in animals, the ability to maintain a relatively constant internal temperature (about 37° C 【99° F】 for mammals, about 40° C 【104° F】 for birds), regardless of the environmental temperature. The ability to maintain an internal temperature distinguishes these animals from cold-blooded, or poikilothermic, animals, which usually have about the same temperature as their environment. Warm-blooded animals are able to remain active in situations in which cold-blooded ones cannot. Body temperatures of homoiotherms are kept at a constant value by regulatory mechanisms that counteract the effects of the external environment. In cold environments, regulatory mechanisms maintain body temperature by increasing heat production and decreasing heat loss. In hot environments, regulatory mechanisms maintain body temperatures by increasing heat loss. Within a neutral range of several degrees (27° to 31° C 【81° to 88° F】 for man), neither heat gain nor heat loss is necessary to maintain body temperature.

Shivering, a regulatory mechanism of many warm-blooded animals, increases heat production. hibernation, another mechanism used by certain warm-blooded animals, decreases heat loss by means of a general slowing-down of bodily functions. panting and perspiring are mechanisms for increasing heat loss.