Most furry mammals pant to regulate their body temperature. Other animals, like ectotherms — lizards, amphibians, and insects — have other behaviors that help keep them cool. Humans, however, are in a category of our own. We are the only mammal that relies on secreting water onto the surface of our skin to stay cool: We call it sweating. But how did we develop this ability? When did we ditch the fur of our primate ancestors in favor of sweaty skin?
At some point in humanity's past, we, too, likely panted to thermoregulate. Our closest primate relatives — chimpanzees and gorillas — dump excess body heat by panting, so it stands to reason that early human ancestors would have panted, too, explains Yana Kamberov, assistant professor of genetics at the Perelman School of Medicine at the University of Pennsylvania.
"Basically, all cooling in mammals involves, to a large extent, the heat that's needed to convert water from a liquid to a gas and the energy that's lost in doing that," Kamberov says. "Furry animals pant in order to take air in, and [they] use that air to dissipate body heat."
For humans, however, something changed over the course of evolution that altered how we as a species thermoregulate and sent us down a unique path. The million-dollar question, Kamberov says, is why.
"One possibility is that it enabled us to explore a niche that was free of predators," she suggests. "If you cool off the way a human does, you can go out during the hottest periods of the day, when most predators are going to be hiding themselves from heat ... We, on the other hand, are able, under very strong radiant heat, to sweat to cool ourselves off. This opens up an avenue for us ... to exploit a niche that otherwise wouldn't be available."
Another hypothesis is that, about 2 million years ago, with the inception of the genus Homo, humans started to develop adaptations that made them good endurance runners. This allowed them to do persistent hunting and gathering, which generates a tremendous amount of body heat, so they needed a way to dump that heat load.
Humans are not the only species that sweat, but our sweat differs from other species, Kamberov notes. Humans have sweat glands that secrete water onto the surface of the skin. Horses also sweat, but they have a different type of gland common in running animals. These are called apocrine glands, and they're associated with the hair on a horse's body.
"What they secrete is not water, but actually rather a mixture of water, lipids, fats, [and] proteins," Kamberov explains. "It's a different kind of sweating; it's a different substance that's being secreted. The horse's main mechanism to cool off is [still] going to be panting."
Other species, such as reptiles, amphibians, and insects, don't have a sweat mechanism, but rely on "an entire suite of traits that they can use to cool off," says Rory Telemeco, a postdoctoral scholar in the Department of Biological Sciences at Auburn University. "Probably the foremost thing is selecting a cooler area or a warmer area and shuttling back and forth between them."
For a lizard or any animal that lives in a hot, dry climate, sweating would be disadvantageous because they would have to replenish the water in their bodies somehow.
"One of the main events that happened in the evolution of reptiles, in particular in the land-dwelling amniotes, was ways to retain water," Telemeco explains. "That's why they have that really heavy, thick, scaly skin. It enables them to retain water. They do still use evaporative cooling some. If you really heat up a desert-dwelling lizard, as it starts to get towards the temperatures that are potentially dangerous, they'll open up their mouth to allow the membranes of the mouth, which are very wet, to have evaporation occur across them, which particularly cools the head down and allows the brain to cool off."
Insects have a high thermal tolerance, but they, too, can get overheated and "break down just like any other animal," Telemeco says. "Butterflies are a neat example because a good ways below that, before they get so hot to where they just can't right themselves or to do anything, they stop being able to fly, because their flight muscles exert [so much] heat."
As it turns out, humans benefit by being exposed to the heat, according to Kamberov. Sweating, in fact, "is a very, very good thing," she says.
"There are studies by Japanese scientists [from] the 1930s and 1940s which demonstrated that where you spend the first two years of life dictates how many of the sweat glands you are born with will become activated, which makes sweating very important," Kamberov explains. "Keeping your children purely in air conditioning is going to affect their ability to thermoregulate later in life."
So, some late summer advice to parents: Get those little ones out of the house in the hot weather and let them sweat.