An endotherm is an organism that produces heat through internal means, such as muscle shivering or increasing its metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

 (Greek: endon = "within", thermē = "heat"). The opposite of endothermy is ectothermy.

Generating and conserving heat

Many endotherms have a larger number of mitochondria per cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

 than ectotherms, which enables them to generate heat by increasing the rate at which they metabolize fat
Fats consist of a wide group of compounds that are generally soluble in organic solvents and generally insoluble in water. Chemically, fats are triglycerides, triesters of glycerol and any of several fatty acids. Fats may be either solid or liquid at room temperature, depending on their structure...

s and sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...

s. 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
Hypothermia is a condition in which core temperature drops below the required temperature for normal metabolism and body functions which is defined as . Body temperature is usually maintained near a constant level of through biologic homeostasis or thermoregulation...

, called hibernation
Hibernation is a state of inactivity and metabolic depression in animals, characterized by lower body temperature, slower breathing, and lower metabolic rate. Hibernating animals conserve food, especially during winter when food supplies are limited, tapping energy reserves, body fat, at a slow rate...

 or torpor
Torpor, sometimes called temporary hibernation is a state of decreased physiological activity in an animal, usually characterized by a reduced body temperature and rate of metabolism. Animals that go through torpor include birds and some mammals such as mice and bats...

, 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.

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
Insulation means:* Building insulation, added to buildings for comfort and energy efficiency* Soundproofing, also known as acoustic insulation, any means of reducing the intensity of sound...

 in the form of fur
Fur is a synonym for hair, used more in reference to non-human animals, usually mammals; particularly those with extensives body hair coverage. The term is sometimes used to refer to the body hair of an animal as a complete coat, also known as the "pelage". Fur is also used to refer to animal...

 or feather
Feathers are one of the epidermal growths that form the distinctive outer covering, or plumage, on birds and some non-avian theropod dinosaurs. They are considered the most complex integumentary structures found in vertebrates, and indeed a premier example of a complex evolutionary novelty. They...

s. Aquatic, warm-blooded animals, such as seals
Pinnipeds or fin-footed mammals are a widely distributed and diverse group of semiaquatic marine mammals comprising the families Odobenidae , Otariidae , and Phocidae .-Overview: Pinnipeds are typically sleek-bodied and barrel-shaped...

 generally have deep layers of fat under the skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...

 for insulation, since fur or feathers would spoil their streamlining. Penguin
Penguins are a group of aquatic, flightless birds living almost exclusively in the southern hemisphere, especially in Antarctica. Highly adapted for life in the water, penguins have countershaded dark and white plumage, and their wings have become flippers...

s have both feathers and fat, since their need for streamlining limits the degree of insulation which feathers alone can give them. Birds, especially wader
Waders, called shorebirds in North America , are members of the order Charadriiformes, excluding the more marine web-footed seabird groups. The latter are the skuas , gulls , terns , skimmers , and auks...

s, have blood vessels in their lower legs
Rete mirabile
A rete mirabile is a complex of arteries and veins lying very close to each other, found in some vertebrates. The rete mirabile utilizes countercurrent blood flow within the net...

 which act as heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...

s. 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
Vasoconstriction is the narrowing of the blood vessels resulting from contraction of the muscular wall of the vessels, particularly the large arteries, small arterioles and veins. The process is the opposite of vasodilation, the widening of blood vessels. The process is particularly important in...

 in response to cold to reduce heat loss. As a result, they blanch (become paler).

Avoiding overheating

In equatorial climates and during temperate summers, overheating
Hyperthermia is an elevated body temperature due to failed thermoregulation. Hyperthermia occurs when the body produces or absorbs more heat than it can dissipate...

 (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
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....

 in the breath, and/or flushing, increasing the blood flow to the skin so the heat will radiate
Radiant energy
Radiant energy is the energy of electromagnetic waves. The quantity of radiant energy may be calculated by integrating radiant flux with respect to time and, like all forms of energy, its SI unit is the joule. The term is used particularly when radiation is emitted by a source into the...

 into the environment. Hairless and short-hair
Hair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....

ed mammals also sweat
Perspiration is the production of a fluid consisting primarily of water as well as various dissolved solids , that is excreted by the sweat glands in the skin of mammals...

, since the evaporation of the water in sweat removes heat. Elephants keep cool by using their huge ear
The ear is the organ that detects sound. It not only receives sound, but also aids in balance and body position. The ear is part of the auditory system....

s like radiator
Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in automobiles, buildings, and electronics...

s in automobiles. Their ears are thin and the blood vessel
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...

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.

Advantages and disadvantages 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 to the optimum temperature for enzyme activity.

The overall rate of an animal's metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

 increases by a factor of about two for every 10 C-change rise in temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

, limited by the need to avoid hyperthermia
Hyperthermia is an elevated body temperature due to failed thermoregulation. Hyperthermia occurs when the body produces or absorbs more heat than it can dissipate...

. 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 season
A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...

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. 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
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...

. A recent study
suggests fungi are fundamentally ill-equipped to thrive at mammalian temperatures. The high temperatures afforded by endothermy might have provided an evolutionary advantage.

Facultative endothermy

Many insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

 species are able to maintain an abdominal temperature above the ambient temperature using exercise. These are known as facultative or exercise endotherms. The honey bee
Honey bee
Honey bees are a subset of bees in the genus Apis, primarily distinguished by the production and storage of honey and the construction of perennial, colonial nests out of wax. Honey bees are the only extant members of the tribe Apini, all in the genus Apis...

, for example, does so by contracting antagonistic flight muscles without moving its wings. This form of thermogenesis is, however, only efficient above a certain temperature threshold, and below about 9–14 °C (48.2–57.2 °F), the honey bee reverts to ectothermy.
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