变温动物也称为外温动物，俗称冷血动物，是没有体内调温系统的动物。一般体温不平，或者以行动来调节体温。如蛇、鳄鱼等较大的变温动物早上需要晒太阳以使体温升高，这样他们才能活动，因此它们几乎活动，夜间休息。调温的方法包括：

尽管同样的环境可以有十倍于温血动物的变温动物存活，可是温血动物大多时候可以把变温动物逼到绝灭，因为温血动物可以找食物的时间比较多。

An ectotherm, from the Greek εκτός (ektós) "outside" and θερμός (thermós) "hot", is an organism in which internal physiological sources of heat are of relatively small or quite negligible importance in controlling body temperature. Such organisms (for example frogs) rely on environmental heat sources, which permit them to operate at very economical metabolic rates. Colloquially, some refer to these organisms as "cold blooded" though such a term is not technically correct, as the blood temperature of the organism varies with ambient environmental temperature. Some of these animals live in environments where temperatures are practically constant, as is typical of regions of the abyssal ocean. In contrast, in places where temperature varies so widely as to limit the physiological activities of other kinds of ectotherms, many species habitually seek out external sources of heat or shelter from heat; for example, many reptiles regulate their body temperature by basking in the sun, or seeking shade when necessary in addition to a whole host of other behavioral thermoregulation mechanisms. In contrast to ectotherms, endotherms rely largely, even predominantly, on heat from internal metabolic processes.