What does endotherm mean?
An endotherm is an animal that generates most of its own body heat through internal metabolic processes rather than depending on the surrounding environment, allowing it to maintain a relatively stable body temperature across a wide range of external conditions. Mammals and birds are the classic examples: their high metabolic rates keep them warm in cold weather and active regardless of the season, in contrast to ectotherms such as fish, amphibians, and reptiles, whose body temperature tracks that of their surroundings. The term comes from Greek roots meaning "heat within," and it appears mainly in scientific writing on biology, physiology, and evolution, where everyday speech would use the looser label "warm-blooded." Endothermy carries significant costs — endotherms must eat far more than similarly sized ectotherms to fuel their internal furnace — but it buys independence from climate and sustained activity, one of the great trade-offs in animal evolution.
nounAn animal that regulates its body temperature internally, maintaining it largely constant through metabolic heat production rather than relying on external sources.
- An animal that maintains a relatively constant body temperature through internal metabolic heat production; a warm-blooded animal.
"Unlike reptiles, birds and mammals are endotherms, keeping their bodies warm even in freezing conditions."
"As an endotherm, the Arctic fox can remain active throughout the polar winter despite temperatures far below freezing."
"Scientists once assumed all dinosaurs were ectotherms, but evidence of rapid growth suggests some may have been endotherms."
Standard regular plural; used when referring to multiple warm-blooded animals or to endotherms as a class.
"Endotherms can forage and hunt at night and in winter, when most ectotherms are sluggish."
You're an endotherm — which is why you burn through roughly 60% of your food energy just staying warm.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of endotherm
Endotherm derives from Greek elements: "endo-" meaning "within" and "-therm" from "thermos," meaning "heat." The word was coined in the nineteenth century within comparative physiology as scientists formalized the distinction between animals that produce their own heat and those that absorb it externally. It belongs to a family of learned Greco-Latin coinages sharing the therm- root, including thermostat, hypothermia, and exothermic, all formed during the modern era of scientific terminology.
Related word forms
How endotherm is actually used
A technical term used chiefly in biology, zoology, and physiology. In everyday speech, "warm-blooded animal" is more common. Strictly speaking, endothermy (internal heat production) and homeothermy (stable body temperature) are distinct concepts — some animals are endothermic without being fully homeothermic — but the terms are often treated as near-equivalent informally.
Easily confused with endotherm
An ectotherm depends on external heat sources to regulate body temperature, whereas an endotherm generates its own heat metabolically.
"Endothermic" is an adjective describing processes (especially chemical reactions) that absorb heat, while "endotherm" is the noun for a warm-blooded animal.