The relationship between animals and their thermal environment has been gaining prominence in the fields of ecology and physiology with the increasing concern over climate change. Yet, despite a large body of knowledge on the thermoregulation of temperate and cold-climate endotherms, our functional knowledge of endotherms in warmer climates remains incredibly scarce. In particular, their use of facultative heterothermy, lowering or raising body temperatures to conserve energy and/or water, has been chronically understudied. As well as providing a general overview of some of the forms of heterothermy observed in warm climates, we review the results of field studies on a highly heterothermic nocturnal mammal (the greater hedgehog tenrec, Setifer setosus) and a homeothermic (yet thermally labile) diurnal mammal (the large treeshrew, Tupaia tana). We discuss the costs and benefits of the body temperature variability in warm climates and the evolution of homeothermy in mammals.