| Thermoregulation in animals |
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Warm-blooded is a term referring toanimalspecies whose bodies maintain a temperature higher than that of their environment. In particular,homeothermic species (includingbirds andmammals) maintain a stable body temperature by regulatingmetabolic processes. Other species have various degrees ofthermoregulation.
Because there are more than two categories of temperature control utilized by animals, the termswarm-blooded andcold-blooded have been deprecated in the scientific field.
In general, warm-bloodedness refers to three separate categories ofthermoregulation.
A significant proportion of creatures commonly referred to as "warm-blooded," like birds and mammals, exhibit all three of these categories (i.e., they are endothermic, homeothermic,and tachymetabolic). However, over the past three decades, investigations in the field of animal thermophysiology have unveiled numerous species within these two groups that do not meet all these criteria. For instance, many bats and small birds become poikilothermic and bradymetabolic during sleep (or, in nocturnal species, during the day). For such creatures, the termheterothermy was introduced.
Further examinations of animals traditionally classified ascold-blooded have revealed that most creatures manifest varying combinations of the three aforementioned terms, along with their counterparts (ectothermy, poikilothermy, and bradymetabolism), thus creating a broad spectrum of body temperature types. Somefish have warm-blooded characteristics, such as theopah.Swordfish and somesharks havecirculatory mechanisms that keep theirbrains andeyes above ambient temperatures and thus increase their ability to detect and react toprey.[1][2][3] Tunas and some sharks have similar mechanisms in their muscles, improving their stamina when swimming at high speed.[4]
Body heat isgenerated bymetabolism.[5] This relates to the chemical reaction incells that break downglucose into water andcarbon dioxide, thereby producingadenosine triphosphate (ATP), a high-energy compound used to power other cellular processes. Muscle contraction is one such metabolic process generating heat energy,[6] and additional heat results from friction as blood circulates through the vascular system.
All organisms metabolize food and other inputs, but some make better use of the output than others. Like all energy conversions, metabolism is rather inefficient, and around 60% of the available energy is converted to heat rather than to ATP.[7] In most organisms, this heat dissipates into the surroundings. However, endothermic homeotherms (generally referred to as "warm-blooded" animals) not only produce more heat but also possess superior means of retaining and regulating it compared to other animals. They exhibit a higher basal metabolic rate and can further increase their metabolic rate during strenuous activity. They usually have well-developed insulation in order to retain body heat: fur andblubber in the case of mammals andfeathers in birds. When this insulation is insufficient to maintain body temperature, they may resort toshivering—rapid muscle contractions that quickly use up ATP, thus stimulating cellular metabolism to replace it and consequently produce more heat. Additionally, almost alleutherian mammals (with the only known exception beingswine) havebrown adipose tissue whosemitochondria are capable ofnon-shivering thermogenesis.[8] This process involves the direct dissipation of the mitochondrial gradient as heat via anuncoupling protein, thereby "uncoupling" the gradient from its usual function of driving ATP production viaATP synthase.[9]
In warm environments, these animals employ evaporative cooling to shed excess heat, either throughsweating (some mammals) or bypanting (many mammals and all birds)—mechanisms generally absent in poikilotherms.
It has been hypothesized that warm-bloodedness evolved in mammals and birds as a defense againstfungal infections. Very few fungi can survive the body temperatures of warm-blooded animals. By comparison, insects, reptiles, and amphibians are plagued by fungal infections.[10][11][12][13] Warm-blooded animals have a defense against pathogens contracted from the environment, since environmental pathogens are not adapted to their higher internal temperature.[14]
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