What does ectotherm mean?
An ectotherm is an animal whose body temperature is governed chiefly by heat from its environment rather than by internal metabolic processes. Reptiles, amphibians, fish, and most invertebrates fall into this category: a lizard warms itself by basking in sunlight, while a fish's internal temperature closely tracks the surrounding water. This contrasts sharply with endotherms — birds and mammals — which burn significant amounts of energy to maintain a constant internal warmth. Because they do not fuel their own heating systems, ectotherms typically need far less food than comparably sized endotherms, though their activity levels are constrained by external temperatures. The word derives from Greek roots meaning 'outside heat' and is standard in scientific writing; the older popular label 'cold-blooded' persists but is discouraged as imprecise, since some ectotherms achieve quite stable body temperatures through behavioral regulation.
nounAn animal that regulates its body temperature primarily by external means, relying on environmental heat sources such as sunlight or warm surfaces rather than generating much metabolic heat of its own.
- An organism whose body temperature depends largely on external heat sources rather than internal metabolic heat production.
"As an ectotherm, the lizard basks on the sun-warmed rock each morning to raise its body temperature before hunting."
"Because it is an ectotherm, the desert tortoise spends cold mornings sheltering underground until the sun warms its burrow."
"The textbook chapter contrasted the energy demands of an ectotherm with those of a similarly sized endotherm."
Regular plural; used when discussing multiple organisms or groups of animals collectively.
"Most reptiles and amphibians are ectotherms."
A snake can survive on far less food than a mammal its size precisely because, as an ectotherm, it doesn't burn calories keeping itself warm.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of ectotherm
Ectotherm comes from the combining forms 'ecto-' (from Greek ektos, 'outside') and '-therm' (from Greek thermē, 'heat'), literally meaning 'heat from outside.' It was coined within modern zoology as a counterpart to 'endotherm,' reflecting the same Greek roots seen in words like thermal and thermostat. Its cognates include exothermic and ectoderm, which share either the 'outside' prefix or the 'heat' root.
How ectotherm is actually used
Ectotherm is a technical term used mainly in biology and zoology; in everyday speech, 'cold-blooded' is more common, though scientists often avoid that term because it is misleading — many ectotherms maintain fairly stable body temperatures behaviorally.
Easily confused with ectotherm
An endotherm generates its own body heat internally through metabolism (e.g., mammals and birds), whereas an ectotherm relies on external sources for warmth.