View source for Endotherm

{{Short description|Organism that maintains body temperature largely by heat from internal bodily functions}}
{{hatnote group|{{About|biological thermoregulation|chemical reactions|Endothermic|the fictional character|Endotherm (comics)}}
{{distinguish|Endoderm}}}}
{{Thermoreg}}
An '''endotherm''' (from [[Ancient Greek|Greek]] ἔνδον ''endon'' "within" and θέρμη ''thermē'' "heat") is an organism that maintains its body at a metabolically favorable temperature, largely by the use of heat released by its internal bodily functions instead of relying almost purely on ambient heat. Such internally generated heat is mainly an incidental product of the animal's routine [[metabolism]], but under conditions of excessive cold or low activity an endotherm might apply special mechanisms adapted specifically to heat production. Examples include special-function muscular exertion such as [[shivering]], and [[Uncoupler|uncoupled]] oxidative metabolism, such as within [[brown adipose tissue]].

Only [[bird]]s and [[mammal]]s are considered truly endothermic groups of animals. However, [[Argentine black and white tegu]], [[leatherback sea turtle|leatherback sea turtles]], [[Fish#Endothermy|lamnid sharks, tuna and billfishes]], [[cicadas]], and [[Operophtera brumata|winter moths]] are [[Mesotherm|mesothermic]]. Unlike mammals and birds, some reptiles, particularly some species of [[Pythonidae|python]] and [[tegu]], possess seasonal reproductive endothermy in which they are endothermic only during their [[reproductive season]].

In common parlance, endotherms are characterized as "[[warm-blooded]]". The opposite of endothermy is [[ectothermy]], although in general, there is no absolute or clear separation between the nature of endotherms and ectotherms.

==Origin==
Endothermy was thought to have originated towards the end of the [[Permian]] Period'''''<ref>{{cite journal | vauthors = Rey K, Amiot R, Fourel F, Abdala F, Fluteau F, Jalil NE, Liu J, Rubidge BS, Smith RM, Steyer JS, Viglietti PA, Wang X, Lécuyer C | display-authors = 6 | title = Oxygen isotopes suggest elevated thermometabolism within multiple Permo-Triassic therapsid clades | journal = eLife | volume = 6 | date = July 2017 | pmid = 28716184 | pmc = 5515572 | doi = 10.7554/eLife.28589 | doi-access = free }}</ref>'''''. One recent study claimed the origin of endothermy within [[Synapsida]] (the mammalian lineage) was among [[Mammaliamorpha]], a node calibrated during the [[Late Triassic]] period, about 233 million years ago.'''''<ref>{{cite journal | vauthors = Araújo R, David R, Benoit J, Lungmus JK, Stoessel A, Barrett PM, Maisano JA, Ekdale E, Orliac M, Luo ZX, Martinelli AG, Hoffman EA, Sidor CA, Martins RM, Spoor F, Angielczyk KD | display-authors = 6 | title = Inner ear biomechanics reveals a Late Triassic origin for mammalian endothermy | journal = [[Nature (journal)|Nature]] | volume = 607 | issue = 7920 | pages = 726–731 | date = July 2022 | pmid = 35859179 | doi = 10.1038/s41586-022-04963-z | s2cid = 236245230 }}</ref>''''' Another study instead argued that endothermy only appeared later, during the [[Middle Jurassic]], among crown-group mammals.<ref>{{cite journal |last1=Newham |first1=Elis |last2=Gill |first2=Pamela G. |last3=Corfe |first3=Ian J. |date=16 February 2022 |title=New tools suggest a middle Jurassic origin for mammalian endothermy |journal=[[BioEssays]] |volume=44 |issue=4 |pages=e2100060 |doi=10.1002/bies.202100060 |pmid=35170781 |s2cid=247599805 |doi-access=free }}</ref>

Evidence for endothermy has been found in basal synapsids ("[[pelycosaur]]s"), [[Pareiasaur|pareiasaurs]], [[Ichthyosaur|ichthyosaurs]], [[Plesiosaur|plesiosaurs]], [[Mosasaur|mosasaurs]], and basal [[Archosauromorpha|archosauromorphs]].<ref name=":1">{{cite journal | vauthors = Grigg G, Nowack J, Bicudo JE, Bal NC, Woodward HN, Seymour RS | title = Whole-body endothermy: ancient, homologous and widespread among the ancestors of mammals, birds and crocodylians | journal = Biological Reviews of the Cambridge Philosophical Society | volume = 97 | issue = 2 | pages = 766–801 | date = April 2022 | pmid = 34894040 | pmc = 9300183 | doi = 10.1111/brv.12822 }}</ref><ref>{{cite journal | vauthors = Legendre LJ, Guénard G, Botha-Brink J, Cubo J | title = Palaeohistological Evidence for Ancestral High Metabolic Rate in Archosaurs | journal = Systematic Biology | volume = 65 | issue = 6 | pages = 989–996 | date = November 2016 | pmid = 27073251 | doi = 10.1093/sysbio/syw033 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Benton MJ |date= December 2021 |title=The origin of endothermy in synapsids and archosaurs and arms races in the Triassic |journal=Gondwana Research |language=en |volume=100 |pages=261–289 |doi=10.1016/j.gr.2020.08.003|s2cid= 222247711 |doi-access=free }}</ref> Even the earliest amniotes might have been endotherms.<ref name=":1" />

==Mechanisms==

===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).

===Avoiding overheating===
In [[Tropical rainforest climate|equatorial climates]] and during [[temperate climate|temperate]] summers, overheating ([[hyperthermia]]) is as great a threat as cold. In hot conditions, many warm-blooded animals increase heat loss by panting, which cools the animal by increasing water [[evaporation]] in the breath, and/or flushing, increasing the blood flow to the skin so the heat will [[radiant energy|radiate]] into the environment. Hairless and short-haired mammals, including humans and horses, also [[Perspiration|sweat]], since the evaporation of the water in sweat removes heat. Elephants keep cool by using their huge [[ear]]s like [[radiator]]s in automobiles. Their ears are thin and the [[blood vessel]]s are close to the skin, and flapping their ears to increase the airflow over them causes the blood to cool, which reduces their core body temperature when the blood moves through the rest of the circulatory system.

==Pros and cons of an endothermic metabolism==
The major advantage of endothermy over ectothermy is decreased vulnerability to fluctuations in external temperature. Regardless of location (and hence external temperature), endothermy maintains a constant core temperature for optimal enzyme activity.

Endotherms control body temperature by internal homeostatic mechanisms. In mammals, two separate homeostatic mechanisms are involved in thermoregulation—one mechanism increases body temperature, while the other decreases it. The presence of two separate mechanisms provides a very high degree of control. This is important because the core temperature of mammals can be controlled to be as close as possible to the optimal temperature for enzyme activity.

The overall rate of an animal's [[metabolism]] increases by a factor of about two for every {{convert|10|C-change|F-change|0|abbr=on}} rise in [[temperature]], limited by the need to avoid [[hyperthermia]]. Endothermy does not provide greater speed in movement than ectothermy (cold-bloodedness)—ectothermic animals can move as fast as warm-blooded animals of the same size and build when the ectotherm is near or at its optimal temperature, but often cannot maintain high metabolic activity for as long as endotherms. Endothermic/homeothermic animals can be optimally active at more times during the diurnal cycle in places of sharp temperature variations between day and night and during more of the year in places of great [[season]]al differences of temperature. This is accompanied by the need to expend more energy to maintain the constant internal temperature and a greater food requirement.<ref>{{cite book | vauthors = Campbell NA, Reece JB |year=2002 |title=Biology |url=https://archive.org/details/biologyc00camp |url-access=registration |edition=6th |publisher=Benjamin/Cummings |page=[https://archive.org/details/biologyc00camp/page/845 845] |isbn=978-0-8053-6624-2 }}</ref>  Endothermy may be important during reproduction, for example, in expanding the thermal range over which a species can reproduce, as embryos are generally intolerant of thermal fluctuations that are easily tolerated by adults.<ref>{{cite journal | vauthors = Farmer CG | title = Parental Care: The Key to Understanding Endothermy and Other Convergent Features in Birds and Mammals | journal = The American Naturalist | volume = 155 | issue = 3 | pages = 326–334 | date = March 2000 | pmid = 10718729 | doi = 10.1086/303323 | s2cid = 17932602 }}</ref><ref>{{cite journal | vauthors = Farmer CG | title = Reproduction: the adaptive significance of endothermy | journal = The American Naturalist | volume = 162 | issue = 6 | pages = 826–840 | date = December 2003 | pmid = 14737720 | doi = 10.1086/380922 | s2cid = 15356891 }}</ref> Endothermy may also provide protection against [[fungal]] infection. While tens of thousands of fungal species infect insects, only a few hundred target mammals, and often only those with a compromised [[immune system]]. A recent study<ref>{{cite journal | vauthors = Robert VA, Casadevall A | title = Vertebrate endothermy restricts most fungi as potential pathogens | journal = The Journal of Infectious Diseases | volume = 200 | issue = 10 | pages = 1623–1626 | date = November 2009 | pmid = 19827944 | doi = 10.1086/644642 | doi-access = free }}</ref>
suggests fungi are fundamentally ill-equipped to thrive at mammalian temperatures. The high temperatures afforded by endothermy might have provided an evolutionary advantage.

[[Ectotherms]] increase their body temperature mostly through external heat sources such as [[sunlight]] energy; therefore, they depend on environmental conditions to reach operational body temperatures. Endothermic animals mostly use internal heat production through metabolic active organs and tissues (liver, kidney, heart, brain, muscle) or specialized heat producing tissues like [[brown adipose tissue]] (BAT). In general, endotherms therefore have higher metabolic rates than ectotherms at a given body mass. As a consequence they also need higher food intake rates, which may limit abundance of endotherms more than ectotherms.

Because ectotherms depend on environmental conditions for body temperature regulation, they typically are more sluggish at night and in the morning when they emerge from their shelters to heat up in the first sunlight. Foraging activity is therefore restricted to the daytime (diurnal activity patterns) in most vertebrate ectotherms. In lizards, for instance, only a few species are known to be nocturnal (e.g. many geckos) and they mostly use 'sit and wait' foraging strategies that may not require body temperatures as high as those necessary for active foraging. Endothermic vertebrate species are, therefore, less dependent on the environmental conditions and have developed a high variability (both within and between species) in their diurnal activity patterns.<ref name =Hut>{{cite book|vauthors=Hut RA, Kronfeld-Schor N, van der Vinne V, De la Iglesia H |chapter=In search of a temporal niche |title=The Neurobiology of Circadian Timing|year=2012|volume=199|pages=281–304|pmid=22877672|doi=10.1016/B978-0-444-59427-3.00017-4|series=Progress in Brain Research|isbn=978-0-444-59427-3 }}</ref>

It is thought that the evolution of endothermia was crucial in the development of [[Eutheria|eutherian]] mammalian species diversity in the Mesozoic period. Endothermia gave the early mammals the capacity to be active during nighttime while maintaining small body sizes. Adaptations in [[photoreception]] and the loss of UV protection characterizing modern eutherian mammals are understood as adaptations for an originally nocturnal lifestyle, suggesting that the group went through an evolutionary bottleneck (the [[Nocturnal bottleneck|nocturnal bottleneck hypothesis]]). This could have avoided predator pressure from diurnal reptiles and dinosaurs, although some predatory dinosaurs, being equally endothermic, might have adapted a nocturnal lifestyle in order to prey on those mammals.<ref name=Hut/><ref>{{cite journal | vauthors = Gerkema MP, Davies WI, Foster RG, Menaker M, Hut RA | title = The nocturnal bottleneck and the evolution of activity patterns in mammals | journal = Proceedings. Biological Sciences | volume = 280 | issue = 1765 | pages = 20130508 | date = August 2013 | pmid = 23825205 | pmc = 3712437 | doi = 10.1098/rspb.2013.0508 }}</ref>

==Facultative endothermy==
Many [[insect]] species are able to maintain a thoracic temperature above the ambient temperature using exercise. These are known as facultative or exercise endotherms.<ref>{{cite book | vauthors = Davenport J |year=1992 |title=Animal life at low temperature |url=https://archive.org/details/animallifeatlowt0000dave |url-access=registration |location=London |publisher=Chapman & Hall |isbn=978-0-412-40350-7 }}</ref> The [[honey bee]], for example, does so by contracting antagonistic flight muscles without moving its wings (see [[insect thermoregulation]]).<ref name=Kammer>{{cite journal | vauthors = Kammer AE, Heinrich B | title = Metabolic rates related to muscle activity in bumblebees | journal = The Journal of Experimental Biology | volume = 61 | issue = 1 | pages = 219–227 | date = August 1974 | pmid = 4414648 | doi = 10.1242/jeb.61.1.219 }}</ref><ref name=Lighton>{{cite journal | vauthors = Lighton JR, Lovegrove BG |year=1990 |title=A temperature-induced switch from diffusive to convective ventilation in the honeybee |journal=Journal of Experimental Biology |volume=154 |issue=1 |pages=509–516 |doi=10.1242/jeb.154.1.509 |doi-access=free }}</ref><ref name=Kovac>{{cite journal | vauthors = Kovac H, Stabentheiner A, Hetz SK, Petz M, Crailsheim K | title = Respiration of resting honeybees | journal = Journal of Insect Physiology | volume = 53 | issue = 12 | pages = 1250–1261 | date = December 2007 | pmid = 17707395 | pmc = 3227735 | doi = 10.1016/j.jinsphys.2007.06.019 }}</ref> This form of thermogenesis is, however, only efficient above a certain temperature threshold, and below about {{convert|9|-|14|°C|°F|abbr=on}}, the honey bee reverts to ectothermy.<ref name=Lighton/><ref name=Kovac/><ref name=Southwick>{{cite journal | vauthors = Southwick EE, Heldmaier G |year=1987 |title=Temperature control in honey bee colonies |journal=BioScience |volume=37 |issue=6 |pages=395–399 |doi=10.2307/1310562 |jstor=1310562 }}</ref>

Facultative endothermy can also be seen in multiple snake species that use their metabolic heat to warm their eggs. ''Python molurus'' and ''Morelia spilota'' are two python species where females surround their eggs and shiver in order to incubate them.<ref>{{cite journal | vauthors = Stahlschmidt ZR, Denardo DF | title = Effect of nest temperature on egg-brooding dynamics in Children's pythons | journal = Physiology & Behavior | volume = 98 | issue = 3 | pages = 302–306 | date = September 2009 | pmid = 19538977 | doi = 10.1016/j.physbeh.2009.06.004 | s2cid = 7670440 }}</ref>

== Regional endothermy ==
Some [[ectotherm]]s, including several species of [[fish]] and [[reptile]]s, have been shown to make use of regional endothermy, where muscle activity causes certain parts of the body to remain at higher temperatures than the rest of the body.<ref name=":0">{{Cite book|title=Environmental Physiology of Animals| vauthors = Willmer P, Stone G, Johnston I |publisher=Wiley|year=2009|isbn=978-1-4051-0724-2 |pages=[https://archive.org/details/environmentalphy00will/page/190 190]|url-access=registration|url=https://archive.org/details/environmentalphy00will/page/190}}</ref> This allows for better locomotion and use of the senses in cold environments.<ref name=":0" />

==Contrast between thermodynamic and biological terminology==
Students encounter a source of possible confusion between the terminology of physics and biology. Whereas the thermodynamic terms "[[exothermic]]" and "[[endothermic]]" respectively refer to processes that give out heat energy and processes that absorb heat energy, in biology the sense is effectively reversed. The [[Metabolism|metabolic]] terms "ectotherm" and "endotherm" respectively refer to organisms that rely largely on external heat to achieve a full working temperature, and to organisms that produce heat from within as a major factor in controlling their body temperatures.<ref>{{Cite journal| vauthors = Humphries MH |date=2010-11-01|title=Endotherm Metabolism Too Hot to Handle|journal=Journal of Experimental Biology|volume=213|issue=21|pages=iv|doi=10.1242/jeb.051300|doi-access=free}}</ref>

== See also ==
* [[Warm-blooded]]

== References ==
{{reflist}}

== External links ==
*[https://www.journals.uchicago.edu/journals/pbz/endothermy-collection ''Physiological and Biochemical Zoology'' Focused Collection: Evolution and Advantages of Endothermy]

[[Category:Animal physiology]]
[[Category:Thermoregulation]]
[[Category:Polyphyletic groups]]