Ectothermy

An ectotherm (from the Greek ἐκτός (ektós) "outside" and θερμός (thermós) "hot"), is an organism in which internal physiological sources of heat are of relatively small or quite negligible importance in controlling body temperature. Such organisms (for example frogs) rely on environmental heat sources, which permit them to operate at very economical metabolic rates. Colloquially, some refer to these organisms as "cold blooded" though such a term is not technically correct, as the blood temperature of the organism varies with ambient environmental temperature. Some of these animals live in environments where temperatures are practically constant, as is typical of regions of the abyssal ocean. In contrast, in places where temperature varies so widely as to limit the physiological activities of other kinds of ectotherms, many species habitually seek out external sources of heat or shelter from heat; for example, many reptiles regulate their body temperature by in the sun, or seeking shade when necessary in addition to a whole host of other behavioral thermoregulation mechanisms. In contrast to ectotherms, endotherms rely largely, even predominantly, on heat from internal metabolic processes.

In ectotherms, fluctuating ambient temperatures may affect the body temperature. Such variation in body temperature is called poikilothermy, though the concept is not widely satisfactory and the use of the term is declining. In small aquatic creatures such as Rotifera, the poikilothermy is practically absolute, but other creatures (like crabs) have wider physiological options at their disposal, and they can move to preferred temperatures, avoid ambient temperature changes, or moderate their effects. Ectotherms can also display the features of homeothermy, especially within aquatic organisms. Normally their range of ambient environmental temperatures are relatively constant, and there are few in number that attempt to maintain a higher internal temperature due to the high associated costs.

Various patterns of behavior enable certain ectotherms to regulate body temperature to a useful extent. To warm up, reptiles and many insects find sunny places and adopt positions that maximise their exposure; at harmfully high temperatures they seek shade or cooler water. In cold weather, honey bees huddle together to retain heat. Butterflies and moths may orient their wings to maximize exposure to solar radiation in order to build up heat before take-off. Gregarious caterpillars, such as the Forest Tent caterpillar, benefit from basking in large groups for thermoregulation. Many flying insects, such as honey bees and bumble bees, also raise their internal temperatures endothermally prior to flight, by vibrating their flight muscles without violent movement of the wings (see insect thermoregulation). Such endothermal activity is an example of the difficulty of consistent application of terms such as poikilothermy and homiothermy.

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