Poikilotherms (Greek poikilos - 'various, spotted', and therme - 'heat') are defined as organisms with variable body temperature (Tb) (Figure 1). Typically, Tb in poikilotherms changes as a function of the temperature of their surroundings under normal physiological and environmental conditions. The opposite of this term is homeotherm (Greek homo - 'the same', and therme -'heat'), which refers to organisms maintaining constant or nearly constant body temperature. The concept of poiki-lothermy originated in and was predominantly used for animal studies, but is in principle applicable to any organism. Poikilothermy is determined by the combination of extrinsic factors (i.e., variation of environmental temperature) with intrinsic (physiological) constraints of the organism such as low levels of metabolic heat production, rapid heat dissipation due to the small body size, high heat conductivity of the external medium (for soil dwellers and aquatic organisms), or excessive energy costs associated with the maintenance of the constant Tb. Although there is no explicit consensus about how broad the variations in Tb should be for an organism to be considered a poikilotherm, it is generally accepted that if the body temperature changes by more than 1.5-2 °C under physiological conditions, the organism is a poikilotherm. Of course, this rule of thumb excludes pathological conditions such as fever or specialized adaptations such as the regulated drop in Tb during entrance into hibernation or torpor in mammals and small birds, when a change in body temperature can considerably exceed 2 °C.
Poikilotherm is one of the oldest terms in ecological physiology, and probably also one with the longest history of confusion. It would be impossible to understand the place of the concept of poikilothermy in thermal biology without clearing away some old but very persistent fallacies. In many texts as well as in vernacular language, poikilotherms are often incorrectly called 'cold-blooded' organisms, in contrast to the 'warm-blooded' homeotherms. In addition to muddling the meaning of poikilothermy, this statement is simply not true. In fact, poikilotherms can have body temperature similar to or even higher than in homeotherms. For example, some desert reptiles and insects can have Tb up to 40-44 °C so that there is nothing 'cold-blooded' about these poikilotherms. Overall, the cold-blooded and warmblooded terminology has no real value for understanding of biological mechanisms or physiological consequences of thermoregulation and is best avoided. Similarly, poiki-lothermy should not be confused with ectothermy, which indicates the predominant reliance of an organism on external heat sources for thermoregulation. Although ectothermic thermoregulation is rarely efficient enough to provide constant Tb in thermally variable environments and most ectothermic organisms are in practice poikilotherms, there are several important exceptions from this rule, and poiki-lotherms can be found among endotherms as well as ectotherms (see Temperature Regulation).
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