Endotherms are animals that generate their own body heat through metabolism. They are also refered to as homeotherms since they regulate and maintain their internal temperature, or 'warm blooded' since their internal temperature is often higher than ectotherms. Mammals and birds are endotherms, as are you(I would hope).

The endotherm is by nature a more complex creature than the ectotherm, since in addition to much of the same wetware the endotherm must add mechanisms for the accelerated dissipation of heat as well as its retention, so as not to be overwhelmed by environments outside the 'acceptable' temperature range. All mammals are endothermic, as are all birds. Endotherms are able to (on average) expend much more energy over a longer period of time than ectotherms, who must rely on the availability of heat energy in the environment to 'move' their metabolic cycle.

One common feature shared among most endotherms is that of 'flexible' skin covering such as feathers, hair or fur. When the creature in question is cold, these all serve to trap warm air near the skin and prevent loss of heat to wind chill. When it warms up, feathers can be fluffed, as can fur; hair stands on end. All of these serve to release trapped air and allow circulation of ambient air nearer to the skin, increasing heat transfer outward (cooling).

It is the mammals' endothermic nature that allows them to be active at night as well as during the day for long continuous cycles. While mammals simply eat to intake energy, an ectotherm must typically eat and then find a spot in which to bask (typically, a vulnerable one or exposed one) in order to collect heat for digestion. In order to not waste heat, ectotherms typically are inactive when it is night and/or cold. This leaves them vulnerable to warm-blooded predators, if they are land-based.

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