poikilothermen

Poikilothermen are animals whose internal body temperature is not maintained at a constant level by metabolic heat production. Instead, their core temperature fluctuates with the ambient environment, rising with external heat and falling in cooler conditions. As a result, physiological processes and activity are strongly influenced by temperature, and body temperature often lags behind rapid changes in surroundings.

Groups that typically exhibit poikilothermy include most invertebrates, fishes, amphibians, and reptiles. They rely on behavioral strategies to regulate heat, such as basking in sunlight, seeking shade, changing body orientation, or moving between sun and shade. Their distribution and daily activity patterns are closely tied to local climate, season, and habitat thermal regimes. In cold climates, their metabolic rate and growth can be slow; in hot conditions, overheating and dehydration can be limiting.

Poikilothermy contrasts with homeothermy, where organisms such as birds and mammals maintain a relatively constant body temperature through metabolic heat production, insulation, and behavior. Some organisms display heterothermy, allowing body temperature to vary during certain life stages, seasons, or activities. Poikilothermy is common among many aquatic and terrestrial species and is considered an adaptation to energy efficiency, lifestyle, and environmental constraints.

Terminology note: the term poikilothermen is a plural form used in some languages; in English, poikilotherms is the standard plural. The concept is commonly described as poikilothermy or ectothermy.