An animal that generates its own body heat internally, maintaining a relatively constant body temperature regardless of environmental conditions.
From Greek 'endo-' (within) and 'therme' (heat); coined in the early 1900s to replace the older term 'warm-blooded,' which scientists found imprecise and misleading.
Mammals and birds are endotherms, which is why you can survive in freezing temperatures while a reptile would slow down completely—but this heat-generating superpower requires eating a lot more food than cold-blooded animals!
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