An increase in maximum temperatures as well as the duration and frequency of extreme heat waves is predicted to occur during the 21st century due to anthropogenic climate change. Being able to accurately predict the response of species to climate change is vital in determining species vulnerability to future climate change. In addition, accurate prediction is necessary for the development of meaningful mitigation strategies. When environmental temperatures exceed normothermic body temperatures, endotherms dissipate heat via evaporative water loss. The efficiency of evaporative water loss in dissipating heat, and thus maintaining core body temperatures below lethal limits is dependent on the physiology of an organism and the environmental conditions the organism finds itself in. For instance, and of relevance to this study, thermal conditions within tree cavities may differ vastly from terrestrial and subterranean microsites. In this project we will test the hypothesis that endotherm thermal physiology has evolved in response to the microclimates in tree cavities at both an intra- and interspecific level, using two study systems: 1 - Southern Yellow-billed hornbill (Tockus leucomelas), hereafter referred to as SYBH, and 2 - desert-dwelling rodents. The SYBH exhibits a peculiar breeding strategy whereby the female seals herself in the nest cavity as a protective mechanism against predators. During incubation and part of the chick-rearing period the females are confined to the nest and, as a consequence, the male hornbills are solely responsible for the provision of food to the females and chicks; thus, male and female SYBH experience markedly different environmental conditions. The peculiar reproductive biology of the SYBH provides the opportunity to investigate whether sex-specific utilisation of microclimates has led to the evolution of sex-specific differences in heat tolerance and evaporative cooling efficiency. Similarly, interspecific variation in microsite use among desert rodents provides us with the opportunity to investigate whether these variations in microsite use has led to evolution of species-specific differences in heat tolerance and evaporative cooling efficiency.