The thermal status of an animal is the result of a combination of physical and physiological factors. In poikilotherms, it may be possible to separate these more easily than in homeotherms, where the presence of control mechanisms can mask the processes occurring. The thermal time constant of a poikilotherm has been shown to be a useful measure of its thermal behaviour, and to vary with the physiological status of the animal. A simple model is developed to show how the thermal time constant is related to the physics of heat exchange. The derived thermal time constant is shown to scale as body mass raised to the power 2/3, and this is compared with results on lizards heating and cooling in water, taken from the literature.
When heat exchange in air is considered, the concept of boundary layer resistance leads to a useful simplification. The thermal time constants in air taken from the literature show that the boundary layer resistance is approximately constant.