Introduction

Endothermy in extant vertebrates

Birds and mammals generally maintain aerobic metabolism at rates about 5 to 10-fold those of reptiles of equivalent size and body temperature. These high rates of endogenous heat production, or endothermy, along with the insulation afforded by feathers and fur, enable birds and mammals to maintain thermal homeostasis over a wide range of ambient temperatures. As a result, these taxa are able to thrive in environments with cold or highly variable thermal conditions and in nocturnal niches generally unavailable to ectothermic vertebrates. Furthermore, the increased aerobic capacity associated with endothermy allows them to sustain activity levels well beyond the capacity of ectotherms (Bennett, 1991). With some noteworthy exceptions, ectotherms, such as reptiles, typically rely on non-sustainable, anaerobic metabolism for all activities beyond relatively slow movements. Although capable of often spectacular bursts of intense exercise, ectotherms generally fatigue rapidly as a result of lactic acid accumulation. Alternatively, endotherms are able to sustain relatively high levels of activity for extended periods of time, enabling these animals to forage widely and to migrate over extensive distances. The physiological capacity of birds and bats to sustain long-distance powered flight is far beyond the capabilities of modern ectotherms (Ruben, 1991).

Some large-size fish (including some billfish, tunas and lamnid sharks) and a few snakes (e.g. Python) maintain somewhat greater than ambient core or deep body temperatures (Block, 1991; van Mierop & Barnard, 1978).