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 centers on whether the underlying power law describing 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, failed to control for phylogeny. We used phylogenetic generalized least-squares models to estimate allometric slopes for both basal metabolic rate (BMR) and field metabolic rate (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 were met. Analyses that assumed no phylogenetic signal in the data (species-level analysis) or a strong phylogenetic signal (independent contrasts), therefore, 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.
Highlights
The empirical values of scaling exponents relating physiological variables to body size are held to reflect important general constraints (Schmidt-Nielsen 1984, Peters 1986, O’Connor et al 2007; see Dodds et al [2001] and Glazier [2005] for reviews on the scaling of metabolic rates)
maximum likelihood (ML) k of body mass alone was high for both the basal metabolic rate (BMR) and the field metabolic rate (FMR) data set (Table 1; Appendix B: Table B1 and B2)
These results indicate that both absolute metabolic rates and body mass taken independently exhibit a strong phylogenetic signal
Summary
The empirical values of scaling exponents relating physiological variables to body size are held to reflect important general constraints (Schmidt-Nielsen 1984, Peters 1986, O’Connor et al 2007; see Dodds et al [2001] and Glazier [2005] for reviews on the scaling of metabolic rates). An early theoretical model proposes that metabolic rate of organisms maintaining a constant body temperature is proportional to the rate of heat loss through their body surface area. Because surface area scales to the two-thirds power of volume and mass, the model predicts that the allometric exponent of metabolic.
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