Fires of Life: Endothermy in Birds and Mammals

Abstract

This book tackles one of the most important and debated innovations in the evolution of life, the evolution of endothermy in mammal and bird lineages. Barry Lovegrove presents a lucid critique of current models of the evolution of endothermy, particularly single-cause models, and presents a novel “triphasic” model for which endothermy evolves in pulses throughout the history of mammals and birds in response to several selective factors. The ideas presented within this book will surely generate discussions and help advance the field. I will confess that my major professor during my Ph.D. program was John Ruben, coauthor of “Endothermy and activity in vertebrates” (Bennett and Ruben 1979), an early and influential presentation of the aerobic capacity model for the evolution of endothermy, so I am vested in the logic behind that argument. Consequently, I naturally approached this book with aerobic capacity model preconceptions and a critical eye. The book is divided into 2 parts: the first 9 chapters collectively focus on fossils and paleoclimates relevant to the evolution of endothermy, and the second 9 focus on patterns of thermoregulation in extant birds and mammals and their evolutionary implications. Three appendices provide descriptions of heat-generating pathways available to vertebrates, discussions of nasal evaporative cooling, water balance and energetics, and a vertebrate phylogeny including aforementioned fossil and extant organisms. Lovegrove defines endothermy as “the capacity to produce heat on demand from within an animal” (p. 6), and argues that basal metabolic rate (i.e. minimum existence metabolic rate) is primarily generated from metabolic activity in central organs (e.g., heart, kidney, liver, intestines) rather than muscles. Considering basal and exercise or thermogenic metabolic rates as processes controlled primarily by central and exercise organs, respectively, is a useful generalization. It is, however, not strictly correct, perhaps especially for birds whose large flight muscle masses require resting maintenance costs that often contribute significantly to basal metabolic rate (e.g., Chappell et al. 1999). Nevertheless, the idea that basal and maximal metabolic rates may not be tightly coupled phenotypically, at least in extant vertebrates (e.g., Petit et al. 2013, Swanson et al. 2017), as predicted by the aerobic capacity model, provides fodder for other evolutionary scenarios. Chapter 2 reviews terrestrial adaptations in early tetrapods that facilitated the evolution of endothermy, including the amniotic egg and adaptations to reduce water loss, to allow for terrestrial locomotion, and to digest land applyparastyle "fig//caption/p[1]" parastyle "FigCapt"