Human sweat is weird. And I’m talking from an evolutionary point of view, not just because it smells. Unlike us, most mammals don’t use sweat to keep cool. Dogs pant, cats lick themselves, and humans, along with a handful of other primates, sweat1.
As gross as this strategy can be, it does work extremely well. One under-appreciated human superpower is that we’re pretty good at dealing with heat. In fact, our early ancestors may have hunted their prey by exhausting them.
Where did our super sweat come from? Are the other sweaty primates as good at it as us? New research reveals the answer to some of these questions.
How to make sweat super
Although humans are fairly unique in sweating to stay cool, other animals still sweat. However, they’re doing it differently.
There are two basic types of sweat gland, apocrine and eccrine. We have eccrine glands over most of our body and apocrine glands in a few limited areas. In most other animals, it’s the other way round with their body dominated by apocrine glands1.
Exactly what they’re doing with these other glands is debated. Horses are one of the few animals using them for heat management, but beyond that, we don’t really know. What we do know is that the difference in glands is a large part of the reason we’re better at staying cool than them.
Eccrine glands continuously release a liquid composed mostly water when activated, making them great for staying cool. Apocrine glands have more fat in their mixture; which bacteria eat, causing body odour. They also don’t secrete their liquid continuously, which is probably for the best given their connection to body odour.
Constant, watery sweat isn’t the only reason our glands are superior at staying cool.
Comparisons with other species reveal our eccrine glands have more blood vessels feeding them (and removing waste products). They also tend to have greater stores of glycogen, the chemical which powers the glands1.
All of this makes our sweat very impressive. But how did it evolve?
Looking at other apes and monkeys, sweating ability seems random. Some have some have the same, super-sweat adaptations as us, others don’t. Yet others seem to mix and match them! It seems evolution was working on shuffle-setting as primates evolved1.
At least, until you look at the climate the various primates grew up in.
Shocking as it may seem, living in a hotter climate was correlated with the evolution of better sweat glands. Interestingly, for many species, this was a bigger predictor of sweating ability than lineage. Two closely related primates would wind up with different glands if they lived in different areas. This explained why sweating ability seemed so random1.
So where do humans fit in?
Despite the fact that family doesn’t matter, our glands still appear to be most similar to chimps. We both have lots of eccrine glands, feed them with many vessels, and keep them full of glycogen.
Humans and chimps, sweating in a tree
Around 7 million years ago humans and chimps were part of the same, ape-like species. This species lived in a hotter climate, so evolved better sweat glands. Over time our two lineages drifted apart but kept our great sweat glands. Likely because we both still had to deal with hot environments.
However, that doesn’t mean we’re both equally good at staying cool. Since then, there’s been another big change. We lost our hair, helping make our thermoregulation even more effective (with caveats). As great as chimps are, we’re still better at dealing with heat than them.
So when the inevitable ape rebellion comes, we should be able to at least run away from them over long distances.
- Best, A. and Kamilar, J.M., 2018. The evolution of eccrine sweat glands in human and nonhuman primates. Journal of human evolution, 117, pp.33-43.