Born to Sweat: Sweating and Human Evolution

Humans – The Long Shot

If you had to bet on the species that would one day rule the planet, you'd be forgiven for not picking Homo sapiens. 300,000 years ago, our early ancestors weren’t the fastest, strongest, or most agile predators around. There are many disadvantages to being human. Among a few: a lion cub makes its first hunting attempt within a year, yet my kids require years of development before they are remotely self-sufficient (and still ask for help opening snacks). The fastest human ever topped out below 30 miles per hour; unable to run down even a lumbering wildebeest in a sprint and easy prey for those lions (both topping out around ~50 mph). Unlike most mammals, physiologically we can’t synthesize vitamin C (or a host of other vitamins) despite dying (painfully) without it; my dog hasn’t developed scurvy despite never eating an orange in his life. And the human brain, though impressive, comes with its own challenge: representing only about 2% of total body weight, it accounts for roughly 20% of the oxygen and calories the body consumes at rest, calories that must be consumed constantly to prevent death.

So, despite all our drawbacks, how is it that humans became the dominant species on the planet? Our big brains, and the teamwork, tool making, culture, and social cooperation they enable, rightfully get the most credit. But that isn’t the full story. After all, anatomically (i.e., physically, if not culturally) modern humans walked the Earth a quarter of a million years before anyone thought to create advanced projectile weapons; certainly long enough for our ancient ancestor’s lineage to end with a lion’s lunch. Another factor in our success, one that doesn't get much attention, is the millions of microscopic structures covering our skin: our sweat glands.

This article examines why we sweat the way we do, how human sweat is unique among all other animals, and what those of us who work in the heat can do when sweating, our greatest biological cooling advantage, is no longer sufficient.

Human's Second-Best Trait: Sweat

There are well over 6,000 known mammal species on Earth. When it’s hot, they pant, wallow in the mud, or seek shade and chill out during the hottest parts of the day. Almost none cool themselves primarily by sweating. There are a few exceptions; horses sweat, as do some primates. But no species on the planet has the ability to actively cool like humans. In fact, even comparing humans to the few other animal species that sweat feels disingenuous, because there really is no comparison.

Let’s compare with what is arguably the animal world’s closest sweat contender, the horse. On a total sweat volume basis, exercising horses sweat much more than humans. At first glance this would appear to make the horse a “better” sweater, but a horse is six times heavier (meaning more muscle generating heat), with only about 2.5 times more skin surface area to dissipate heat. On a pound for pound basis, human sweat is much more effective at cooling us. To better understand why, we must briefly explore two different types of sweat glands: eccrine and apocrine.

Not All Sweat is Equal

Horses, along with every other mammal except a few primates[1], almost exclusively have apocrine glands. Apocrine glands are associated with fur (i.e., hair follicles) and produce a relatively thick, oily sweat that less easily evaporates into the surrounding air. We retain a few of these apocrine glands ourselves, almost exclusively in the armpit and groin areas – places we retain “fur”. Fun side fact: the oily composition of sweat from these glands, and resulting bacteria growth, is why we use deodorant.

Apocrine sweat typically must pass through a layer of fur before it can evaporate and cool the animal (remember, sweat must evaporate to cool). In the few animals that thermoregulate using apocrine gland sweat, this presents a problem. For example, horse sweat has a high concentration of a protein, latherin, that acts like a slippery soap, allowing water in sweat to pass through a horse’s coat and which creates the lathering observed on working horses. It’s no coincidence “lathering” on a horse looks much like soap lather. From a thermoregulatory standpoint, this makes apocrine sweat much less efficient; it requires a cocktail of proteins and oils just to pass water beyond the fur for evaporation into the air.  

In contrast, humans have almost exclusively eccrine glands. Eccrine glands don’t require a hair follicle; they exist on bare skin and dump sweat directly onto the skin surface. It’s no coincidence we lack a fur coat even though we actually have about the same density of body hair follicles as chimpanzees. Our much thinner and shorter hair doesn’t prevent sweat evaporation.

Along with the development of sweat glands, the loss of fur in our evolutionary ancestors went a long way in enabling the nearly unmatched thermoregulation in hot environments that we enjoy today. Hair loss complemented our sweating ability. Sweat evaporation from our bare skin surface is somewhere between two and three times more efficient than from fur-covered skin. And with no layer of fur to bypass, we have no need for oily additions to our sweat, leaving eccrine sweat almost purely water – typically over 99% water, in fact.[2]

It’s not just that we have efficient eccrine glands for sweating – we have loads of them. Somewhere between two and four million, more than ten times as many as a chimpanzee. A thumbnail-sized section of your skin contains somewhere around 100 eccrine sweat glands, although density varies by body location. The palms of your hands and feet have the most (around 500 glands in each thumbnail-sized patch), while the same area on your legs has only about 60.

These millions of sweat glands allow us to sweat heavily. Military literature, such as this Army Research Institute for Environmental Medicine report, often reference a typical maximum of around 2 liters of sweat production per hour (the same value given in studies of marathon runners in the heat). More typically, a 2019 study of hundreds of athletes found adult male athletes produced, on average, 1.24 liters of sweat an hour, with women producing just under 1 liter.[3] Some literature finds that acclimatized individuals are capable of sweat rates up to 3 liters an hour – nearly a gallonof sweat.

As a bonus, eccrine sweat glands reabsorb much of the electrolytes that would otherwise be lost in sweat. This keeps eccrine sweat overwhelmingly water, minimizing the electrolyte cost (and potential physiological issues of electrolyte depletion) of sustained sweating. Heat acclimatization makes this system even more efficient, allowing the body to sustain high sweat rates for longer before electrolyte deficiency becomes a limiting factor.

Why all the sweat? The Persistence Hunting Hypothesis

Let’s return to how sweating helped propel us to the top of the animal kingdom. Of all the athletic abilities in the animal world – a cheetah’s speed, gorilla’s strength, or lion’s teeth and claws – how does simple sweating stack up?

Let’s be clear: it’s the human brain that ultimately allowed us to get where we are now. But, as mentioned, the human brain is power-hungry and requires cooling. Sweating is one of the ways our evolutionary ancestors addressed the need to maintain a thermoregulatory balance that enabled such a small part of our body to remain constantly active.

But sweating did more than just cool our brain. By having an innate ability to maintain a constant body temperature even on hot days, our ancestor’s world expanded. Large mammal predators like lions are most active in the relatively coolest parts of the day or at night, forcing prey animals (our early ancestors among them) to be active in daytime heat. (Go to the zoo on a summer day. The big cats are sleeping in the shade.) Faced with a tradeoff between nighttime predation and daytime heat stroke, early humans decided to own the day.

By evolving to own the heat, early humans were able to engage in persistence hunting, a technique described by Harvard paleoanthropologist Daniel Lieberman in his book The Story of the Human Body.  How does an early hunter use persistence hunting to take down a much faster antelope or much larger wildebeest? By running them to death.

The idea goes like this: humans, while not particularly fast, are designed to walk and run long distances, thanks largely to our ability to avoid overheating by sweating. Large four-legged mammals, while faster and more agile, need to pant to cool down. Critically, animals like wildebeest and antelope cannot pant while running, since their breathing is tied to stride cycles when running (even dogs, which can pant while running, do so much less effectively – panting is most efficient with rapid, shallow breaths, while running certainly is not). By targeting one animal and maintaining the chase at a steady pace, early humans could quite literally run a prey animal to exhaustion by raising the target’s core temperature beyond what it could endure. As a bonus, persistence hunting occurred at a time when competing predators were busy staying cool in the shade!

In the central Kalahari desert, persistence hunting continued until modern times. A fascinating first-hand account of the technique is available here, with details reflecting the central role of sweating and the human ability to operate in the heat. Hunts would take place at the hottest time of the day, with temperatures even over 100^oF. Hunters would first drink as much water as possible, then choose a single target to chase.

The target animal (for example, a kudu, a type of large African antelope), would sprint off before finding a place to rest in the shade. The key to success in this hunt is tracking – if hunters maintain or regain contact before the animal cooled off, they catch up and force it to sprint away again in the heat before recovering. The cycle repeated until the animal was too overheated to continue. In this manner, in as little as two hours, a small group of hunters could take down the larger, faster antelope.

It’s also relevant to briefly digress into the topic of recent global temperature changes. The period of time during human evolution saw some relatively wide swings in global average temperature, with the warmest period about 125,000 years ago during the last interglacial period. Depending on who you ask (there is always uncertainty in scientific study), our current environment is either approaching the warmth experience during this last interglacial or has already exceeded it. The implication? We are likely on the cusp of an environment warmer than the one our ancient ancestors evolved in.

The Downside

If sweating is such a superpower, why is it that only humans do it so well? The full answer is nuanced (losing fur has drawbacks, and bipedalism, which reduces solar exposure and makes distance travel highly efficient, may have been a requirement), but there is one obvious drawback to copious sweating – water loss.

The total amount of daily water loss during physical work in the heat can exceed 10 liters (with 16 liters a day mentioned in one report – more than four gallons!). Ten liters of water is over 20 pounds, more than 10% the body weight of many adult men, and the loss of more than 2% of our weight in water results in a loss of performance and thermoregulatory ability. All that sweating creates a huge requirement to replace all the water we lose.

Additionally, acclimatization to the heat – our best natural defense against heat illnesses like heat stroke – actually increases the rate that we sweat! This is a bit of a catch 22, since our improved ability to operate in the heat means we have an increased need to replace the water we lose to sweat.

Humans Today

While my ancestors might have evolved to run down gazelle on the African savanna, I’m under no impression I could do the same today. I’m in decent shape (I’ll do the occasional 10k race) and might retain the inherent genetics for it, but I certainly don’t have the lifetime of heat acclimatization (the physiological requirement) or the knowledge to choose prey and track it (the cultural requirement). It’s a safe guess that you don’t, either.

However, many of us still exert ourselves in the heat to provide the family meal. We just do it less directly than hunting, using the tools needed for construction, farming, logging, repair, or public safety to earn our dinner.

Which brings us back to our big brains. Our cultural and technological capacity to innovate and thrive in the heat still helps us, even if most of us aren’t able to run down prey animals in the summer. Qore Performance’s ICEAGE ECOSYSTEM is a prime example of how we extend our innate human ability to operate in heat, providing a superhuman boost to our natural ability to cool by sweat.

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About the author: Dr. Erik Patton holds a PhD from Duke University where he conducted research on the challenges rising temperatures pose for military training. An Army veteran, Erik has served in a variety of extreme climates ranging from deserts in the U.S. Southwest and Middle East (120^oF) to Arctic conditions in central Alaska (-42^oF).