Phoenix from the Ashes: Fire, Torpor, and the Evolution of Mammalian Endothermy

The evolution of endothermy in mammals and birds has been widely debated. Endothermy is characterized by high endogenous heat production via combustion of metabolic fuels. This differs from ectothermy in most living organisms, which generally do not produce substantial amounts of internal heat for thermoregulation (Tattersall et al., 2012; Withers et al., 2016). Endogenous heat production is energetically very costly. In comparison to ectothermic terrestrial vertebrates, namely the amphibians and reptiles, the minimum metabolic rate (MR) of normothermic or homeothermic (high constant body temperature, T_b) animals at rest is about 4–8-fold higher in the endotherms. This difference is even more pronounced at low ambient temperatures (T_a) at which the T_b of ectotherms follows T_a, and the MR decreases to even lower levels. In contrast, the T_b of homeothermic endotherms remains high and constant over a wide range of T_a. Therefore, to compensate for increased heat loss at low T_a, MR of especially small mammals and birds must increase substantially and can be 100-fold or more of that in ectotherms (Bartholomew, 1982). Of course this high MR requires a substantial uptake of food and in endotherms much of this chemical energy is simply converted into heat for thermoregulation rather than growth or reproduction as in ectotherms.