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An ectotherm (), more commonly referred to as a " cold-blooded animal", is an animal in which internal physiological sources of heat, such as blood, are of relatively small or of quite negligible importance in controlling body temperature.Davenport, John. Animal Life at Low Temperature. Publisher: Springer 1991. Such organisms (, for example) rely on environmental heat sources,Jay M. Savage; with photographs by Michael Fogden and Patricia Fogden. (2025). 9780226735382, University of Chicago Press. ISBN 9780226735382 which permit them to operate at very economical metabolic rates.Milton Hildebrand; G. E. Goslow, Jr. Principal ill. Viola Hildebrand. (2025). 9780471295051, Wiley. ISBN 9780471295051
Some of these animals live in environments where temperatures are practically constant, as is typical of regions of the abyssal ocean and hence can be regarded as homeothermic ectotherms. 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 regulate their body temperature by in the sun, or seeking shade when necessary in addition to a host of other behavioral thermoregulation mechanisms.
In contrast to ectotherms, rely largely, even predominantly, on heat from internal metabolic processes, and use an intermediate strategy.
Because there are more than two categories of temperature control utilized by animals, the terms warm-blooded and cold-blooded have been deprecated as scientific terms.
In addition to behavioral adaptations, physiological adaptations help ectotherms regulate temperature. Diving reptiles conserve heat by heat exchange mechanisms, whereby cold blood from the skin picks up heat from blood moving outward from the body core, re-using and thereby conserving some of the heat that otherwise would have been wasted. The skin of bullfrogs secretes more mucus when it is hot, allowing more cooling by evaporation.
During periods of cold, some ectotherms enter a state of torpor, in which their metabolism slows or, in some cases, such as the wood frog, effectively stops. The torpor might last overnight or last for a season, or even for years, depending on the species and circumstances.
Owners of reptiles may use an ultraviolet light system to assist their pets' basking.
Because ectotherms depend on environmental conditions for body temperature regulation, as a rule, they are more sluggish at night and in early mornings. When they emerge from shelter, many Diurnality ectotherms need to heat up in the early sunlight before they can begin their daily activities. In cool weather the foraging activity of such species is therefore restricted to the day time in most vertebrate ectotherms, and in cold climates most cannot survive at all. In lizards, for instance, most nocturnal species are geckos specialising in "sit and wait" foraging strategies. Such strategies do not require as much energy as active foraging and do not require hunting activity of the same intensity. From another point of view, sit-and-wait predation may require very long periods of unproductive waiting. Endotherms cannot, in general, afford such long periods without food, but suitably adapted ectotherms can wait without expending much energy. Endothermic vertebrate species are therefore less dependent on the environmental conditions and have developed a higher variability (both within and between species) in their daily patterns of activity. (2025). 9780444594273 ISBN 9780444594273
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 animal such as , poikilothermy is practically absolute, but other creatures (like ) 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 is relatively constant, and there are few in number that attempt to maintain a higher internal temperature due to the high associated costs.Willmer, Pat; Stone, Graham; Johnston, Ian. Environmental Physiology of Animals. Hoboken: Wiley, 2009. Ebook Library. Web. 01 Apr. 2016.
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