Physiologically, animals can be divided into three groups: homeotherms, which maintain a fairly constant internal temperature regardless of the external temperature; poikilotherms, which have a body temperature that varies according to the surrounding environmental temperature; and heterotherms, which sometimes maintain a fairly constant body temperature and sometimes do not.
Poikilotherms, such as amphibians and insects, have a high thermal or heat conductance between the body and the environment, and a low metabolic rate. For this reason body temperature__and thus tissue temperature changes with environmental temperature. Being ectothermic and manitaining body temperature by using sources of heat energy such as solar radiation rather than metabolism has advantages. Prisoners of environmental temperatures, poikilotherms of temperate regions, such as snakes, become highly active only when temperature is adequately warm. Because their metabolic activity declines with decreasing temperature, these animals become sluggish in the cool of morning and evening. Similarly, they have to restrict their activity to the late spring, summer and early fall. During periods of intense physical activity, when energy consumption is high, poikilotherms depend on the anaerobic (without oxygen) breakdown of glycogen (sugars stored for energy). These breakdown results in an accumulation of lactic acid in the tissues, and the lactic acid can be oxidised only after activity ceases. Anaerobic metabolism severely limits bursts of poikilothermic activity to a few minutes, because of physical exhaustion. This tendancy to become exhausted is one reason that so many predatory terrestrial poikilotherms, such as snakes and alligators, secure prey by ambush rather than by chase.
Because they do not depend on internally generated body heat, poikilotherms can reduce metabolic activity during periods of temperature extremes and of food or water shortage. Low energy demands enable poikilotherms to colonize areas of limited food and water, such as deserts. Because they do not have the problem of metabolic heat loss, poikilotherms are not limited to any minimum size or definite shape. Many are single celled, such as paramecia and amoebas; others such as earthworms, millipedes, and snakes have cylindrical bodies. Such characteristics enable poikilotherms to exploit resources and habitats unavailable to homeotherms. On the other hand, the same metabolic restrictions imposed an upper size limit: poikilotherms would not be able to obsorb enough heat to warm a very large body. For this reason, some paleontologists argue that large dinosaurs had to be endothermic (maintaining a constant body temperature by means of metabolism). A counter argument is that large ectotherms could develop and maintain body temperatures above air temperatures in a tropical environment because their low surface-to-volume ratio would limit cooling.
Although poikilotherms as a group do not have a lower limit to body size, size is nevertheless an important aspect of their life. The rate of heating and cooling in a poikilotherm decreases as the size increases. Because of its small size, a beetle heats and cools quickly, but rarely can it raise its uniform temperature above that of the surrounding environmental temperature. However, it does have the ability to control its temperature by moving in and out of the sunlight.
For very large terrestrial poikilotherms, such as tortoises, life is considerably different. Because they heat slowly, they need a much higher environmental temperature than smaller poikilotherms need to reach the same temperature. Further, because large poikilotherms have difficulty locating shelter from environmental extremes, they are more or less restricted to environments with small seasonal fluctuations in temperatures. Large poikilotherms, however, have the advantage of moving freely about in space and time during the day because their large body mass buffers their body temperatures.
Many poikilothermic animals of temperate and arctic regions must endure long periods of below-freezing temperatures in winter. Some intertidal invertebrates of high latitudes and certain aquatic insects survive the cold by actually freezing and then thawing when the temperature moderates. Other animals, particularly Arctic and Antarctic fish and many insects, resist freezing because of their presence of glycerol in their body fluids. Glycerol protects against damage from freezing and lowers the animal's freezing point.
In addition, many insects exhibiting frost hardiness enter a resting stage diapause, characterized by a cessation of feeding, growth, mobility and reproduction. Among many insects, diapause is a genetically determined, obligatory resting stage before development can proceed——it prevents the appearance of a sensitive stage of development at a time when low temperatures would kill.