Phylogeny and metabolic scaling in mammals.

Abstract

The scaling of metabolic rates to body size is widely considered to be of great biological and ecological importance, and much attention has been devoted to determining its theoretical and empirical value. Most debate centres on whether the underlying power law determining metabolic rates is 2/3 (as predicted by scaling of surface area/volume relationships) or 3/4 ('Kleiber's Law'). Although recent evidence suggests that empirically derived exponents vary among clades with radically different metabolic strategies, such as ectotherms and endotherms, models, such as the Metabolic Theory of Ecology, depend on the assumption that there is at least a predominant, if not universal, metabolic scaling exponent. Most analyses claimed to support the predictions of general models however fail to control for phylogeny. We used phylogenetic generalised least squares models to estimate allometric slopes for both basal metabolic rates (BMR) and field metabolic rates (FMR) in mammals. Metabolic rate scaling conformed to no single theoretical prediction, but varied significantly among phylogenetic lineages. In some lineages we found a 3/4 exponent in others a 2/3 exponent, and in yet others exponents differed significantly from both theoretical values. Analysis of the phylogenetic signal in the data indicated that the assumptions of neither species-level analysis nor independent contrasts are met. Analyses that assumed no phylogenetic signal in the data (species level analysis) or a strong phylogenetic signal (independent contrasts) returned estimates of allometric slopes that were erroneous in 30% and 50% of cases respectively. Hence, quantitative estimation of the phylogenetic signal is essential for determining scaling exponents. The lack of evidence for a predominant scaling exponent in these analyses suggests that general models of metabolic scaling, and macro-ecological theories that depend on them, have little explanatory power.