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An animal that is dependent on or capable of the internal generation of heat.
However, the problem is severe for an endotherm, particularly a small endotherm, which must maintain a steady internal temperature and has a considerably greater demand for respiratory oxygen.
Extant birds are accomplished endotherms and many maintain the highest body temperatures.
Endotherms are animals that maintain a constant body temperature even when environmental temperatures fluctuate. In contrast, the environment regulates the body temperature of ectotherms and thus their metabolic rate tends to be lower than the metabolic rates of endotherms, which must maintain a high, constant body temperature.
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