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

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Abstract
The fact that animals evolved to regulate stable body temperatures 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. Temperature-independent activity rates in ectotherms are 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 byproduct of high activity rates. Large and highly active animals inevitably heat up during strenuous activity; to sustain high rates of activity their enzymes and other macromolecules must function at higher than ambient temperatures. Biochemical restructuring for activity at high tissue temperatures evolved because it 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. Animals that evolved preactivity warm-up by shivering and basking also became preadapted for homeothermy; 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 an additional positive selective pressure if there is an advantage to broadening 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.