, where the Q10 can be determined from the intercepts. In the
present case Q10=1.93, which is similar to the Q10
values derived from the pre-exercise and maximum fH values in
Table 1. If the average slope
of the pooled regression equations for the two temperatures is used (see
Table 2), the relationship
between s
O2,
fH and Tb is shown in
Fig. 5 and is described by the
following equation:
![]() | (5) |
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An interesting aspect of the present data is the fact that the increase in
sO2 in response
to an increase in Tb is the result of an increase in
fH, with no significant change in sO2 pulse. However,
during exercise at both temperatures, the increases in fH are
insufficient to provide all of the additional O2 required and there
are significant increases in the sO2 pulses. Consequently, the
situation arises (as illustrated in Fig.
2) whereby an fH value of around 60 beats
min-1 is related to an
s
O2 value of
approximately 0.4 ml g-1 h-1 at 27°C, when the
animal is exercising maximally, and to an
s
O2 value of
approximately 0.1 ml g-1 h-1 at 36°C during the
pre-exercise period. This means, of course, that the sO2 pulse is
fourfold greater during the former than during the latter. It is apparent from
equations 1-4 and 6-9 and Fig. 9 of Bennett
(1972
) that a similar
phenomenon occurs in S. hispidus and in Varanus gouldii,
when activity is the result of electrical stimulation, although in the latter
species, the values of fH during `exercise' at 27°C and while at
`rest' at 36°C do not actually overlap. On the basis of a study on
Iguana iguana and Varanus exanthematicus at 35°C, it
would seem that the major contribution to the increase in sO2 pulse
during exercise is a twofold increase in
CaO2C
O2
(Gleeson et al., 1980
).
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Acknowledgments |
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References |
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