poikilothermic

Poikilothermy is the condition in which an animal's body temperature varies considerably and generally tracks the ambient environmental temperature. The term comes from the Greek poikilos "varied" and thermos "heat." In poikilotherms, metabolic rate and physiological processes are strongly influenced by external temperatures, unlike homeotherms, which maintain a relatively constant internal temperature through metabolic heat production.

Most poikilotherms are ectothermic, relying primarily on environmental heat sources to regulate body temperature. They commonly employ behavioral strategies such as basking in the sun to raise body temperature, seeking shade to avoid overheating, and adjusting activity patterns with the time of day or season.

Temperature governs metabolism, growth, digestion, reproduction, and activity. Development rates are often temperature-dependent; higher temperatures speed up development up to species limits, while low temperatures slow metabolism or halt activity. Consequently, the geographic range and life cycles of poikilotherms are closely tied to climate.

Reptiles, amphibians, most fishes, and many invertebrates are poikilothermic. The term is commonly contrasted with endothermy, used to describe mammals and birds that maintain stable body temperatures through metabolic heat.