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In a story I am writing, some animals (including sentient ones) can touch fire without being harmed.

Not any kind of fire, though. They can caress the fire of a candle with a finger, and they can have their paws or hands inside a fireplace for a few seconds with only some disconfort and pain but no injuries. More than that harms them. They can still die from burns in a house fire (though they are more likely to survive one, and with less severe wounds) and their bodies can be cremated into ashes.

How could mammals evolve to be resistant to fire like that?

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    $\begingroup$ Related: Fire Resistant Fauna $\endgroup$ – Secespitus Sep 30 '17 at 15:09
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    $\begingroup$ Check out the adaptations of thermophiles that live around volcanic vents on earth. Many of those can be adapted $\endgroup$ – pojo-guy Sep 30 '17 at 16:02
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    $\begingroup$ Different, but maybe of interest: Some trees have nuts that germinate after being burned. They grow after forest fires. $\endgroup$ – mviereck Sep 30 '17 at 22:00
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Mammals are not known to be thermophiles (a type of extremophile or organism that lives and thrives in extremely hot temperatures). However, most thermophiles are Archaea (single-celled organisms) and they can survive extreme heat very well. Perhaps a symbiotic relationship would be appropriate for your mammals.

The Pompeii Worm makes its home next to hydrothermal vents in the ocean. By themselves, these worms would likely not survive very long, but they have a coating of bacteria on their backs that affords a degree of insolation. In return, the worms provide for the bacteria through their mucus secretions that the bacteria feed on.

This explanation has a few advantages, as it scrapes a lot of time off your worlds evolutionary development and frees up some of the design elements of your creatures. It also matches your descriptions of heat tolerance to a point, but more extreme temperatures will still cause damage. The downside is that to support the bacteria in sufficient numbers, the mammals would likely be slimy to the touch.

Works Cited: 5 Animals That Can Take the Heat: National Geographic | Alivinella pompejana: Wikipedia | Thermophile: Wikipedia | Archaea: Wikipedia

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Perhaps the animals evolved in an area where localized hot spots were common. Constant micro-meteorite rain or a planetary crust that is honeycombed by tiny lava tubes might work. More realistically, perhaps a semi-sentient prey animal started using small fires as a defense mechanism and the local predators had to evolve small flame resistance or starve.

  • The first line of defense to evolve might be a fire resistant fluid excreted by the pores in their hands.
  • Thicker cell walls in the outer epidermis and improved cellular repair/replacement would also be helpful.
  • A faster circulatory system and heat-trapping blood could distribute the fire's energy throughout the predator's body.
  • Finally, a semi-magical enhancement to their cellular mitochondria, allowing them to actually digest the fire's energy and store it for future cellular function. (You might want to look into these heat-loving creatures for insights on how digesting heat works).

That is a pretty heavy shopping list for conventional evolution, but with the help of a benevolent god (author) or a team of creative genetic-scientists, it might be doable.

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The first thing I thought is leathery skin, or armadillo-like armor. Otherwise they could have developed a skin feature that constantly makes them exhale hydrogen or something to keep their skin wet.

Obviously for this to happen they probably evolved in someplace where they are often exposed to fire and burns, or in a place where heat burns grass and shrubs on a regular basis, or they evolved somewhere completely unrelated but where they needed similar characteristics to those required to resist fire.

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They are wet.

Not just wet, but furry and wet. Water has a high specific heat

The specific heat of water is 1 calorie/gram °C = 4.186 joule/gram °C which is higher than any other common substance. As a result, water plays a very important role in temperature regulation. The specific heat per gram for water is much higher than that for a metal, as described in the water-metal example.

So: water will absorb heat and heat up slowly, giving you the degree of fire resistance you want. The best way I can think of to have a lot of water on body surfaces is to have it held there by fine fur, or maybe matted fur like my cat. If the water is salty too it will evaporate less easily, although the specific heat is slightly less. Your creatures will be furry like a cat but sweat like a horse. Or a human. Except humans don't have hair on their palms unless... anyway, most humans don't.

If you don't want them wet all the time and don't want furry hands, you could have them sweat like mad in the presence of heat. Which I certainly do. Also if I am in a high place (or even think about it) my hands sweat. For better grip? Anyway, your creatures could pour sweat out of any body part in the fire. That would work well. It might be a little hard to explain in prose - harder than saying they are are covered in soaking wet fur clumps.

Maybe you don't want your creatures to be wet all the time and you don't like the sweat on demand idea. Instead, you could have them be oily. Some furry things are oily, especially aquatic mammals, and the oil stays on the fur. Olive oil (as an example of an oil) has a specific heat about half that of water, which is not bad. http://www.engineeringtoolbox.com/specific-heat-capacity-d_391.html It could serve the same function. A problem with oil on the fur is that if it did get hot enough to catch fire, these poor creatures would really burn hot and the fire would spread over them fast.

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The ecosystem has a dominant predator species which hunts by breathing fire.

Any species which is resistant to fire has a higher chance to survive an encounter with this predator. So fireproof skin becomes an evolutionary advantage and might develop independently in multiple species.

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Humans are already pretty nicely adapted to heat. Using the immense cooling power of evaporation of water, we can survive even in extreme heats.

So, as long as the climate isn't just insanely hot, but rather plagued with local, short-term heat-bursts (lava & hot steam), an animal could survive in it using an improved version of sweating.

The amount we sweat would be higher, and the boiling temperature of our sweat would be lower. Coming in contact with something extremely hot would lead to evaporation surrounding our body, acting as a gaseous heat shield (see Leidenfrost-effect).
Additionally, the skin would be more or less fire-resistant - multiple layers of dead skin, mixed in with substances to weaken thermal conductivity.
Finally, below those layers, tissues with good blood circulation to dissipate any excess heat left.

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