Endothermic
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In biology, an endothermic organism is one that maintains a constant body temperature. An endotherm is an animal that is able to keep its internal temperature at a stable level by controlling its metabolic rates. The only organisms that are endotherms are mammals and birds. Because endotherms can control their body temperature they are able to keep their body functioning in environments where ectotherms couldn't survive. A negative of this is that they are forced to keep their body at just the right temperature or they will die. It takes a considerable amount of energy for endotherms to do this. Endotherms have it especially hard in the winter. There isn't enough food to give them energy to keep their metabolic rates stable twenty-four hours a day. To compensate for the lack of food the organism may go into a controlled state of hypothermia called hibernation, or torpor. This purposely drops the body temperature so that not as much energy is being used and the animal won't starve during the winter. Endotherms also expend a great deal of energy in order not to overheat. To avoid overheating, an organism may pant, sweat, lick, or seek shelter or water.
Contrast with: Ectotherm.
In chemistry, an endothermic reaction is one that that requires heat to break the bonds of the reactants.
See also: homeothermic, homoiothermic, exothermic, thermodynamics, chemical thermodynamics.