# Endotherm

An endotherm (from 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 set free 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 uncoupled oxidative metabolism such as within brown adipose tissue. Only birds and mammals are extant universally endothermic groups of animals. Certain lamnid sharks, tuna and billfishes are also endothermic.

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.

## Mechanisms

### Generating and conserving heat

Sustained energy output of an endothermic animal (mammal) and an ectothermic animal (reptile) as a function of core temperature
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 number of mitochondria per cell than ectotherms. This enables them to generate heat by increasing the rate at which they metabolize fats and sugars. 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 cycles called torpor, or they might occur in much longer, even seasonal, cycles called hibernation. The body temperatures of many small birds (e.g. hummingbirds) and small mammals (e.g. tenrecs) fall dramatically during daily inactivity, such as nightly in diurnal animals or during the day in 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.[1]

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.[2]

Heat loss is a major threat to smaller creatures, as they have a larger ratio of surface area to volume. Small warm-blooded animals have insulation in the form of fur or feathers. Aquatic warm-blooded animals, such as seals, generally have deep layers of blubber under the skin and any pelage that they might have; both contribute to their insulation. Penguins have both feathers and blubber. Penguin feathers are scale-like and serve both for insulation and for streamlining. Endotherms that live in very cold circumstances or conditions predisposing to heat loss, such as polar waters, tend to have specialised structures of blood vessels in their extremities that act as heat exchangers. 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 waders, often have very well-developed heat exchange mechanisms in their legs—those in the legs of emperor penguins are part of the adaptations that enable them to spend months on Antarctic winter ice.[3][4] 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 equatorial climates and during 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 radiate into the environment. Hairless and short-haired mammals, including humans, also sweat, since the evaporation of the water in sweat removes heat. Elephants keep cool by using their huge ears like radiators in automobiles. Their ears are thin and the blood vessels 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 optimum 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 optimum temperature for enzyme activity.

The overall rate of an animal's metabolism increases by a factor of about two for every 10 °C (18 °F) 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 optimum 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 seasonal differences of temperature. This is accompanied by the need to expend more energy to maintain the constant internal temperature and a greater food requirement.[5] 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.[6][7] Endothermy may also provide a 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[8] suggests fungi are fundamentally ill-equipped to thrive at mammalian temperatures. The high temperatures afforded by endothermy might have provided an evolutionary advantage.

Ectotherms will increase their body temperature mostly through external heat sources such as sunlight energy, therefore they depend on the occurring 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 would 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 day time (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.[9]

It is thought that the evolution of endothermia was crucial in the development of eutherian mammalian species diversity in the Mesozoic period. Endothermia gave the early mammals the capacity to be active during night time 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 bottle neck (the 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.[9][10]

## 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.[11] The honey bee, for example, does so by contracting antagonistic flight muscles without moving its wings (see insect thermoregulation).[12][13][14] This form of thermogenesis is, however, only efficient above a certain temperature threshold, and below about 9–14 °C (48–57 °F), the honey bee reverts to ectothermy.[13][14][15]

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.[16]

## Regional endothermy

Some ectotherms, including several species of fish and reptiles, 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.[17] This allows for better locomotion and use of the senses in cold environments.[17]

## Contrast between thermodynamic and biological terminology

Because of historical accident, 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 inverted. The 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.

## References

1. ^ Refinetti, Roberto (2010). "The circadian rhythm of body temperature". Frontiers in Bioscience. 15: 564–594. doi:10.2741/3634.
2. ^ "Heat Transport". users.rcn.com. Retrieved 2015-11-04.
3. ^ Thomas, D.B.; Fordyce, R.E. (2008). "The heterothermic loophole exploited by penguins". Australian Journal of Zoology. 55 (5): 317–321. doi:10.1071/ZO07053.
4. ^ Thomas, D.B., D.T. Ksepka and R.E. Fordyce. 2010. Penguin heat-retention structures evolved in a greenhouse Earth. Biology Letters (published online before print December 22, 2010, doi:10.1098/rsbl.2010.0993)
5. ^ Campbell, N. A.; Reece, J. B.; et al. (2002). Biology (6th ed.). Benjamin/Cummings. p. 845.
6. ^ Farmer, C. G. (2000-03-01). "Parental Care: The Key to Understanding Endothermy and Other Convergent Features in Birds and Mammals". The American Naturalist. 155 (3): 326–334. doi:10.1086/303323. ISSN 0003-0147. PMID 10718729.
7. ^ Farmer, C. G. (2003-12-01). "Reproduction: The Adaptive Significance of Endothermy". The American Naturalist. 162 (6): 826–840. doi:10.1086/380922. ISSN 0003-0147. PMID 14737720.
8. ^ Robert, Vincent A. and Casadevall, Arturo (2009). "Vertebrate Endothermy Restricts Most Fungi as Potential Pathogens". The Journal of Infectious Diseases. 200 (10): 1623–1626. doi:10.1086/644642. PMID 19827944.CS1 maint: Multiple names: authors list (link)
9. ^ a b Hut RA, Kronfeld-Schor N, van der Vinne V, De la Iglesia H (2012). In search of a temporal niche: environmental factors. Progress in Brain Research. 199. pp. 281–304. doi:10.1016/B978-0-444-59427-3.00017-4. ISBN 9780444594273. PMID 22877672.
10. ^ Gerkema, Menno P.; Davies, Wayne I. L.; Foster, Russell G.; Menaker, Michael; Hut, Roelof A. (2013-08-22). "The nocturnal bottleneck and the evolution of activity patterns in mammals". Proc. R. Soc. B. 280 (1765): 20130508. doi:10.1098/rspb.2013.0508. PMC 3712437. PMID 23825205.
11. ^ Davenport, J. (1992). Animal life at low temperature. London: Chapman & Hall.
12. ^ Kammer, A. E.; Heinrich, B. (1974). "Metabolic rates related to muscle activity in bumblebees". Journal of Experimental Biology. 6 (1): 219–227.
13. ^ a b Lighton, J. R. B.; Lovegrove, B. G. (1990). "A temperature-induced switch from diffusive to convective ventilation in the honeybee". Journal of Experimental Biology. 154 (1): 509–516.
14. ^ a b Kovac, H.; Stabentheiner, A.; Hetz, S. K.; Petz, M.; Crailsheim, K. (2007). "Respiration of resting honeybees". Journal of Insect Physiology. 53 (12): 1250–1261. doi:10.1016/j.jinsphys.2007.06.019. PMC 3227735. PMID 17707395.
15. ^ Southwick, E. E.; Heldmaier, G. (1987). "Temperature control in honey bee colonies". BioScience. 37 (6): 395–399. doi:10.2307/1310562. JSTOR 1310562.
16. ^ Stahlschmidt, Z. R.; DeNardo, D. F. (2009). "Effect of nest temperature on egg-brooding dynamics in Children's pythons". Physiology & Behavior. 98 (3): 302–306. doi:10.1016/j.physbeh.2009.06.004. PMID 19538977.
17. ^ a b Willmer, Pat; Stone, Graham; Johnston, Ian (2009). Environmental Physiology of Animals. Wiley. p. 190. ISBN 9781405107242.
BattleBots (season 1)

Season 1 of the American competitive television series BattleBots premiered on Comedy Central on August 23, 2000. Season 1 of BattleBots was hosted by Bil Dwyer and Sean Salisbury. Legendary boxing ring announcer and radio host Mark Beiro acted as the BattleBots arena announcer. Donna D'Errico, along with twins Randy and Jason Sklar were the arena-side correspondents.The pilot, known as episode one was originally aired as a Pay-per-view event before being pitched to network executives.

Bradyaerobic is a term used in biology that describes an animal that has low levels of oxygen consumption.By necessity a bradyaerobic animal can engage in short low or high low-level aerobic exercise, followed by brief anaerobically powered bursts of energy. Bradyaerobes can be sprinters, but not long-distance animals.

Bradymetabolism refers to organisms with a high active metabolism and a considerably slower resting metabolism. Bradymetabolic animals can often undergo dramatic changes in metabolic speed, according to food availability and temperature. Many bradymetabolic creatures in deserts and in areas that experience extreme winters are capable of "shutting down" their metabolisms to approach near-death states, until favorable conditions return(see hibernation and estivation).

Several variants of bradymetabolism exists. In mammals, the animals normally have a fairly high metabolism, only dropping to low levels in times of little food. In most reptiles, the normal metabolic rate is quite low, but can be raised when needed, typically in short bursts of activity in connection with capturing prey.

Dimethyldioctadecylammonium bromide (also dioctadecyldimethylammonium bromide or DODAB) is a double-chained quaternary ammonium surfactant that forms unilamellar vesicles (ULVs) in water. Among various preparation methods, the ‘‘hot-water” method offers a simple procedure to prepare DODAB cationic vesicles by simply dissolving the DODAB in hot water above 50 °C, i.e., chain melting (main) transition, Tm.

In general, the DSC thermograms of the unsonicated DODAB dispersions are dominated by two endotherms; the pre- (35–36 °C) and main transition (42.7–45 °C) peaks. Moreover, in literature reported the presence of a third

endotherm (post transition) at 52.2 °C.

The main transition (Tm) is ascribed to gel to liquid-crystalline phase transition in which the alkyl chains transform from solidlike to liquid-like state.

The 10 mM DODAB is a critical concentration, below which the dispersions consist of large polydispersed unilamellar vesicles (ULVs) that exhibit a local (chain melting) transition at 43 °C, beyond which a structural transition occurs: ULVs --> MLVs (multilamellar vesicles) as indicated by the sudden increase in the dynamic moduli. However, above 10 mM DODAB, the dispersions are mostly formed by ULVs in coexistence with lamellar fragments resulting in a network that shows a rheogram similar to that of hexagonal liquid-crystalline phase.

Ectotherm

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. 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 reptiles regulate their body temperature by basking in the sun, or seeking shade when necessary in addition to a whole host of other behavioral thermoregulation mechanisms.

For home captivity of pet reptiles, owners can use a UVB/UVA light system to assist the animals' basking behaviour.In contrast to ectotherms, endotherms rely largely, even predominantly, on heat from internal metabolic processes, and mesotherms use an intermediate strategy.

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 is relatively constant, and there are few in number that attempt to maintain a higher internal temperature due to the high associated costs.

Eurytherm

A eurytherm is an organism, often an endotherm, that can function at a wide range of ambient temperatures. To be considered a eurytherm, all stages of an organism's life cycle must be considered, including juvenile and larval stages. These wide ranges of tolerable temperatures are directly derived from the tolerance of a given eurythermal organism's proteins. Extreme examples of eurytherms include Tardigrades (Tardigrada), the desert pupfish (Cyprinodon macularis), and green crabs (Carcinus maenas), however, nearly all mammals, including humans, are considered eurytherms. Eurythermy can be an evolutionary advantage: adaptations to cold temperatures, called cold-eurythemy, are seen as essential for the survival of species during ice ages. In addition, the ability to survive in a wide range of temperatures increases a species' ability to inhabit other areas, an advantage for natural selection.

Eurythermy is an aspect of thermoregulation in organisms. It is in contrast with the idea of stenothermic organisms, which can only operate within a relatively narrow range of ambient temperatures. Through a wide variety of thermal coping mechanisms, eurythermic organisms can either provide or expel heat for themselves in order to survive in cold or hot, respectively, or otherwise prepare themselves for extreme temperatures. Certain species of eurytherm have been shown to have unique protein synthesis processes that differentiate them from relatively stenothermic, but otherwise similar, species.

Exergonic process

An exergonic process is one which there is a positive flow of energy from the system to the surroundings. This is in contrast with an endergonic process. Constant pressure, constant temperature reactions are exergonic if and only if the Gibbs free energy change is negative (∆G < 0). "Exergonic" (from the prefix exo-, derived for the Greek word ἔξω exō, "outside" and the suffix -ergonic, derived from the Greek word ἔργον ergon, "work") means "releasing energy in the form of work". In thermodynamics, work is defined as the energy moving from the system (the internal region) to the surroundings (the external region) during a given process.

All physical and chemical systems in the universe follow the second law of thermodynamics and proceed in a downhill, i.e., exergonic, direction. Thus, left to itself, any physical or chemical system will proceed, according to the second law of thermodynamics, in a direction that tends to lower the free energy of the system, and thus to expend energy in the form of work. These reactions occur spontaneously.

A chemical reaction is also exergonic when spontaneous. Thus in this type of reactions the Gibbs free energy decreases. The entropy is included in any change of the Gibbs free energy. This differs from a exothermic reaction or a endothermic reaction where the entropy is not included. The Gibbs free energy is calculated with the Gibbs–Helmholtz equation:

${\displaystyle \Delta G=\Delta H-T\cdot \Delta S}$

where:

T = temperature in kelvins (K)
ΔG = change in the Gibbs free energy
ΔS = change in entropy (at 298 K) as ΔS = Σ{S(Product)} − Σ{S(Reagent)}
ΔH = change in enthalpy (at 298 K) as ΔH = Σ{H(Product)} − Σ{H(Reagent)}

A chemical reaction progresses only spontaneously when the Gibbs free energy decreases, in that case the ΔG is negative. In exergonic reactions the ΔG is negative and in endergonic reactions the ΔG is positive:

${\displaystyle \Delta _{\mathrm {R} }G<0}$ exergon
${\displaystyle \Delta _{\mathrm {R} }G>0}$ endergon

where:

${\displaystyle \Delta _{\mathrm {R} }G}$ equals the change in the Gibbs free energy after completion of a chemical reaction.
Exothermic reaction

An exothermic reaction is a chemical reaction that releases energy through light or heat. It is the opposite of an endothermic reaction.Expressed in a chemical equation: reactants → products + energy.

Exothermic Reaction means "exo" (derived from the greek word: "έξω", literally translated to "out") meaning releases and "thermic" means heat. So the reaction in which there is release of heat with or without light is called

exothermic reaction.

Gigantothermy

Gigantothermy (sometimes called ectothermic homeothermy or inertial homeothermy) is a phenomenon with significance in biology and paleontology, whereby large, bulky ectothermic animals are more easily able to maintain a constant, relatively high body temperature than smaller animals by virtue of their smaller surface area to volume ratio. A bigger animal has proportionately less of its body close to the outside environment than a smaller animal of otherwise similar shape, and so it gains heat from, or loses heat to, the environment much more slowly.The phenomenon is important in the biology of ectothermic megafauna, such as large turtles, and aquatic reptiles like ichthyosaurs and mosasaurs. Gigantotherms, though almost always ectothermic, generally have a body temperature similar to that of endotherms. It has been suggested that the larger dinosaurs would have been gigantothermic, rendering them virtually homeothermic.

Glanosuchus

Glanosuchus is a genus of scylacosaurid therocephalian from the Late Permian of South Africa. The type species G. macrops was named by Robert Broom in 1904. Glanosuchus had a middle ear structure that was intermediate between that of early therapsids and mammals. Ridges in the nasal cavity of Glanosuchus suggest it had an at least partially endothermic metabolism similar to modern mammals.

Heterothermy

Heterothermy or heterothermia (from Greek ἕτερος heteros "other" and θέρμη thermē "heat") is a physiological term for animals that vary between self-regulating their body temperature, and allowing the surrounding environment to affect it. In other words, they exhibit characteristics of both poikilothermy and homeothermy.

Homeothermy

Homeothermy or homothermy is thermoregulation that maintains a stable internal body temperature regardless of external influence. This internal body temperature is often, though not necessarily, higher than the immediate environment (from Greek ὅμοιος homoios "similar" and θέρμη thermē "heat"). Homeothermy is one of the three types of thermo regulation in warm-blooded animal species. Homeothermy's opposite is poikilothermy.

Homeotherms are not necessarily endothermic. Some homeotherms may maintain constant body temperatures through behavioral mechanisms alone, i.e., behavioral thermoregulation. Many reptiles use this strategy. For example, desert lizards are remarkable in that they maintain near-constant activity temperatures that are often within a degree or two of their lethal critical temperatures.

Kleptothermy

Kleptothermy is any form of thermoregulation by which an animal shares in the metabolic thermogenesis of another animal. It may or may not be reciprocal, and occurs in both endotherms and ectotherms. Its most common form is huddling.

Mesotherm

A mesotherm (from Greek μέσος mesos "intermediate" and thermē "heat") is a type of animal with a thermoregulatory strategy intermediate to cold-blooded ectotherms and warm-blooded endotherms.

Stenothermic

A stenotherm (from Greek στενός stenos "narrow" and θέρμη therme "heat") is a species or living organism only capable of living or surviving within a narrow temperature range.

The opposite is a eurytherm, an organism that can function at a wide range of different body temperatures.

Thermolabile

Thermolabile refers to a substance which is subject to destruction, decomposition, or change in response to heat. This term is often used to describe biochemical substances.For example, many bacterial exotoxins are thermolabile and can be easily inactivated by the application of moderate heat.

Enzymes are also thermolabile and lose their activity when the temperature rises.

Loss of activity in such toxins and enzymes is likely due to change in the three-dimensional structure of the toxin protein during exposure to heat.

In pharmaceutical compounds, heat generated during grinding may lead to degradation of thermolabile compounds.

This is of particular use in testing gene function. This is done by intentionally creating mutants which are thermolabile. Growth below the permissive temperature allows normal protein function, while increasing the temperature above the permissive temperature ablates activity, likely by denaturing the protein.

Thermolabile enzymes are also studied for their applications in DNA replication techniques, such as PCR, where thermostable enzymes are necessary for proper DNA replication. Enzyme function at higher temperatures may be enhanced with trehalose, which opens up the possibility of using normally thermolabile enzymes in DNA replication.

Thermoregulation

Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature as its own body temperature, thus avoiding the need for internal thermoregulation. The internal thermoregulation process is one aspect of homeostasis: a state of dynamic stability in an organism's internal conditions, maintained far from thermal equilibrium with its environment (the study of such processes in zoology has been called physiological ecology). If the body is unable to maintain a normal temperature and it increases significantly above normal, a condition known as hyperthermia occurs. For humans, this occurs when the body is exposed to constant temperatures of approximately 55 °C (131 °F), and with prolonged exposure (longer than a few hours) at this temperature and up to around 75 °C (167 °F) death is almost inevitable. Humans may also experience lethal hyperthermia when the wet bulb temperature is sustained above 35 °C (95 °F) for six hours. The opposite condition, when body temperature decreases below normal levels, is known as hypothermia. It results when the homeostatic control mechanisms of heat within the body malfunction, causing the body to lose heat faster than producing it. Normal body temperature is around 37 °C (99 °F), and hypothermia sets in when the core body temperature gets lower than 35 °C (95 °F). Usually caused by prolonged exposure to cold temperatures, hypothermia is usually treated by methods that attempt to raise the body temperature back to a normal range.It was not until the introduction of thermometers that any exact data on the temperature of animals could be obtained. It was then found that local differences were present, since heat production and heat loss vary considerably in different parts of the body, although the circulation of the blood tends to bring about a mean temperature of the internal parts. Hence it is important to identify the parts of the body that most closely reflect the temperature of the internal organs. Also, for such results to be comparable, the measurements must be conducted under comparable conditions. The rectum has traditionally been considered to reflect most accurately the temperature of internal parts, or in some cases of sex or species, the vagina, uterus or bladder.Occasionally the temperature of the urine as it leaves the urethra may be of use in measuring body temperature. More often the temperature is taken in the mouth, axilla, ear or groin.Some animals undergo one of various forms of dormancy where the thermoregulation process temporarily allows the body temperature to drop, thereby conserving energy. Examples include hibernating bears and torpor in bats.

Warm-blooded

Warm-blooded animal species can maintain a body temperature higher than their environment. In particular, homeothermic species maintain a stable body temperature by regulating metabolic processes. The only known living homeotherms are birds and mammals, though ichthyosaurs, plesiosaurs and non-avian dinosaurs are believed to have been homeotherms. Other species have various degrees of thermoregulation.

Animal body temperature control varies by species, so the terms "warm-blooded" and "cold-blooded" (though still in everyday use) suggest a false idea of there being only two categories of body temperature control, and are no longer used scientifically.

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