Global variation in thermal physiology of birds and mammals: evidence for phylogenetic niche conservatism only in the tropics

AimPhysiological traits that approximate the fundamental climatic niche - the climatic conditions where a species can survive - are the outcome of adaptation to the environment under historical and current environmental constraints. If a large amount of the variation in physiological traits among species can be explained by their phylogeny rather than by contemporary environmental conditions, this would indicate phylogenetic conservatism in physiological traits, i.e. the tendency of species to retain their ancestral physiology over time. Here, we evaluate the relative contributions of phylogeny and environment to explain the variation in physiological traits of birds and mammals at the global level, as well as separately for tropical versus temperate species. LocationGlobal. MethodsWe compiled a large data set from the literature, on the thermal traits and basal metabolic rates of 552 endotherms (255 bird and 297 mammal species) as measured in physiological experiments, along with phylogenetic, geographical and climatic data. Our analyses, which were performed separately for birds and mammals, partitioned the variation in comparative physiological data into the relative contributions of phylogenetic and environmental distance matrices. ResultsOverall, the current environment explained a larger amount of variation in thermal traits among species than the phylogeny. However, we found that phylogeny was much more important than current environment for explaining the variation in physiological traits in the tropics, whereas environment was more important than phylogeny in temperate species. Main conclusionsWhile evidence for phylogenetic conservatism in physiological traits at the global level was weak, results for tropical species suggest phylogenetic conservatism in their physiological traits. These results indicate a stronger tendency in tropical species to retain their ancestral thermal traits, which might in turn imply a lower physiological adaptability of tropical species to ongoing and future climate change.