Field test of a paradigm: hysteresis of heart rate in thermoregulation by a free-ranging lizard (Pogona barbata)

The discovery that changes in heart rate and blood flow allow some reptiles to heat faster than they cool has become a central paradigm in our underst anding of reptilian thermoregulation. However, this hysteresis in heart rat e has been demonstrated only in simplistic laboratory heating and cooling t rials, leaving its functional significance in free-ranging animals unproven . To test the validity of this paradigm, we measured heart rate and body te mperature (T-b) in undisturbed, free-ranging bearded dragons (Pogona barbat a), the species in which this phenomenon was first described. Our field dat a confirmed the paradigm and we found that heart rate during heating usuall y exceeded heart rate during cooling at any T-b. Importantly, however, we d iscovered that heart rate was proportionally faster in cool lizards whose T -b was still well below the 'preferred T-b range' compared to lizards whose T-b was already close to it. Similarly, heart rate during cooling was prop ortionally slower the warmer the lizard and the greater its cooling potenti al compared to lizards whose T-b was already near minimum operative tempera ture. Further, we predicted that, if heart rate hysteresis has functional s ignificance, a 'reverse hysteresis' pattern should be observable when lizar ds risked overheating. This was indeed the case and, during heating on thos e occasions when T-b reached very high levels (>40 degrees C), heart rate w as significantly lower than heart rate during the immediately following coo ling phase. These results demonstrate that physiological control of thermor egulation in reptiles is more complex than has been previously recognized.