Exploring the Evolution of the Basal Metabolic Rate in Bats

Abstract Rates of energy expenditure are thought to play a crucial role in shaping the evolution of the behavior, ecology, and physiology of organisms. The most frequently measured rate of energy expenditure is the basal metabolic rate (BMR). Mammals show an enormous range in BMR, with the variability spanning five orders of magnitude. Here we review current hypotheses proposed to explain this variation and test these hypotheses using data from 95 species of bats from 10 families. Our analysis of the evolution of BMR in bats indicates that there is a significant phylogenetic component to BMR: closely related species have more similar rates of basal metabolism than more distantly related species. After controlling for this effect, the most important determinant of BMR in bats is body size, explaining 84% of the variation. We also found that several other life-history and ecological factors (independent of phylogeny and body size) played an important role in the evolution of BMR in bats, offering mixed support to current hypotheses. We also explore whether variation in BMR has allowed rates of diversification to change over evolutionary time but find no evidence to suggest that this has occurred. INTRODUCTION It is widely assumed that energy plays a pivotal role in shaping the behavior, ecology, and physiology of organisms. In fact, much of contemporary ecological theory attempts to understand the link between energetics and factors such as patterns of species richness, reproductive effort, distribution, activity patterns, and other life-history traits (e.g., Alexander, 1999; McNab, 1992a; Thompson, 1992). Among the several energetic parameters used to investigate such links, basal metabolic rate (BMR) stands as one of the most important. Originally defined as a way to index the minimum rate of energy necessary to maintain homeostasis, BMR is by far the most widely measured energetic parameter. In mammals, BMR accounts for more than 50% of the total free-ranging energy expenditure (Nagy et al., 1999; Speakman, 2000) and consequently may have overt ecological and evolutionary significance. Operationally, BMR is defined as occurring in postabsorptive animals within.