An animal that can generate and maintain heat within its body independently of the environmental temperature. Mammals and birds are endotherms and hence are often described as being warm-blooded. Moreover, some fishes, insects, and reptiles can generate heat internally to maintain their body temperature to at least some degree. For example, certain billfishes, tunas, and certain sharks conserve metabolic heat using vascular countercurrent heat exchangers; these are known as regional endotherms. See homoiothermy. Compare ectotherm; heterotherm.