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。

A poikilotherm is an organism whose internal temperature varies considerably. It is the opposite of a homeotherm, an organism which maintains thermal homeostasis. Usually the variation is a consequence of variation in the ambient environmental temperature. Many terrestrial ectotherms are poikilothermic. However some ectotherms remain in temperature-constant environments to the point that they are actually able to maintain a constant internal temperature (i.e. are homeothermic). It is this distinction that often makes the term "poikilotherm" more useful than the vernacular "cold-blooded", which is sometimes used to refer to ectotherms more generally.