Warm-blooded

"Hot blooded" redirects here. For the song, seeHot Blooded.

For the warmblood sport horse breeds, seeWarmblood.

Thermographicimage: acold-bloodedsnake is shown eating a warm-blooded mouse

Warm-bloodedis an informal term referring toanimalspecieswhose bodies maintain a temperature higher than that of their environment. In particular,homeothermicspecies (includingbirdsandmammals) maintain a stable body temperature by regulatingmetabolicprocesses. Other species have various degrees ofthermoregulation.

As there are more than two categories of temperature control utilized by animals, the termswarm-bloodedandcold-bloodedhave been deprecated in the scientific field.

Terminology

In general, warm-bloodedness refers to three separate categories ofthermoregulation.

• Endothermyis the ability of some creatures to control their body temperatures through internal means such as muscle shivering or increasing theirmetabolism.The opposite of endothermy isectothermy.
• Homeothermymaintains a stable internal body temperature regardless of external influence and temperatures. The stable internal temperature is often higher than the immediate environment. The opposite ispoikilothermy.The only known living homeotherms aremammalsandbirds,as well as one lizard, theArgentine black and white tegu.Some extinct reptiles such asichthyosaurs,pterosaurs,plesiosaursand some non-aviandinosaursare believed to have been homeotherms.
• Tachymetabolismmaintains a high "resting" metabolism. In essence, tachymetabolic creatures are "on" all the time. Though their resting metabolism is still many times slower than their active metabolism, the difference is often not as large as that seen inbradymetaboliccreatures. Tachymetabolic creatures have greater difficulty dealing with a scarcity of food.

Varieties of thermoregulation

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,andtachymetabolic). 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 termheterothermywas introduced.

Further examinations of animals traditionally classified ascold-bloodedhave 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. Somefishhave warm-blooded characteristics, such as theopah.Swordfishand somesharkshavecirculatorymechanisms that keep theirbrainsandeyesabove ambient temperatures and thus increase their ability to detect and react toprey.Tunas and some sharks have similar mechanisms in their muscles, improving their stamina when swimming at high speed.

Heat generation

Body heat isgeneratedbymetabolism.This relates to the chemical reaction incellsthat break downglucoseinto 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,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.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 andblubberin the case of mammals andfeathersin 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 alleutherianmammals havebrown adipose tissuewhosemitochondriaare capable ofnon-shivering thermogenesis.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.

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.

Defense against fungi

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.Warm-blooded animals have a defense against pathogens contracted from the environment, since environmental pathogens are not adapted to their higher internal temperature.

See also

• Argentine black and white tegu § Endothermic behavior
• Mesotherm
• Thermogenic plant

References

Footnotes

Citations

External links

• What is Warm Blooded??
• The Reptipage: What is cold-blooded?