Why Have Some Animals Evolved to Regulate a High Body Temperature?

The evolution of thermoregulation in endothermic animals is analyzed by asking two different but related questions: Why have animals evolved to regulate stable body temperatures, and why are the evolved temperature set points generally much higher than ambient temperature? The answer to the first question is probably related to enzyme specialization for the maintenance of high activity rates. By specializing to operate at specific temperatures (either high or low) enzymes improve their potential to promote high rates of substrate turnover while still remaining subject to necessary mechanisms of metabolic control. On the other hand, temperature-independent activity rates in ectotherms have been shown to be associated with comparatively low rates of aerobic metabolism, resulting probably from inefficiency that is a "cost" of biochemical temperature modulations. High temperature set points may have evolved from inability to rapidly dissipate all of the heat produced as a by-product of high activity rates. Large and highly active animals such as birds, mammals, as well as many insects and some present-day reptiles, inevitably heat up during strenuous activity. In order to sustain their high rates of activity for durations in excess of several minutes their enzymes and other macromolecules must function at higher than ambient temperatures. It is concluded that biochemical restructuring for activity at high tissue temperatures has evolved, in part, because it has extended the ability to be maximally active beyond the short time otherwise required to "overheat." The higher the temperature set point, the greater the endurance of high-rate aerobic activity at high ambient temperatures. An animal that has committed its biochemistry to operate at a relatively high body temperature during activity can only initiate activity if it has concomitantly evolved the ability to warm itself. Animals that evolved preactivity warm-up by shivering and basking, for example, also became preadapted for homeothermy, for at least some of the identical mechanisms used for preactivity warm-up are also applied between bouts of activity to achieve homeothermy. The ability to maintain a constant body temperature despite fluctuations of ambient temperature should be under additional positive selective pressure if there is advantage to broaden the temperature niche. The breadth of this niche will depend on energetic constraints limiting the durations and costs pre- and interactivity warm-up and on the ability to dissipate endogenous or exogenous heat loads.