A homeothermic organism has a generally constant internal temperature. Other organisms are called poikilotherms, or, for the sake of consistency, heterotherms. Only birds and mammals are true homeotherms, for they are also endotherms (they use an internal heat source) and their temperature is regulated. They are called " warm-blooded ".
The difference between poikilotherms and homeotherms is not as clear as it seems to be. Some homeotherms (such as the hedgehog or young black swift) can let their body temperature drop, and some poikilotherms (such as tuna) can warm their body.
The difference between poikilotherms and endotherms should not be confused with the difference between endotherms and ectotherms. The heat source of an ectothermic organism comes exclusively or almost exclusively from the exterior. Its internal temperature is therefore the same temperature as the surrounding environment.
Fish are not far from being homeotherms when the water temperature varies little. A poikilothermic animal can also be an endotherm. Even if its body temperature varies, it can, in fact, remain higher than that of the surrounding environment.
In animals, behavioural strategies can compensate for the absence of internal regulation. Beehives and termite mounds are thermoregulated by bees and termites. A lizard understands how to take advantage of the sun's warmth or, conversely, the cool shade.
Only birds and mammals are true homeotherms. Credits: DR.