Endotherme

An endotherm (from Greek ἔνδον endon "within" and θέρμη thermē "heat") is an organism that maintains its body at a metabolically favorable temperature, largely by the use of heat released by its internal bodily functions instead of relying almost purely on ambient heat. Such internally generated heat is mainly an incidental product of the animal's routine metabolism, but under conditions of excessive cold or low activity an endotherm might apply special mechanisms adapted specifically to heat production. Examples include special-function muscular exertion such as shivering, and uncoupled oxidative metabolism, such as within brown adipose tissue. Only birds and mammals are extant universally endothermic groups of animals. However, Argentine black and white tegu, leatherback sea turtles, lamnid sharks, tuna and billfishes, cicadas, and winter moths are also endothermic. Unlike mammals and birds, some reptiles, particularly some species of python and tegu, possess seasonal reproductive endothermy in which they are endothermic only during their reproductive season. In common parlance, endotherms are characterized as "warm-blooded". The opposite of endothermy is ectothermy, although in general, there is no absolute or clear separation between the nature of endotherms and ectotherms. Endothermy was thought to have originated towards the end of the Permian Period. One recent study claimed the origin of endothermy within Synapsida (the mammalian lineage) was among Mammaliamorpha, a node calibrated during the Late Triassic period, about 233 million years ago. Another study instead argued that endothermy only appeared later, during the Middle Jurassic, among crown-group mammals. Evidence for endothermy has been found in basal synapsids ("pelycosaurs"), pareiasaurs, ichthyosaurs, plesiosaurs, mosasaurs, and basal archosauromorphs. Even the earliest amniotes might have been endotherms. Many endotherms have a larger amount of mitochondria per cell than ectotherms. This enables them to generate heat by increasing the rate at which they metabolize fats and sugars.