Temperature regulation (or thermoregulation) can be defined as the control of the temperature(s) of a body under finite environmental conditions. Regulation is achieved by controlling heat gain and heat loss between the body and the environment through the utilization of autonomic and behavioral mechanisms. Birds and mammals have evolved a battery of behavioral and autonomic motor outputs (i.e., effectors) to regulate their core body temperatures within narrow limits when subjected to a wide range of ambient temperatures (Prosser and Heath, 1991). In some cases, reptiles, fish, and amphibians are able to regulate their body temperatures by means of behavioral responses. Invertebrates are temperature conformers, meaning that their body temperatures usually are about the same as that of their surrounding environment. Even the most primitive organisms display thermotropism (i.e., the tendency to turn toward or away from a heat source), and many temperature conformers have distinct behavioral thermoregulatory responses (Whittow, 1970; Prosser, 1973). It should be remembered that ambient temperature is probably the most critical environmental factor in limiting an organism's choices among possible habitats. Thus the development of temperature regulation undoubtedly has played a major role in evolution.

A brief historical perspective

The existence of thermoregulatory systems probably was one of the earliest discoveries of an involuntary homeostatic process, as reviewed by Lomax (1979), Folk (1974), and Hensel (1981).