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Basically I want to create a world in which some of the humans have developed a higher body temperature in order to survive cold, harsh climates, such as the poles or even tall mountains. The reason behind it is rather selfish: I just want them to be a sort of barbarian like nation wearing skimpy outfits, something along the lines of Tarzan (please do not dwell on this aesthetic aspect though, just the scientific implications).

I know that the human body goes into heat stroke at temperatures of over 40 degrees Celsius/107 degrees Fahrenheit, but I was wondering if there's any way for it to evolve to overcome that. How would that affect a human being's appearance? Would they need a better vascularized skin, maybe a larger heart? Would this have any impact on their height or muscular development? I was thinking about the fact that an increased pilosity could help them maintain body heat better, but the idea of ape-looking humans does not appeal to me. Perhaps they could have a thicker skin instead? Also, I want my evolved humans to be mostly hunters, so quite active, therefore I need to know whether this increase in body temperature would have any impact on their diet as well.

I remember reading that monks in Tibet can increase the temperature in their extremities (such as hands and feet) by up to a third in order to prevent frostbite, but the explanation for that was meditation. Obviously my frost barbarians won't have time to meditate, so I was thinking of a more permanent solution.

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    $\begingroup$ Note: you will still need to obey the laws of thermodynamics. If you want a warmer creature without any protection against the frigid elements, that creature is going to have to consume a remarkable amount of calories. There's a reason why Eskimo can withstand extraordinary exposure, but still elect to wear lots of warm clothes when they can. You'll need a lot of high energy food available. $\endgroup$ – Cort Ammon Oct 9 '15 at 22:45
  • $\begingroup$ Oh yes, that should go without saying, which is why these humans would be hunters rather than gatherers, also because the climate would get in the way. I just don't know how many calories per day would they require. I really wish there was some sort of algorithm to calculate the link between maintaining a certain body temperature and the calorie intake but so far I haven't found any. $\endgroup$ – Feidhelm Oct 9 '15 at 22:48
  • $\begingroup$ Its actually not a simple curve. The body is not well modeled as a lump for thermodynamics. Have you considered what they would do with the excess energy? Societies are not wasteful. They won't burn calories just to keep warm if they can use those calories to accomplish something and stay warm as a side effect. $\endgroup$ – Cort Ammon Oct 9 '15 at 22:58
  • $\begingroup$ I did not want to dwell on an explanation as to what their purpose would be in such a universe, as I have already nailed that down and it would expand too much on the background of my project. What I do need to figure out is how they could have achieved this state, preferably with as much scientific background as possible. $\endgroup$ – Feidhelm Oct 9 '15 at 23:02
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    $\begingroup$ How do you think having a higher body temperature would help in a cooler climate? All it would do is increase the temperature differential, causing you to lose heat faster and have more issues not less! $\endgroup$ – NPSF3000 Oct 10 '15 at 6:09
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The answer to this is simpler than you think: don't make their body temperature special at all.

Consider that you don't have to raise core temperatures at all. In fact, it's not really all that helpful. The difference in temperature between the core and the outside environment is so monumental, that raising body temperature doesn't help all that much. Consider: the average winter temperature in the arctic is −30°F. The human body is 98.6° F, round up to 100°F, there's a 130°F difference between the outside and the core. Now, lets super super super charge our humans, so that instead of operating at 100 degrees, they operate at 135°F (the highest recorded land temperature ever). Now the difference is 165°F. Going from "normal human" to "ridiculously hot" only bought us a 22% increase in thermal flux!

We can, however, change the distributions. The core is always the warmest part of the body, and the body gets colder as you approach the extremities. Humans have evolved to keep the core warm, and we so by cutting off blood flow to regions that get cold, basically sacrificing their warmth so that the core can stay warm. In fact, we can cut blood flow to the skin to almost zero, in environments of extreme cooling (source). Its the cutting of the blood flow that allows things like frostbite to happen. Once we cease to warm up a body part, it begins to drop in temperature far more rapidly.

If your Barbarians simply adjusted this process so that they didn't engage in as much vasoconstriction due to cold, they'd easily keep their toes and fingers from freezing. This would be the non-meditating version of what the Tibetan monks do.

Humans have evolved to be efficient with our energy, and our cold response is part of that. If you don't care about efficiency at all (no known species on the planet), or you can be very active and use waste heat to stay warm, there's no particular reason you need any special core temperature or weather gear. Just make sure the heat gets to the outside, and you wont have any trouble at all.

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    $\begingroup$ Well, in some parts of the world, we have more a problem of not burning enough calories … $\endgroup$ – celtschk Jul 28 '16 at 10:29
  • $\begingroup$ So, you'd end up with systemic hypothermia in no time. Sure, your fingers and toes wouldn't get frostbitten, but you wouldn't live as long in cold conditions as a normal human who does get frostbite. Your body heat would go to keeping your extremities warm, and would be lost, eventually leading to death, whereas a normal human might get frostbitten and perhaps lose a few toes, but would live longer. Evolution is about compromises that enhance the chances of procreating more, not preserving relatively unnecessary digits at the cost of lowering the chances of future reproduction. $\endgroup$ – Monty Wild Jul 28 '16 at 23:15
  • $\begingroup$ @MontyWild The issue with hypothermia only exists if the body doesn't adapt to this behavior. If you simply generate more body-heat, you can make up for the extra lost heat. Of course, as you point out, if this is evolved, there's going to need to be a very important use for those extra-warm digits to "pay for" the cost of the extra calories burned. Of course, if you are very active all the time, those calories are basically free beause you generated them doing useful work. $\endgroup$ – Cort Ammon Jul 28 '16 at 23:23
  • $\begingroup$ @CortAmmon, calories are never free. Additionally, in sufficiently cold conditions, you might be unable to convert stored energy to heat fast enough to maintain your body temperature in the face of the heat lost from your warm extremities, and even if you could, you'd run out of energy faster than a normal - but frostbitten - human. It'd take an evolutionary pressure where people who are missing digits due to frostbite won't ever be able to reproduce again to achieve something like this. $\endgroup$ – Monty Wild Jul 28 '16 at 23:40
  • $\begingroup$ @MontyWild Calories can be free. If you already needed to expend them to do work, the waste heat from that action is... well... waste. If you can harness that waste heat to heat your extremities better, then they're yours to take. I agree that the situation is highly unlikely, but you can create hypothetical scenarios where it makes sense. If the main source of food for the humanoids can only be found through tiny variances in surface textures, the value of keeping those sensory neurons functioning would come from the acquisition of calories that were otherwise unattainable. $\endgroup$ – Cort Ammon Jul 29 '16 at 0:06
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TL;DR: NO!

This is a case of 'you can't get there from here' evolution - at least in any realistic sense.

'Cold-blooded' creatures such as lizards and frogs have many different proteins that do each task, each operating optimally in a different temperature regime. However, with the evolution of homeothermy, a particular temperature was chosen, and the DNA coding for the now-irrelevant proteins was lost. A frog cell's DNA is far longer than a human's for that reason.

The problem is that to evolve proteins that can all operate in a higher temperature regime is quite difficult, whereas to simply select the right proteins from an existing set is far simpler - except that humans no-longer have those proteins, or the DNA that codes for them, but reptiles and amphibians did - and still do.

The Alternative:

The solution to your problem - scantily-clad barbarians in a cold climate - is actually simpler, and could well add to your desired trope:

Make them big. Really big, say seven to nine feet tall and four or five hundred pounds, with masses of muscle covered by a good padding of fat. They wouldn't look like Arnold Schwarzenegger, they'd look more like big wrestlers or weight lifters. The advantage of this is described by the square-cube law. More volume in total means less surface are per unit volume, thus reduced heat loss.

In addition, having a more active metabolism would mean that they'd be producing more heat, and they'd be able to allow more blood to flow through their extremities in cold weather, thus greatly reducing the chances of frostbite. In fact, wearing too many clothes would be uncomfortable for them, especially in warmer climates - they'd be at risk of overheating.

On the downside, being this big and having this high a metabolism would require a lot of fuel, and we can expect our big, muscular barbarians to be big eaters too. Fortunately, the meat that they hunt is a good source of energy and protein, both of which they'd require in abundance.

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  • $\begingroup$ If there was an adaptive reason for higher body temperatures, we could evolve them. We would just evolve proteins that would function at those higher temperatures. There probably is an upper limit, but it is probably nearer 50 than 37, $\endgroup$ – James K Jul 28 '16 at 13:19
  • $\begingroup$ @JamesK, realistically, no. Normal body temperature is tied so closely into the way that we work that to increase it would lead to all sorts of problems, not the least that only a relatively small further increase in body temperature could lead to death. $\endgroup$ – Monty Wild Jul 28 '16 at 23:08
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Increasing body temperature only means you'd lose more heat to the outside. It won't increase your survival odds.

What you want is faster circulation, as in more beats per second and better insulation. In land animals, gap between individual hairs traps air to prevent heat loss. The other way around that is to have air pockets trapped in fat as in whales or large deep sea animals. So, your options are: hairy barbarians, fairy barbarians (feathers), or flabby barbarians.

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    $\begingroup$ add information about that en.wikipedia.org/wiki/Brown_adipose_tissue , as not all humans loose it as adult (some aborigines from Australia - have it, as I head) but need to digg, to see if it's true or not (heard from 2 sources, but can't be sure they are independent) $\endgroup$ – MolbOrg Jul 28 '16 at 11:00

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