coldbloodedness

Cold-bloodedness is a non-technical term describing animals whose body temperature largely follows the surrounding environment rather than being generated metabolically. In biology, this condition is called ectothermy or poikilothermy.

Most ectotherms are fish, amphibians, and reptiles. They regulate temperature mainly through behavior—basking in sunlight, seeking shade, or moving between microhabitats—rather than by producing heat internally. Endothermy, by contrast, involves maintaining a relatively constant body temperature through internal heat production, as in birds and mammals. The term "cold-blooded" is a simplification: body temperatures in ectotherms can vary with conditions, and some species maintain a restricted range or use localized heat sources.

Consequences of cold-bloodedness include a metabolic rate that tends to scale with ambient temperature, so growth, development, activity, and reproduction are strongly temperature-dependent. Temperature also shapes habitat preference, geographic distribution, and vulnerability to climate variability.

Evolution and usage: Ectothermy is widespread in fishes, amphibians, and reptiles and is considered an ancestral trait in these groups; endothermy evolved independently in birds and mammals. The phrase persists in everyday language and in historic scientific literature, though many scientists prefer the terms ectothermy and poikilothermy to avoid ambiguity. In culture, "cold-blooded" is also used metaphorically to describe perceived emotional distance or lack of empathy, separate from physiology.