Meeting Abstract
The evolutionary success of endothermy is an enduring enigma. The energetic cost of maintaining a constant, high internal body temperature is so severe that endotherms must consume as much as thirty times the energy per unit time than an ectotherm of similar body size. What source, or sources, of selection could have favored such an energetically costly strategy? Several hypotheses for the evolution of endothermy have been proposed, including the “thermoregulation,” “aerobic capacity,” and “parental care” models. None of these models enjoy unequivocal support, and debate continues over the true cause or causes of the origin and maintenance of endothermy over evolutionary time. Here, I argue in favor of another potentially important source of selection in the evolution of endothermy. I draw on recent work demonstrating that fever enhances the performance of immune systems to increase the efficiency by which individuals can defend themselves against pathogens. Thus, fever-range temperatures represent the thermal optimum for immune function, and endotherms permanently maintain internal body temperatures very close to this optimum, essentially ‘priming’ their immune systems for a rapid response to infection. Conversely, ectotherms employ behavioral fever, whereby they must move into warmer microclimates in order to increase their body temperatures during an infection. Behavioral fever is slow, has opportunity costs, and is strongly constrained in homogeneous thermal environments, all of which suggest that it is an inefficient way to mount a rapid immune response. An evolutionary arms race in an ancient ectotherm, whereby better behavioral thermoregulators were favored by increasingly virulent pathogens (and vice versa) could have led to the evolution of endothermy in the stem groups of modern mammals and birds.
