An endotherm is an organism that produces heat through internal means, such as muscle shivering or increasing its metabolism (Greek: endon = "within", thermē = "heat"). The opposite of endothermy is ectothermy.
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Many endotherms have a larger number of mitochondria per cell than ectotherms, which enables them to generate heat by increasing the rate at which they metabolize fats and sugars. These animals require a much greater quantity of food than ectothermic animals to sustain their higher metabolism.
In many endothermic animals, a controlled state of hypothermia, called hibernation or torpor, conserves energy by lowering the body temperature. Many birds' and small mammals' (e.g. tenrecs) body temperature drops during daily inactivity, such as at night for diurnal animals or during the day for nocturnal animals, thus reducing the energy cost of maintaining body temperature. Human metabolism also slows down slightly during sleep.
The resting human body generates about two-thirds of its heat through metabolism inside 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.[1]
Heat loss is a major threat to smaller creatures, as they have a larger ratio of surface area to volume. Most 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 fat under the skin for insulation, since fur or feathers would spoil their streamlining. Penguins have both feathers and fat, since their need for streamlining limits the degree of insulation which feathers alone can give them. Birds, especially waders, have blood vessels in their lower legs which act as heat exchangers. The veins are adjacent to the arteries, and thus extract heat from the arteries and carry it back into the trunk. Many warm-blooded animals reduce blood flow to the skin by vasoconstriction in response to cold to reduce heat loss. As a result, they blanch (become paler).
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.
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 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.[2] 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[3] suggests fungi are fundamentally ill-equipped to thrive at mammalian temperatures. The high temperatures afforded by endothermy might have provided an evolutionary advantage.
Many insect species are able to maintain an abdominal temperature above the ambient temperature using exercise. These are known as facultative or exercise endotherms.[4] The honey bee, for example, does so by contracting antagonistic flight muscles without moving its wings.[5][6][7] 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.[6][7][8]
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