Editing Endotherm (section)

===Generating and conserving heat===
[[File:Homeothermy-poikilothermy.png|thumb|right|Sustained energy output of an endothermic animal ([[mammal]]) and an ectothermic animal ([[reptile]]) as a function of core temperature]]
[[File:Thermal Regulation Graph.svg|thumb|This image shows the difference between endotherms and ectotherms. The mouse is endothermic and regulates its body temperature through homeostasis. The lizard is ectothermic and its body temperature is dependent on the environment.]]
Many endotherms have a larger amount of [[mitochondria]] per [[cell (biology)|cell]] than ectotherms. This enables them to generate heat by increasing the rate at which they metabolize [[fat]]s and [[sugar]]s. Accordingly, to sustain their higher metabolism, endothermic animals typically require several times as much food as ectothermic animals do, and usually require a more sustained supply of metabolic fuel.

In many endothermic animals, a controlled temporary state of [[hypothermia]] conserves energy by permitting the body temperature to drop nearly to ambient levels. Such states may be brief, regular [[Circadian rhythm|circadian cycles]] called [[torpor]], or they might occur in much longer, even seasonal, cycles called [[hibernation]]. The body temperatures of many small birds (e.g. [[hummingbird]]s) and small mammals (e.g. [[tenrec]]s) fall dramatically during daily inactivity, such as nightly in [[diurnality|diurnal]] animals or during the day in [[nocturnality|nocturnal]] animals, thus reducing the energy cost of maintaining body temperature. Less drastic intermittent reduction in body temperature also occurs in other larger endotherms; for example human metabolism also slows down during sleep, causing a drop in core temperature, commonly of the order of 1 degree Celsius. There may be other variations in temperature, usually smaller, either endogenous or in response to external circumstances or vigorous exertion, and either an increase or a drop.<ref>{{cite journal | vauthors = Refinetti R | title = The circadian rhythm of body temperature | journal = Frontiers in Bioscience | volume = 15 | issue = 2 | pages = 564–594 | date = January 2010 | pmid = 20036834 | doi = 10.2741/3634 | doi-access = free }}</ref>

The resting human body generates about two-thirds of its heat through metabolism in internal organs in the thorax and abdomen, as well as in the brain. The brain generates about 16% of the total heat produced by the body.<ref>{{Cite web | vauthors = Kimball JW | date = 25 June 2014 |title = The Transport of Heat|url = https://www.biology-pages.info/H/HeatTransport.html |work = Kimball's Biology Pages | via = www.biology-pages.info }}</ref>

Heat loss is a major threat to smaller creatures, as they have a larger ratio of [[Surface-area-to-volume ratio|surface area to volume]]. Small warm-blooded animals have [[thermal insulation|insulation]] in the form of [[fur]] or [[feather]]s. Aquatic warm-blooded animals, such as [[pinniped|seals]], generally have deep layers of [[blubber]] under the [[skin]] and any [[Fur|pelage]] (fur) that they might have; both contribute to their insulation. [[Penguin]]s have both feathers and blubber. Penguin feathers are scale-like and serve both for insulation and streamlining. Endotherms that live in very cold circumstances or conditions predisposing to heat loss, such as polar waters, tend to have [[rete mirabile|specialised structures of blood vessels in their extremities]] that act as [[heat exchanger]]s. The veins are adjacent to the arteries full of warm blood. Some of the arterial heat is conducted to the cold blood and recycled back into the trunk. Birds, especially [[wader]]s, often have very well-developed [[rete mirabile|heat exchange mechanisms]] in their legs&mdash;those in the legs of [[emperor penguin]]s are part of the adaptations that enable them to spend months on Antarctic winter ice.<ref>{{cite journal | vauthors = Thomas DB, Fordyce RE | year = 2008 | title = The heterothermic loophole exploited by penguins | journal = Australian Journal of Zoology | volume = 55 | issue = 5| pages = 317–321 | doi = 10.1071/ZO07053 }}</ref><ref>{{cite journal | vauthors = Thomas DB, Ksepka DT, Fordyce RE | title = Penguin heat-retention structures evolved in a greenhouse Earth | journal = Biology Letters | volume = 7 | issue = 3 | pages = 461–464 | date = June 2011 | pmid = 21177693 | pmc = 3097858 | doi = 10.1098/rsbl.2010.0993 }}</ref> In response to cold, many warm-blooded animals also reduce blood flow to the skin by [[vasoconstriction]] to reduce heat loss. As a result, they blanch (become paler).