The relationships among species' physiological capacities and the geographical variation of ambient climate are of key importance to understanding the distribution of life on the Earth. Furthermore, predictions of how species will respond to climate change will profit from the explicit consideration of their physiological tolerances. The climatic variability hypothesis, which predicts that climatic tolerances are broader in more variable climates, provides an analytical framework for studying these relationships between physiology and biogeography. However, direct empirical support for the hypothesis is mostly lacking for endotherms, and few studies have tried to integrate physiological data into assessments of species' climatic vulnerability at the global scale. Here, we test the climatic variability hypothesis for endotherms, with a comprehensive dataset on thermal tolerances derived from physiological experiments, and use these data to assess the vulnerability of species to projected climate change. We find the expected relationship between thermal tolerance and ambient climatic variability in birds, but not in mammals-a contrast possibly resulting from different adaptation strategies to ambient climate via behaviour, morphology or physiology. We show that currently most of the species are experiencing ambient temperatures well within their tolerance limits and that in the future many species may be able to tolerate projected temperature increases across significant proportions of their distributions. However, our findings also underline the high vulnerability of tropical regions to changes in temperature and other threats of anthropogenic global changes. Our study demonstrates that a better understanding of the interplay among species' physiology and the geography of climate change will advance assessments of species' vulnerability to climate change.

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Aim Physiological 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. Location Global. Methods We 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. Results Overall, 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 conclusions While 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.

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Aim To understand how climatic conditions influence the geographical distributions of species and their potential responses to climate change, we investigated the relationships between the thermal tolerances of species and the size and limits of their distributions. We tested two hypotheses for endotherms: the climatic variability hypothesis, which predicts increases in range size with increasing breadth of thermal tolerance, and the climatic extreme hypothesis, which predicts that range limits are related to thermal tolerance limits. Furthermore, we tested whether these relationships differ between temperate and tropical areas. Location Global. Time period Present. Major taxa studied Birds and mammals. Methods We compiled data on thermal tolerances that had been measured in physiological experiments for 453 endothermic species, along with information on geographical ranges and climatic conditions. We applied phylogenetic generalized least square regressions to test for relationships between thermal tolerance and (a) range size or limits and (b) breadth and extremes of the climatic conditions that each species experiences across its distribution. Results We found that range size was not related to the breadth thermal tolerance for endotherms. However, the range limits at high latitudes as well as the minimum temperatures experienced by species were closely related to the physiological cold tolerances of species. These relationships were particularly strong in temperate regions, but these patterns were not found in the tropics. Main conclusions Our results are inconsistent with the predictions of the climatic variability hypothesis, but are in line with the predictions of the climatic extreme hypothesis. Furthermore, the factors determining species distributions do not appear to be the same in tropical and temperate regions. Our study emphasizes the need to combine spatially explicit distribution models with information from physiological experiments in order to capture regional differences and improve predictions of the responses of species to climate change.