Poikilotherms

Poikilotherms are animals whose body temperature varies with the temperature of their environment. In contrast to homeotherms, poikilotherms do not maintain a constant internal temperature, and their metabolic rate generally depends on ambient temperature. Most poikilotherms are ectothermic, obtaining much of their heat from external sources; however, some may regulate their temperature behaviorally to stay within favorable ranges.

They regulate body temperature primarily by behavior, such as basking in sunlight to warm up or seeking shade, burrowing, or changing activity patterns with the time of day or season. Temperature affects physiological processes in poikilotherms: enzyme activity, metabolism, growth, development, and reproduction are typically faster at higher environmental temperatures and slower at cooler ones; extreme temperatures can be lethal.

Many poikilotherms display heterothermy, showing periods of reduced metabolic rate or body temperature, such as torpor, estivation, or seasonal diapause. They can often occupy environments with limited energy resources because they rely less on energetic heat production, but their activity and geographic range are constrained by climate and the availability of suitable microhabitats.

Examples include fishes, amphibians, reptiles, and many invertebrate groups. The term is sometimes contrasted with homeothermy, in which animals regulate a relatively constant body temperature through metabolic heat production or evaporative cooling. The usage of “cold-blooded” is dated and misleading, as poikilothermy refers to temperature variability, not perceived external heat.