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First published online June 6, 2005
Journal of Experimental Biology 208, 2269-2276 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01657
Digestive state influences the heart rate hysteresis and rates of heat exchange in the varanid lizard Varanus rosenbergi
1 Adaptational and Evolutionary Respiratory Physiology Laboratory,
Department of Zoology, La Trobe University, Melbourne, Victoria 3086,
Australia
2 School of Biosciences, University of Birmingham, Birmingham B15 2TT,
UK
* Author for correspondence (e-mail: timothy.clark{at}latrobe.edu.au)
Accepted 18 April 2005
To maximize the period where body temperature (Tb)
exceeds ambient temperature (Ta), many reptiles have been
reported to regulate heart rate (fH) and peripheral blood
flow so that the rate of heat gain in a warming environment occurs more
rapidly than the rate of heat loss in a cooling environment. It may be
hypothesized that the rate of cooling, particularly at relatively cool
Tbs, would be further reduced during postprandial periods
when specific dynamic action (SDA) increases endogenous heat production (i.e.
the heat increment of feeding). Furthermore, it may also be hypothesized that
the increased perfusion of the gastrointestinal organs that occurs during
digestion may limit peripheral blood flow and thus compromise the rate of
heating. Finally, if the changes in fH are solely for the
purpose of thermoregulation, there should be no associated changes in energy
demand and, consequently, no hysteresis in the rate of oxygen consumption
(
O2). To test
these hypotheses, seven individual Varanus rosenbergi were heated and
cooled between 19°C and 35°C following at least 8 days fasting and
then approximately 25 h after consumption of a meal (mean 10% of fasted body
mass). For a given Tb between the range of 19-35°C,
fH of fasting lizards was higher during heating than
during cooling. Postprandial lizards also displayed a hysteresis in
fH, although the magnitude was reduced in comparison with
that of fasting lizards as a result of a higher fH during
cooling in postprandial animals. Both for fasting and postprandial lizards,
there was no hysteresis in
O2 at any
Tb throughout the range although, as a result of SDA,
postprandial animals displayed a significantly higher
O2 than fasting
animals both during heating and during cooling at Tbs
above 24°C. The values of fH during heating at a given
Tb were the same for fasting and postprandial animals,
which, in combination with a slower rate of heating in postprandial animals,
suggests that a prioritization of blood flow to the gastrointestinal organs
during digestion is occurring at the expense of higher rates of heating.
Additionally, postprandial lizards took longer to cool at
Tbs below 23°C, suggesting that the endogenous heat
produced during digestion temporarily enhances thermoregulatory ability at
lower temperatures, which would presumably assist V. rosenbergi
during cooler periods in the natural environment by augmenting
temperature-dependent physiological processes.
Key words: metabolic rate, metabolism, digestion, rate of oxygen consumption, body temperature, reptile, goanna, fasting, postprandial
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