Physiological control of warming and cooling during simulated shuttling and basking in lizards.

Differences in warming and cooling rates in basking lizards have long been thought to be brought about by adjustments in heart rate and blood flow. We examined the physiological control of warming and cooling in Iguana iguana, Sceloporus undulatus, and three species of Cordylus by measuring time constants, heart rate, and superficial capillary blood flow. Previously, techniques have not been available to measure time constants in shuttling animals. Using a combination of rapid measurements of temperature and blood flow and numerically intensive parameter-fitting methods, we measured dominant and subdominant time constants in lizards subjected to periods of both simulated basking and simulated shuttling. Cutaneous blood flow and heart rate were measured using laser Doppler flowmeters. Of the three, only the larger I. iguana measurably altered rates of warming and cooling during basking. During shuttling, none of the species effectively controlled warming and cooling. During both basking and shuttling, blood flow and heart rate tended to change in predicted directions. Superficial blood flow correlated with surface temperature while heart rate correlated more closely with core temperature. Changes in superficial blood flow and heart rate varied relatively independently in I. iguana. The techniques used here provide a better understanding of the ability of these species to control thermoregulation.