Physiological control of heat exchange in lizards

Lizards can control rates of warming and cooling by changing blood flow rates and distributions as measured by differences in time constants. I present a new model that estimates a dominant and three subdominant time constants for an animal. Dominant time constants are those typically measured by physiologists. Subdominant time constants describe how spatial temperature gradients in the animal decay, and they are shorter than the dominant time constant. This method allows one to determine time constants for lizards subjected to step changes in environmental temperature. It also allows for determination of time constants for an animal experiencing a complex thermal environment, e.g., shuffling between two microhabitats. Finally, it allows investigators to determine time scales over which control of warming and cooling may be important. Dominant and subdominant time constants were measured during long and short periods of warming and cooling in Iguana iguana, Sceloporus undulatus, and three species of Cordylus. In response to long warming and cooling, only in Iguana did time constants for warming and cooling differ consistently. The limbs are the major site for the control of heat exchange in I. iguana. Insulating the limbs with cotton gauze abolished the difference between warming and cooling in I. iguana. When subjected to simulated shuffling none of the species altered time constants for heating and cooling. This suggests that small (<70 g) lizards are unable to control warming or cooling in response to long or short periods of warming and cooling, while larger animals are able to control warming and cooling only during long periods. Blood flow and heart rate were measured in I. iguana and S. undulatus using a laser doppler blood flow meter. During warming, blood flow increased at both the limbs and torso in I. iguana and at the torso of S. undulatus and Cordylus spp. Blood flow usually returned to prewarming levels upon cooling and appeared to be nearly linearly related to superficial body temperatures. Heart rate followed a pattern similar to blood flow in both species, but appeared to be more closely related to deep body temperatures than to superficial temperatures.