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My world is extremely acidic. Upon contact with liquid water, the acid forms and can lead to several problems that the natives have to face. One of these problems is that the farther from the extremes of the world (in a part equivalent to the equator), the higher the temperatures tend to be throughout the year. In this scenario, perspiring through the skin (something that apparently only mammals do; correct me if I'm wrong) can be lethal, both in the short and long term, as the acid will form from this excreted liquid.

How could mammals in my world develop to deal with this problem? Sweating differently, like dogs?

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"perspiring through the skin (something that apparently only mammals do; correct me if I'm wrong) " If you define perspiration as squeezing liquid water our via the skin... many things do that. Many Mammals, amphibians, snails, some TREES, etc..

If water contact with the air or dust forms acid, your inhabitants should stop breathing. Because the inside of your lung is permanently damp. If it dries, you die. Immediately.

How to deal with it? Easy. Just base the creature's whole metabolism AT the same acidity level as the environment. 1 or 2 pH difference can be accommodated by metabolic chemistry, but anything more is simply not doable.

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    $\begingroup$ most furred animals do not sweat. $\endgroup$ – Trish Oct 23 '20 at 11:55
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I will ignore the fact that realistically in your world life would either have developed around this acid such that its interaction with water wouldn't be a problem at all (kind of like we aren't cowering from the dread power of oxygen, it's everywhere and a basic part of our biochemistry, some organisms might need to avoid it but most have biochemical protections to tolerate it and even actively use it), or would not have developed at all. Because water is fundamental to biochemistry, making it so that liquid water always becomes acidic after being exposed to air will have effects that go far beyond issues with sweating. As MarvinKitfox points out if you have problems with sweating you'll have worse ones with breathing, and let's not even get into eating and drinking.

Ignoring all that: the reason organisms sweat to cool down is to use the properties of evaporative cooling. Dogs cool down the exact same way, panting just evaporates the saliva in their mouth instead of sweat on their skin. Some alternatives could be: using a different chemical (not that easy because water has unique properties that make it very good for this but presumably there are plausible alternatives. Maybe ammonia?). Secreting very alkaline sweat to counteract the acid, but that kind of gets back to the notion that your biochemistry would have developed to deal with that acid to begin with. One could suppose alternate ways of controlling the temperature, but unfortunately while trying to find examples I ran across this paper that says... basically that evaporative cooling is the only possible way animals have to keep their body temperature below the environments':

https://jeb.biologists.org/content/219/14/2145

You could imagine organs that work on principles of compressing/expanding gases or liquids like a fridge does. Or having a reflective covering, though I think that will only take you so far (the fact animals aren't white in the tropics suggests it's not that useful, not enough to outweigh camouflage concerns at least). The other way Earth animals avoid excessive temperatures is behaviorally: seeking out shade and cooler areas when it is particularly hot.

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    $\begingroup$ +1 purely for the highly accurate phrase ‘dread power of oxygen’ $\endgroup$ – Joe Bloggs Oct 23 '20 at 11:47
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I'd have to agree with marvin's answer. If the simple contact with air is enough to form highly acidic substances, your creatures, assuming them to work exactly like vertebrate life on earth can't really:

  • Open their mouths (which are damp since saliva has water. This also means panting like a dog to cool down will likely be an unpleasant experience).

  • Breathe (most of the respiratory system is kept moist at all times).

  • Eat (small amounts of air do enter your digestive track during the eating process, meaning this air could potentially turn the moisture all the way to your stomach into acid, which is a problem).

  • Get hurt in any way which resulted in decent bleeding (at least in humans, plasma is responsible for over half of our blood volume, and plasma is itself 90% water).

  • See with eyes like our own (our eyes can capture light because they have many living cells, and because they're living they need to be kept moist).

  • Smell with noses like our own (we capture smell particles mostly thanks to the mucus in our nose, which is also meant to help moisturizing the air before it enters the lungs. The snake method of smelling also doesn't work, because the tongue and the vomeronasal organ are kept damp as well ).

To sum it up: vertebrates like those found on earth as we know them would, in their majority, have little to no chance of survival, because each and every one of them had their evolution focused around one thing: water. Every single one of them came from the ocean, and adaptations from land are basically separated into 2 groups: moving on land and conserving water inside their systems, but we have little to no adaptations to dealing with said water suddenly turning into acid as far as I'm aware.

If these alien mammals can survive in your world's conditions, that means they don't have to worry about the acid at all, because unlike our world's mammals, your mammals are more than adapted to these acid environments, thanks to the billions of years of evolution selected by said acidic environment. This also accounts any and all: reptiles, fish, birds, fungi, echinoderms, plants, mollusks, arthropods, annelids, euchariote cells, bacteria and any other organic life your world might have.

There's only one scenario I can see in which we have mammals not adapted to the harsh environment suddenly appearing in the planet without any trace of being adapted to it: a group of very stubborn humans who traveled to this world in hopes of colonizing it and yet decided to stay despite knowing that every single time they leave their airtight, filtered stations, they'd need to wear airtight, hamzat-suit style protection suits with self-enclosed breathing apparatuses, and that if said suits or living spaces suffer a breach, the people inside them have a good chance of dying.

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