Poikilothermic

Definition

Describing an organism whose body temperature varies substantially with ambient environmental conditions rather than being internally regulated. In poikilothermic animals, metabolic rate, enzyme kinetics, and locomotor performance are temperature-dependent, leading to behavioral thermoregulation (basking, shade-seeking, burrowing) as compensatory mechanisms. The term is often applied to insects, arachnids, and other ectotherms, though some ectotherms achieve practical homeothermy through microhabitat selection. Distinguished from homeothermic organisms that maintain stable internal temperatures via endogenous heat production.

Etymology

Greek poikilos (varied, variegated) + thermē (heat)

Example

Desert harvestmen (Opiliones) are poikilothermic, becoming torpid during cold desert nights and rapidly warming on sun-exposed rocks each morning to resume foraging.

Synonyms

• ectothermic (partial, often conflated)
• cold-blooded (vernacular, imprecise)

Related Terms

• homeothermic
• ectothermy
• endothermy
• behavioral thermoregulation
• thermal ecology
• Q10 effect

Usage Notes

Poikilothermic emphasizes temperature variability, while ectothermic emphasizes heat source (external). The terms overlap heavily in practice—most ectotherms are poikilothermic, though some achieve homeothermy behaviorally. Avoid 'cold-blooded' in technical writing; it conflates thermal variability with low body temperature and is misleading for tropical poikilotherms operating at 35–40°C.