The broad-scale ecology of energy expenditure of endotherms

Energy expenditure in animals scales allometrically with body mass, but residual variation is not well understood. We examine the existing data on field metabolic rates (FMR) in endotherms for the potential role of environmental conditions. Across latitude, mass-corrected FMR of 248 bird and mammal populations fall between two constraint lines: a lower bound that increases towards the poles and is driven by environmental factors and an upper bound that is invariant with latitude and may represent physiological limitations. This triangular pattern can be explained statistically with a multipredictor model that combines environmental conditions and species biology (including phylogeny). Lower environmental temperature and longer day length increase FMR, while taxonomy and diet explain much of the remaining variation. Combined, these effects appear to form a diversity of 'metabolic niches' that overall decreases from the tropics to the poles. The potential of factors related to latitude acting as constraints on the ecology and evolution of metabolic strategies in endotherms is discussed.