Ectotherm

Definition

An organism whose body temperature is regulated primarily by external environmental heat sources rather than by internally generated metabolic heat. Ectotherms rely on behavioral thermoregulation—such as basking, shade-seeking, or posture changes—and physiological adjustments to maintain activity across varying thermal conditions. This thermal strategy permits low resting metabolic rates but constrains activity to temperatures where ambient heat permits sufficient enzyme function and membrane fluidity.

Etymology

From Greek ektos (outside) + thermē (heat)

Example

Desert locusts (Schistocerca gregaria) are classic ectotherms: they elevate body temperature by basking perpendicular to the sun to reach flight thresholds near 38°C, then seek shade or adopt stilting postures to avoid lethal overheating when soil surfaces exceed 50°C.

Synonyms

• poikilotherm (partial, often conflated)

Related Terms

• endotherm
• poikilotherm
• homeotherm
• Thermoregulation
• behavioral thermoregulation
• critical thermal maximum
• Q10 effect
• torpor

Usage Notes

Ectothermy describes the source of heat (external), not temperature stability. Many ectotherms achieve temporary homeothermy (stable body temperature) through precise behavioral thermoregulation—e.g., tiger beetles running in brief bursts then stopping to cool. Contrast with endotherm (internal heat production, as in bees during flight). 'Cold-blooded' is deprecated in scientific usage as misleading: ectotherms often have warmer bodies than endotherms when active in sunny habitats. Poikilothermy (variable body temperature) frequently co-occurs with ectothermy but is conceptually distinct.