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There are a wide range of environmental temperatures located all around the world, from the low -90° C to high 60° C at the extremes. When looking at the core body temperature however the range is closer together at about 36° C to 40° C. From there we living things like humans can only go so far dealing with outside temperatures. (Near the end of the article 'What environmental changes can we handle?') There are a lot of negative consequences when the core body temperature changes too much. Nature has many ways for endothermic creatures to maintain their body temperature for hot and cold environments like size and shape of the body to having better insulation, fur, sweating, etc.

My question is would it be possible to change the range of endothermic thermoregulation to something higher or lower and by how much? I figure there would be limiting factors like water's boiling and freezing point. I figure there is also an operational range for digesting food in an efficient manner when it comes to enzymes breaking down proteins, fats, and such. But otherwise I do not know if it is possible for an endothermic creature to have a core body temperature of lets say the extremes of near 0° C or 100° C.

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    $\begingroup$ You may find this of interest scienceinschool.org/2011/issue20/arctic as there are creatures that can exist in .... cold environments. The AFP's mentioned should be able to work for land based creatures as well $\endgroup$ – Enigma Maitreya Apr 1 '17 at 0:57
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On this Earth many proteins begin to denaturate around 45° to 48° C, making this an upper bound for internal temperature of homeothermic animals. The lower bound is given by the ambiental temperature (an active homeotherm with internal temperature below ambient uses lots of energy to shed heat) and by the diminished activity of enzymes at low temperatures (at 20° C enzymes have about half of the efficiency they have at 37° C). Consider for example humans with an internal temperature around 37° C -- when the ambiental temperature climbs above 35° C we lose most of our endurance, and if ambiental temperatures go above 45° C we die. So I would expect that the vast majority of homeotherms will have internal temperatures of about 30° to about 45° C.

For extraterrestrial organisms everything goes. After all, they are extraterrestrial and use exotic xenobiochemistry; I can't see how anything quantitative can be said about a xenobiont with unkown xenobiochemistry.

Note: There are indeed thermophile organisms which thrive at high temperatures, even above 100° C; they are all prokaryotes (archaea or bacteria), that is, single celled organisms with no nucleus. All multicellular organisms are eukaryotes, that is, their cells have a nucleus. I don't know of any thermophile eukaryote.

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Living being chemistry is something that doesn't change too much among differente species. This is probably due to the fact that, once a "recipe" has been found to work, you cannot easily change it and still having it work.

So, to make life viable at higher temperatures, you would need to start almost from scratch, and let evolution select proteins stable in the temperature range you are targeting. Probably a good starting point would be some estremophile bacteria, in particular termophile

A thermophile is an organism—a type of extremophile—that thrives at relatively high temperatures, between 41 and 122 °C (106 and 252 °F). Many thermophiles are archaea. Thermophilic eubacteria are suggested to have been among the earliest bacteria.

Thermophiles are classified into

  1. Obligate thermophiles (also called extreme thermophiles) require such high temperatures for growth
  2. Facultative thermophiles (also called moderate thermophiles) can thrive at high temperatures, but also at lower temperatures (below 50 °C (122 °F))
  3. Hyperthermophiles are particularly extreme thermophiles for which the optimal temperatures are above 80 °C (176 °F).
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It depends on the creature's biochemistry. Our proteins, enzymes and biochemical reactions have evolved to fit our body temperature, which is why we die if it changes much. An alien life form could evolve to have a different body temperature, but it would then have different biochemistry.

If it's water based then yes, the liquidity range of water will be important - but at higher pressures water boils at higher temperatures than 100°C. Our body temperature range is much closer to freezing than boiling point, but I don't think there is any reason why this must be the case; suitable biochemistry would make higher temperature life possible. Many molecules are unstable at high temperature, but there are many other molecules which are not - there are organisms on earth (thermophiles) which have functioning metabolisms at very high temperatures, so there's no real reason to assume it's not possible.

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