Poikilotherm(redirected from allotherm)
Also found in: Dictionary, Thesaurus, Medical.
a cold-blooded animal, that is, an animal with a variable body temperature that changes as a function of the temperature of the environment. Poikilotherms include all invertebrates; the only poikilothermic vertebrates are fishes, amphibians, and reptiles.
The body temperature of poikilotherms is generally only 1° or 2° higher than, or equal to, that of the environment. Thermoregulation is imperfect. In many poikilotherms the body temperature rises as a result of muscular work or the absorption of solar heat. For example, the body temperature of bumblebees in flight may reach 38° or even 44°C at an air temperature of 4° to 8°C. However, after the cessation of flight the body cools rapidly to the temperature of the environment. If the external temperature rises above or falls below optimal limits, poikilotherms become torpid or perish. Many of them are in torpor a large part of the year; for example, the turtle Testudo horsfieldi is active only three months of the year. The absence of perfect thermoregulatory mechanisms in poikilotherms may be explained by the relatively poor development of their nervous systems (especially the central nervous system), a level of metabolism approximately 20 to 30 times lower than that of homeo-therms, and other features associated with the fact that poikilotherms have a more primitive organization than birds and mammals.