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Thermoregulation in animals |
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A eurytherm is an organism, often an endotherm, that can function at a wide range of ambient temperatures.[1] To be considered a eurytherm, all stages of an organism's life cycle must be considered, including juvenile and larval stages.[2] These wide ranges of tolerable temperatures are directly derived from the tolerance of a given eurythermal organism's proteins.[3] Extreme examples of eurytherms include Tardigrades (Tardigrada), the desert pupfish (Cyprinodon macularis), and green crabs (Carcinus maenas), however, nearly all mammals, including humans, are considered eurytherms.[4][5][6] Eurythermy can be an evolutionary advantage: adaptations to cold temperatures, called cold-eurythemy, are seen as essential for the survival of species during ice ages.[7] In addition, the ability to survive in a wide range of temperatures increases a species' ability to inhabit other areas, an advantage for natural selection. Eurythermy is an aspect of thermoregulation in organisms. It is in contrast with the idea of
Examples
- Tardigrades, known for their ability to survive in nearly any anhydrobiotic state.[9]
- The desert pupfish, a rare bony fish that occupies places like the Colorado River Delta in Baja California, small ponds in Sonora, Mexico, and drainage sites near the Salton Sea in California, can function in waters ranging from 8º to 42ºC.[10]
- The green crab is a common species of Victoria, Australia in the south, including more temperate regions like Northwest Africa in between.[11] The green crab has been shown to survive in waters at least as cold as 8°C, and at least as warm as 35°C.[12]
- conifers (genus Larix) are the primary plants occupying the boreal forests of Siberia and North America. Although they are conifers, they are deciduous, and therefore lose their needles in Autumn. Species like the black spruce, or tamarak (Larix laricina) occupy wide swaths of land ranging from Indiana in the south, well into the arctic circle in Northern Alaska, Canada, and Siberia in the north.[13]It has been shown that the black spruce can endure temperatures as cold as –85°, and at least as warm as 20°C.
- Killer whales (Orcinus Orca) are found at nearly every latitude on earth.[14] They are able to withstand water temperatures ranging from 0° to 30-35°C.[15] Killer whales are deemed a cosmopolitan species, along with the osprey (Pandion haliaetus) and the house sparrow (Passer domesticus).[15][16][17]
Advantages over Stenotherms
It is thought that adaptations to cold temperatures (cold-eurythermy) in animals, despite the high cost of functional adaptation, has allowed for mobility and agility. This cold eurythermy is also viewed as a near necessity for survival of the evolutionary crises, including ice ages, that occur with relative frequency over the evolutionary timescale. Due to its ability to provide the excess energy and aerobic scope required for endothermy, eurythermy is considered to be the "missing link" between ectothermy and endothermy.[18] The green crab's success demonstrates one example of eurythermic advantage. Although invasive species are typically considered to be detrimental to the environment in which they are introduced, and even considered to be a leading cause of animal extinctions,[19] the ability of an animal to thrive in various environmental conditions is a form of evolutionary fitness, and therefore is typically a characteristic of successful species. A species' relative eurythermality is one of the main factors in its ability to survive in different conditions. One example of eurythermic advantage can be seen in the failure of many of the world's coral reefs. Most species of coral are considered to be stenothermic.[20] The worldwide increase in oceanic temperatures has caused many coral reefs to begin bleaching and dying because the coral have begun to expel the zooxanthellae algae that live in their tissues and provide them with their food and color.[21][22] This bleaching has resulted in a 50% mortality rate in observed corals in the waters off of Cape York in Northeastern Australia, and a 12% bleaching rate in observed reefs throughout the world.[23] Although regulators, especially endotherms, expend a significantly higher proportion of energy per unit of mass, the advantages of endothermy, particularly endogenous thermogenesis, have proven significant enough for selection.[24]
Thermal Coping Mechanisms
The ability to maintain
Key to maintaining homeostasis, individual thermoregulation is the ability to maintain internal body temperature in humans, the most recognizable eurytherm. In humans, deep-body temperature is regulated by
While some organisms are eurythermic due to their ability to regulate internal body temperature, like humans, others have wildly different methods of extreme temperature tolerance. Tardigrades are able to enter an anhydrobiotic state, often called a tun, in order to both prevent dessication and endure extreme temperatures. In this state, tardigrades decrease their bodily water to about 1–3% wt./wt.[34] Although this state allows certain tardigrades to endure temperatures at the extremes of –273° and 150°C at the extremes, tardigrades in their hydrated state are able to withstand temperatures as low as –196°C. This displayed extremotolerance has led scientists to speculate that tardigrades could theoretically survive on Mars, where temperatures regularly fluctuate between –123° and 25°C, as well as even possibly the near absolute zero of interplanetary space. The tardigrade's ability to withstand extremely cold temperatures as a tun is a form of cryptobiosis called cryobiosis. Although the high temperature endurance of tardigrades has been significantly less studied, their cryobiotic response to low temperatures has been well-documented.[35][36] Tardigrades are able to withstand such cold temperatures not by avoiding freezing using antifreeze proteins as a freeze avoidance organism would, but rather by tolerating ice formation in the extracellular body water, activated by ice nucleating proteins. In addition to other organisms, plants (Plantae) can be either stenothermic or eurythermic. Plants inhabiting the boreal and polar climates generally tend to be cold-eurythermic, enduring temperatures as cold as –85°, and as warm as at least 20°C, such as boreal deciduous conifers.[37] This is in direct contrast to plants that typically inhabit more tropical or montane regions, where plants may have purely tolerable range between only about 10° and 25°C, such as the banyan tree.[37]
Eurythermal Protein Adaptation
The tolerance for extreme body temperatures in a given eurythermic organism is largely due to an increased temperature tolerance by the respective organism's homologous proteins. In particular, the proteins of a warm-adapted species may be inherently more eurythermal than a cold-adapted species, with warm-adapted species' proteins withstanding higher temperatures before beginning to denature, therefore avoiding possible cell death.[38][39] Eurythermal species also have shown adaptations in protein synthesis rates compared to non-eurythermal similar species. Rainbow trout(Salmo gairdneri) have shown constant protein synthesis rates over temperatures ranging from 5° to 20°C, after acclimating to any temperature in this range for 1 month. In contrast, carp
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- ^ "Corals Are Dying on the Great Barrier Reef". 2016-03-21. Retrieved 2018-06-08.
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