What does ectothermic mean?
"Ectothermic" is an adjective from biology describing organisms whose body temperature depends primarily on heat absorbed from their surroundings rather than on internal metabolic heat production. Reptiles, amphibians, fish, and most invertebrates are ectothermic, which is why lizards bask in sunlight and become sluggish on cool mornings. The everyday synonym is "cold-blooded," but scientists prefer "ectothermic" because it is precise and free of the unfair connotations of the older term — these animals are not cold, they simply borrow their warmth. The word comes from the Greek roots ektos, meaning "outside," and thermē, meaning "heat." Its opposite is "endothermic," describing warm-blooded creatures such as birds and mammals that maintain a steady internal temperature. Because external dependence makes ectothermic animals highly sensitive to climate, the word appears often in ecological discussions of habitat and global warming.
adjectiveOf or relating to an organism that regulates its body temperature largely by absorbing heat from its external environment rather than by generating heat internally.
- Relating to an organism that depends on external environmental heat to regulate its body temperature (cold-blooded).
"Reptiles are ectothermic animals that bask in the sun to raise their body temperature."
"Because fish are ectothermic, their activity levels rise and fall with the temperature of the water around them."
"The zoo's reptile house maintains heated rocks so its ectothermic residents can thermoregulate throughout the day."
A rattlesnake and a crocodile are essentially solar-powered — as ectotherms, they can survive on a tenth of the food a warm-blooded animal of the same size would need.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of ectothermic
Ectothermic is built on two ancient Greek roots: ektos, meaning "outside" or "external," and thermē, meaning "heat." The same combining form -thermic appears in scientific vocabulary such as endothermic, exothermic, and isothermal, while the root ektos also surfaces in English words like exotic and exodus. The term emerged in nineteenth- and early twentieth-century zoological writing as biologists sought a neutral, technical alternative to the folk label "cold-blooded."
Related word forms
How ectothermic is actually used
Primarily a scientific term used in biology, zoology, and ecology; 'cold-blooded' is the everyday equivalent, though it carries misleading negative connotations that 'ectothermic' avoids.
Easily confused with ectothermic
Endothermic organisms generate their own body heat internally (like birds and mammals), whereas ectothermic ones rely on external sources; in chemistry, the terms also describe reactions that absorb versus release heat.
Exothermic describes a process or reaction that releases heat, not an organism's way of regulating body temperature.