The range of ambient temperature in which normal metabolism provides enough heat to maintain an essentially constant body temperature in homeothermic animals. The limits of the zone depend on the species and breed of an animal and its age, sex, degree of acclimatization, how it is fed, and even the time of day. The zone is narrow for the young of a species (e.g., 29°–30°C for a chick) and wide for a well- fed large adult (e.g., −30° to +25°C for a cow).