AimWe test the hypothesis that endotherm body temperature varies with diet.LocationGlobal terrestrial ecosystems.MethodsWe compile data from the literature on diet and body temperature in mammals and birds. We analyse these and demonstrate global macrophysiological patterns.ResultsIn mammals, carnivores overall have a lower mean body temperature (Tb) than either herbivores or omnivores. However, within carnivores, those taking vertebrate prey have a higher mean Tb than predators of invertebrates. Among herbivores, species eating grass, leaves or seeds have the highest mean Tb, those taking fruit an intermediate mean Tb and those taking flowers or nectar the lowest mean Tb. These patterns are robust to the influence of body mass and phylogenetic non-independence. In birds the relationship between Tb and diet is complicated by a significant inverse relationship between body mass and Tb and strong dietary niche conservation within lineages. After allowing for body mass, herbivores show an identical qualitative pattern to mammals, whereas carnivores show the opposite trend to mammals: those taking invertebrate prey have a higher mean Tb than those taking vertebrates.Main conclusionsThere is a significant relationship between diet and Tb in mammals. Birds show a qualitatively similar but non-significant pattern. Published studies show that in reptiles and fish herbivory is largely confined to species that can maintain a relatively high Tb either by living in warm environments or through behavioural thermoregulation. We therefore propose a general relationship for all vertebrates: herbivory requires a warmer body than carnivory. The basal metabolic rate (BMR) will then be higher in herbivores because of the universal relationship between BMR and Tb, together with the higher maintenance costs of their longer guts. We suggest that the evolution of endothermy was a key factor in the widespread incidence of herbivory in mammals.