Did pathogens facilitate the rise of endothermy?

Abstract

The evolutionary success of endothermy in mammals and birds represents an enduring enigma. Relative to an ectotherm of equivalent body size, endotherms expend many times the energy to maintain high, stable body temperatures. What source or sources of selection could have favored such an energetically costly strategy? Several hypotheses have been proposed for agents of selection that may have facilitated the rise of endothermy, with mixed support for each. Here, I discuss the possibility that an additional agent of selection, pathogens, may have played an important role in the evolution and maintenance of obligate endothermy in mammals and birds. I draw on recent work demonstrating that the immune system is highly thermally sensitive and individuals that maintain warm body temperatures have enhanced ability to reduce pathogen burdens. To further increase immune function when encountering a pathogen, ectotherms often employ the environmentally constrained and costly strategy of behavioral fever, whereas endotherms maintain body temperatures close to the optimum for immune performance at most times, essentially ‘priming’ their immune systems for a rapid response to infection. An evolutionary arms race in an ancient ectotherm, in which better behavioral thermoregulators were favored by increasingly virulent pathogens (and vice versa) could have contributed to the evolution of obligate endothermy in mammals and birds.