Environmental determinants of minimum body temperature in mammals

Physiological regulation of body temperature, set at a high level, is one of the key features of endothermic homeotherms, such as birds and mammals. However, many mammals and some birds have evolved the ability for temporal down-regulation of core body temperature. We investigated how variation in environment temperature and habitat primary productivity determine variation in daily body temperature down-regulation among mammalian species. Nearly half of the variation in minimum daily body temperature among species was explained by variation in both primary productivity and environmental temperature. Mammals expressing low minimum body temperature inhabited regions of low annual temperature with wide daily and seasonal temperature variation. Simultaneously, those regions were characterized by low productivity and low seasonality in productivity. Furthermore, regions characterized by a high level of among-year variation in environmental temperature, but not in primary productivity, were inhabited by species with low minimum body temperature, but only by those adapted to relatively humid conditions. Our results suggest that daily heterothermy can be selectively advantageous in the environmental circumstances when high energetic demands for maintaining endothermic homeothermy, physiological regulation of a high and stable body temperature, cannot be supported. The results corroborate the hypothesis that mammals that have evolved daily down-regulation of body temperature may have higher chances of surviving extinction events caused by climatic changes. Therefore, daily heterothermy adaptation in contemporary mammals represents a mechanism for surviving the ongoing global warming.