Today, only birds and mammals are able both to produce their own body heat (this is known as endothermy) and maintain it at a high, stable temperature (homeothermy). (sciencedaily.com)
For example, from the family cat whose «temperature is about the same whether she is asleep in the shade or running about in the sun» we learn about the principle of endothermy («heat from within») that mammals use to keep their body temperatures more or less constant. (newscientist.com)
«This strategy of deriving energy from the sun is described as ectothermy (outside heat) and gives reptiles the advantage of having a warm body without the high good costs associated with endothermy,» says McGowan. (newscientist.com)