User:Bio-m77/Homeothermy

Homeothermy or homoiothermy (from Greek ὅμοιος homoios "similar" and θέρμη thermē "heat") is a form of thermoregulation in which an organism maintains a constant internal body temperature despite external temperature fluctuations. Homeothermy often results in an organism's internal body temperature being higher than the surrounding environment, but this is not always the case. Homeotherms are often endothermic, but there are also several forms of ectothermic homeotherms. Many ectothermic homeotherms live in areas with stable external temperatures (such as some tropical fish which live in stable coral reefs).

Homeothermy likely evolved as a response to nocturnal lifestyles 200-66 million years ago. Homeotherms can use many different forms of physiological or behavioral thermoregulation in order to maintain a constant body temperature, including but not limited to: vasodilation/vasoconstriction, muscle contractions, burrowing, migration, sweating/panting, piloerection, and non-shivering thermogenesis. Poikilothermy is the opposite of homeothermy, and both have particular advantages and disadvantages which can help the organism survive and function effectively in their environment.

Evolution

While the exact origin of homeotherms is not certain, it is believed that exclusive homeothermy could have evolved between 200 and 66 million years ago. Some researchers have suggested that exclusive homeothermy may have evolved in response to nocturnal lifestyles. Other researchers have suggested that it developed following the mass extinction at the Cretaceous-Paleogene Boundary that eventually lead to the diversification of placentals.

Homeothermy enables higher metabolic capabilities that are not present in poikilotherms, which allows homeotherms to maintain their internal body temperature despite external changes. Strict homeothermy generally relies on heat generated and retained via metabolism and other physiological mechanisms, making it an energetically costly regulatory system. Recent evidence has suggested that the homeothermy observed in most mammals must have evolved from an older heterothermic mechanism. This is because strict heterothermy's high metabolic cost would have been unfavorable to earlier life forms.

Adaptations

Endothermic homeotherms often maintain stable body temperatures using metabolic heat production and other physiological mechanisms. In contrast, ectothermic homeotherms often use behavioral mechanisms and changes in their cardiovascular system to maintain stable body temperatures. Homeotherms have many different behavioral and physiological thermoregulatory mechanisms that they can use to maintain a constant body temperature.Thermoregulation in homeotherms serves two purposes: counteracting large external/internal temperature changes that can disrupt homeostasis, and counteracting continuous minor fluctuations in temperature which occur within the thermoneutral zone. Endotherms use adaptive thermogenesis in order to deal with changes in the temperature of the external environment. Adaptive thermogenesis is the production of heat during cold exposure. However, unlike homeotherms, poikilothermic ectotherms often have to acclimate their cellular metabolism and biochemistry to deal with temperature changes. This acclimation can occur via isozymes, antifreeze proteins, producing transcription factors for metabolic genes, mitochondrial biogenesis, and homeoviscous adaptations.

Behavioral Mechanisms

Many homeotherms make use of both physiological and behavioral mechanisms to maintain a constant internal temperature. Some homeotherms may rely solely on behavioral thermoregulation though. Generally, ectothermic homeotherms are more reliant on behavioral mechanisms than endothermic homeotherms, as ectotherms cannot effectively maintain their internal temperature using physiological means. Often times these ectothermic homeotherms need to live in temperate, stable climates that are similar to their internal body temperature.

In cold environments, homeotherms can use several behavioral mechanisms in order to maintain a constant body temperature and stay warm. Some homeotherms migrate to escape cold conditions, though they may do so to escape the heat as well. Migration makes the maintenance of internal homeostatic temperatures more efficient, as the organism does not have to deal with fluctuating environmental conditions. Many homeotherms may actively seek out warm, sunny, windless areas to maintain a relatively warm, constant body temperature despite the cold. Some organisms may curl up in order to reduce the amount of surface area that is exposed to the cold air, minimizing heat loss. Some homeotherms build warm nests, closed off shelters, or burrows in order to escape the cold. Deep burrows can also be used to cope with exceedingly hot environments.

Homeotherms have many different behavioral mechanisms used to adjust to hotter environmental conditions. These mechanisms let them stay cool and maintain a constant body temperature in said conditions. Some homeotherms enter bodies of water in order to maintain a constant body temperature in both hotter and cooler temperatures, as water temperatures do not change much over time when compared to air temperatures. Some homeotherms may move to cooler, shadier areas in order to cool off. Some animals have been known to orient their body in a certain manner to reduce the amount of surface area exposed to the sun. This allows them to minimize the amount of heat they gain through radiation in exceedingly hot environments (as seen in some insects and birds).

Physiological Mechanisms

Endothermic homeotherms rely mostly on physiological adaptations to maintain a relatively stable and high internal temperature. Many organisms, both ectothermic and endothermic can use vasoconstriction in order to minimize heat loss to the cold environment through convection. Vasodilation is also used by both endotherms and ectotherms in order to increase heat loss in hotter environments. An organism's heart rate may also vary in response to heating and cooling. Organisms may increase their blood flow to dissipate more heat from the body in exceedingly hot environments, and blood flow may decrease to conserve heat in colder environments. Counter-current heat exchange is another technique used by some homeotherms to reduce heat loss in limbs using heat exchange through blood flow.

Mammals and birds are both "warm-blooded" and homeothermic; however, this group of homeothermic animals is polyphyletic.

Homeotherms may increase their metabolic heat production in colder temperatures. Heat production can be generated through muscle "tone", which consists of sporadic and frequent contractions of separate motor units of the skeletal muscles. While thermoregulatory muscle tone bears some resemblance to shivering, it generates far less heat than shivering. In colder temperatures, muscle "tone" will cease to be used for metabolic heat production, and shivering will be used instead, as shivering generates 4-17 times the amount of heat that "tone" produced. However, homeotherms almost never use shivering as a means of continuously dealing with low temperatures. While involuntary muscle activity like shivering can help warm a homeotherm in cooler conditions, voluntary activity can also generate metabolic heat in a similar way. Homeotherms may also generate heat during cold periods using non-shivering thermogenesis. Non-shivering thermogenesis involves the production of heat using the mitochondria found within brown fat. To cope with hot temperatures, some homeotherms decrease the amount of heat produced by heavily limiting muscle activity, which also reduces their metabolic rate. Some homeotherms can reduce the amount of epinephrine in their blood, which reduces their metabolic rate, which helps to maintain a constant internal body temperature in hot environments. Some organisms can use estivation in order to deal with hot, arid conditions.

In hot temperatures many homeotherms may use sweating (evaporation off of the body surface) or panting (evaporation off of the mucosa of the respiratory tract) in order to lose excess body heat through evaporation. Similarly to shivering, homeotherms almost never use this for continuous adjustments to high temperatures. Insulation (fat, fur, hair or feathers) can often be used by homeotherms living in cold environments to better retain heat. Piloerection ("puffing up" the fur and feathers) can allow the insulation to become even more effective, as piloerection allows the insulation to better trap still air, which acts as an effective insulator. In contrast to piloerection, many homeotherms may flatten their fur or feathers against their body to reduce the amount of insulation provided by these structures, letting them better lose heat in hotter environments.

Advantages and Limitations

Poikilotherms are organisms which do not maintain fixed internal body temperatures. Instead, their body temperature fluctuates based on their environment and level of activity.

Advantages of Homeothermy

Enzymes are usually only capable of functioning optimally at fairly narrow temperature ranges. If enzymes are exposed to temperatures outside of this narrow range, it can reduce or entirely stop the rate of the reaction which the enzyme catalyzes. So homeotherms can specialize in enzymes that are efficient at a particular temperature, as homeotherms constantly keep their body temperature in a narrow, specific temperature range. In contrast, poikilotherms often need to operate well below their enzyme's optimum efficiency. Alternatively, poikilotherms may migrate, hibernate, or use additional resources to produce a wider range of enzymes to account for their wide range of possible body temperatures. However, some environments offer much more consistent temperatures than others. For example, the tropics usually have minimal temperature variations. In addition, large bodies of water, such as the ocean and very large lakes, have moderate temperature variations.

Homeotherms tend to have higher metabolic rates than poikilotherms over a wider range of temperatures. Homeotherms are also capable of remaining active at a wider range of temperatures when compared to poikilotherms.

Disadvantages of Homeothermy

Many homeotherms use enzymes that are specialized for a narrow temperature range, meaning hypothermia can rapidly lead to decreases in cellular functions, torpor, loss of consciousness, and death. Hyperthermia results in crucial proteins denaturing, cell membranes being degraded, and eventually the organism's death. Poikilotherms generally tend to tolerate changes in internal body temperature (hypothermia and hyperthermia) better than homeotherms. This is because poikilotherms can use homeoviscous adaptations to change the amount of cholesterol and the types of lipids in their membrane (to avoid the membrane being either degraded or too stiff). Poikilotherms can also use isozymes in order to have functioning enzymes at different internal body temperatures.

Additionally, homeothermy obtained from endothermy often results in homeotherms having to consume far more energy than poikilotherms of a similar size. This means that many environments have a lower carrying capacity for homeotherms due to the amount of food they need relative to a poikilotherm of a similar size. However, behavioral thermoregulation is far less energetically expensive than physiological mechanisms, so this disadvantage mainly applies to endothermic homeotherms. In cold weather, the energy used in order to maintain a constant body temperature can accelerate starvation and may lead to death. As a result, these mechanisms of thermogenesis cannot be used for extended periods of time.

Another disadvantage is that evaporative thermoregulation in homeotherms could result in a significant loss of water and energy. Thermoregulation by evaporation also does not function well in humid environments, which could be detrimental to homeotherms in hot humid environments when compared to poikilotherms in those same environments. Excess sweating/panting can also lead to dehydration and hypovolemia if water isn't replenished, meaning it cannot be used for extended periods of time as a form of thermoregulation in hot weather.

See Also

• Kleptothermy

• Gigantothermy
• Thermoregulation
• Poikilothermy
• Hypothalamus
• Heterothermy
• Thermogenesis