Heat transfer in a microvascular network: The effect of heart rate on heating and cooling in reptiles (Pogona barbata and Varanus varius)

Thermally-induced changes in heart rate and blood flow in reptiles are beli eved to be of selective advantage by allowing animal to exert some control over rates of heating and cooling. This notion has become one of the princi pal paradigms in reptilian thermal physiology. However, the functional sign ificance of changes in heart rate is unclear, because the effect of heart r ate and blood flow on total animal heat transfer is not known. I used heat transfer theory to determine the importance of heat transfer by blood flow relative to conduction. I validated theoretical predictions by comparing th em with field data from two species of lizard, bearded dragons (Pogona barb ata) and lace monitors (Varanus varius). Heart rates measured in free-rangi ng lizards in the field were significantly higher during heating than durin g cooling, and heart rates decreased with body mass. Convective heat transf er by blood flow increased with heart rate. Rates of heat transfer by both blood flow and conduction decreased with mass, but the mass scaling exponen ts were different. Hence, rate of conductive heat transfer decreased more r apidly with increasing mass than did heat transfer by blood flow, so that t he relative importance of blood flow in total animal heat transfer increase d with mass. The functional significance of changes in heart rate and, henc e, rates of heat transfer, in response to heating and cooling in lizards wa s quantified. For example, by increasing heart rate when entering a heating environment in the morning, and decreasing heart rate when the environment cools in the evening a Pogona can spend up to 44 min longer per day with b ody temperature within its preferred range. It was concluded that changes i n heart rate in response to heating and cooling confer a selective advantag e at least on reptiles of mass similar to that of the study animals (0.21-5 .6 kg). (C) 2000 Academic Press.