IRA FLATOW: This is Science Friday. I’m Ira Flatow. The dog days of summer are here. And you know that phrase is generally used to describe the hot, muggy days late in the season, when even our four-legged friends don’t have the energy to do anything except sit around and pant, just trying to keep cool. I’ll bet you didn’t know that that phrase did not originate with dogs. It may have originated with Greek astronomy.

Nevertheless, like dogs, many mammals pant to regulate their body temperature. Other animals, like ectotherms– lizards, amphibians, and insects– have to look to their environment for a cool, shady spot. And, of course, we humans are in a category of our own. Believe it or not, we are the only mammal that relies on secreting water onto the surface of our skin to stay cool– to sweat. But how did we develop this ability? And when did we ditch the fur of our primate ancestors in favor of sweaty skin?

This week, we’re taking a refreshing dive into the world of thermoregulation. That means ways humans and animals evolved to beat the heat. Have you a question about adaptive behaviors for staying cool? Give us a call. Our number, 844-724-8255, 844-SCI-TALK, or tweet us @scifri.

Yana Kamberov is Assistant Professor of Genetics at the Perelman School of Medicine at the University of Pennsylvania. Welcome to Science Friday, Dr. Kamberov.

YANA KAMBEROV: Thank you, Ira. How are you?

IRA FLATOW: Fine, how are you?

YANA KAMBEROV: I’m good.

IRA FLATOW: Good. Rory Telemeco is a post-doctoral scholar in the Department of Biological Sciences at Auburn University. Welcome to Science Friday.

RORY TELEMECO: Thank you.

IRA FLATOW: Yana, let’s talk about why furry animals have to pant to stay cool. What does that do for them?

YANA KAMBEROV: So panting, as you said, is something that furry animals do. And 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. And so furry animals plant in order to take air in, and use that to dissipate body heat.

They do it in their upper respiratory tract. In a dog, this happens– sorry, I guess I would say in a horse, that big snout that a horse has, you’re getting evaporative cooling inside that big snout. And it’s, again, just an animal using convection, essentially, to evaporate water off of the surface.

IRA FLATOW: It’s like when you come out of the shower, you feel cold, because the water is evaporating off of you.

YANA KAMBEROV: Exactly. That’s exactly right.

IRA FLATOW: So what about our primate ancestors? Is that what they did also?

YANA KAMBEROV: Well, if we look at our closest primate relatives– chimpanzees, gorillas, if we even look beyond that into outside of the great apes, and we look at macaques, for example, old world monkeys, we see that panting is the main mechanism by which they thermoregulate, by which they dump their body heat. And so it stands to reason that our ancestors– our last common ancestor with the chimpanzee– would also have panted.

But something changed over the course of human evolution, perhaps having to do with the fact that we became striding bipeds and later evolved endurance running capabilities, which altered how we as a species thermoregulate, and sent us down a very unique path. But so the idea is that, yeah, we– probably very similar to a chimpanzee. Panting as a main mechanism to cool off is probably what our human ancestors did.

IRA FLATOW: You were talking about a horse’s panting, but don’t we all see horses sweating all the time also?

YANA KAMBEROV: They do. Right, exactly. So horses lather up. But so what’s distinguishing about– humans are not the only species that sweats, but it’s the kind of sweat that we have. As you said, we have sweat glands that secrete water onto the surface of the skin.

Horses have a different type of gland. A lot of cursorial animals, running animals, have this. They are called apocrine glands, and they’re associated with the hairs on the horse’s body. And what they secrete is not water, but it’s actually rather a mixture of some water, some lipids, some fats, some protein. So it’s not the same– it’s a different substance that’s being secreted. But the horse’s main mechanism is going to be panting to cool off.

IRA FLATOW: I didn’t know that. I always thought the sweating was–

YANA KAMBEROV: Yeah, it’s a different kind of sweating.

RORY TELEMECO: Now, Rory, your animals that you study don’t sweat.

RORY TELEMECO: Correct.

IRA FLATOW: What do they do?

RORY TELEMECO: Well, they have an entire suite of traits that they can use to cool off. Probably the foremost thing is selecting a cooler area or a warmer area, and shuttling back and forth between them. And humans will do that some, too, [INAUDIBLE] and whatnot.

IRA FLATOW: So we’re talking about lizards and amphibians and insects, and stuff like that?

RORY TELEMECO: Yeah, pretty much every animal other than a bird or a mammal. So even nematodes will thermoregulate some. Fish. Yeah.

IRA FLATOW: So I guess for a lizard, or an animal that lives in a hot, dry climate, sweating would be very disadvantageous, because then you’d have to get the water back in your body somehow.

RORY TELEMECO: Correct. One of the main evolutionary events that happened in the evolution of reptiles, in particular in the land-dwelling amniotes, was ways to retain water. That’s why they have that really heavy, thick, scaly skin, is it enables them to retain water.

They do still use evaporative cooling some. So 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 the head down and allows the brain to cool off. Interesting.

IRA FLATOW: So Dr. Kamberov, why did we develop the ability to sweat instead of pant? What was the advantage there?

YANA KAMBEROV: That’s sort of the million dollar question. So we actually can’t pant, because of change structural changes in the human body. For example, we don’t have that big snout that a horse is going to have. Our face has a very reduced snout.

So there are a couple of hypotheses that exist as to why humans developed such a unique way to dissipate heat. One possibility is that it enabled us to explore a niche that was free of predators. So 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. They’re going to stay out of the sun, essentially.

We, on the other hand, are able, under very strong radiant heat, to just sweat to cool ourselves off. Kind of opens up an avenue for us to– we’re not very fast, right– to go and exploit a niche that otherwise wouldn’t be available.

Another hypothesis is that, about 2 million years ago, really with the inception of the genus Homo, there is evidence that suggests that humans started to developed adaptations that made them good endurance runners. So we could do persistence running, persistent hunting and gathering. And that generates a tremendous amount of body heat. And so you need a way to dump that heat load.

Now, if you’re going to be running– a horse at full gallop cannot pant, so its main mechanism for cooling off is not available because of the way in which the organs are going to push on the body as a quadruped is going to– a four-legged animal is going to slant forward. So again, your main mechanism isn’t available to you.

On the other, hand humans can run around at our full speed and still manage to cool off. So there’s a hypothesis that this allowed us to exploit persistence hunting, for example, or cover, again, long distances in the hot midday sun, in the warm regions where humans first evolved.

IRA FLATOW: Interesting. Meg tweeted us a nice picture of her cat. There’s a lot of cat pictures, but it’s a nice question. She says, how do cats cool down? I read their paws can sweat a little, but my cat pants after a game of mouse.

YANA KAMBEROV: That’s right. So cats cool off by panting. But it is true that– again, humans have a very specialized kind of sweat gland. I mentioned apocrine glands, which exist in horses. But sweat glands, eccrine sweat glands, which are the glands that secrete water onto the surface of human skin, in virtually all other mammals outside of the closest primate relatives to humans, those glands are restricted to the bottom of the paws.

And in those– if you look at the bottom of your cat’s foot, remember what you see is that thick pad, and then in between you see a bunch of hair. That thick pad is just chock full of sweat glands. And what those sweat glands are actually used for in this case is not for thermoregulation, but they’re used for traction, so how much friction there is between the animal and the surface that it’s walking on. You can think about when you have a dry palm, and you try to just slide your palm across a surface, it sort of just skids. So you need a little bit of–

IRA FLATOW: People spit on your hands before they–

YANA KAMBEROV: That’s right. That’s right. Gives you a little traction. Exactly.

IRA FLATOW: That’s interesting. Rory, we never think of insects as getting overheated, but can they? Can it get too hot for some insects?

RORY TELEMECO: Yes, they sure can. They have some of the highest thermal tolerance of animals. They can tolerate temperatures often into the upper 40s C. So they’re getting around into the low 100s Fahrenheit. But they will overheat, and they’ll break down just like any other animal that gets too hot.

IRA FLATOW: So when they break down, does that mean they just collapse and have to wait for it to get cooler? Or does it kill them?

RORY TELEMECO: It depends on exactly how hot it gets. It will first cause them to almost go into a torpor state. They look almost catatonic if you keep them up to just that point. And then you cool them down, they usually can recover, but if you go just a degree or more over that, it will kill them.

IRA FLATOW: Wow. It’s true of butterflies, other–?

RORY TELEMECO: Mmhm.

IRA FLATOW: Really.

RORY TELEMECO: Yup. All of the insects. And butterflies are a neat example, because they– a good ways below that, before they get hot to where they just can’t right themselves or to do anything, they stop being able to really fly, because their flight muscles exert enough heat.

IRA FLATOW: How hot would that be, outside? What temperature would that be?

RORY TELEMECO: Upper 90s, low 100s.

IRA FLATOW: Really?

RORY TELEMECO: Your body temperature gets that hot.

IRA FLATOW: Then we’re going to see a lot of that now, aren’t we?

RORY TELEMECO: Well, potentially, right. But they still can behaviorally thermoregulate. So the first thing that would happen is they would adjust the time of day that they’re out flying, or potentially whether or not they can go into a shady area and let their body cool down, and then fly some more. They also, when they’re up in the air, are being able to cool through convection just by blowing through the air–

IRA FLATOW: Interesting.

RORY TELEMECO: –which can help bring that down a little bit. But, eventually, you’re right, they will get hot enough that they have to land and stop feed or stop ovipositing.

IRA FLATOW: This is Science Friday from PRI, Public Radio International, talking about sweating with Yana Kamberov and Rory Telemeco.

Let me ask you this, Dr. Kamberov. Recent research shows that tattoos, too many tattoos, can interfere with your sweat glands. Have you heard about that?

YANA KAMBEROV: I’ve vaguely heard about this. Someone forwarded me the article. I haven’t actually had a chance to look at it. In a way, it’s not surprising. You’re going to damage the skin a bit where the sweat glands live. And you might be damaging some of the vasculature, some of the blood vessels. You might be damaging some of the nerves that control sweating. So yeah, that would argue that you shouldn’t completely cover yourself in tattoos. That would be very bad if you’re in a hot place.

And Rory, speaking of hot, when it gets hot out, we like to go jump in a lake or a pool or something like that. Is that what animals that can’t regulate, like amphibians, is that what they do also, why we see frogs in the ponds and things like that?

RORY TELEMECO: Yeah, to some extent. The frogs have to stay in the water as well just because if they dry out, they die. But being in the water helps, because as they’re cooling– enough evaporative cooling happens just by sitting in a puddle, because the skin of frogs can shed water so readily. They’re sitting there in their puddle, they don’t dry out, but they’re shedding water, allowing them to cool off. And they rarely will get too hot as long as they stay moist.

IRA FLATOW: That’s quite interesting. Yana, if we are sweating because we want to stay cool, what about the sweating we do when we get nervous? Sweaty palms, your armpits–

YANA KAMBEROV: Sweaty palms. Exactly. So it turns out that the way that those glands are innervated– so remember that the ancient condition, the ancestral condition, is sweat glands in the hands and the feet. And the way that those are innervated is actually different than the thermoregulatory sweat glands that are in the rest of your body, that are primarily responsible for cooling you off, which makes sense. You’re not going to have, as Rory mentioned, a tremendous amount of convection when you’re standing on top of your sweat glands.

So, exactly, when you get nervous, your hands start to sweat. And it’s just the neurotransmitters that are– the main activators of those sweat glands are different than the neurotransmitters of the rest of the body.

IRA FLATOW: So they may have evolved a different way.

YANA KAMBEROV: Ultimately, the way to sort of– going back to your earlier question of when did all this happen, and why did why did humans develop this unique system of thermoregulation– if we know what the genetic causes are for this, then we can actually date when it happened. If we know the genetic changes that are responsible for expanding sweat glands from being just on the bottom of the feet to being everywhere in the human body, then we can learn when this occurred, and have a sense of perhaps why it occurred in humans. But–

IRA FLATOW: Go ahead. I’m sorry. I didn’t mean to interrupt.

YANA KAMBEROV: Oh, no. Yeah. So the basic mechanism of how you build a sweat gland in the palm, there’s got to be a a basic mechanism of how the skin decides to build a sweat gland, versus some other skin-derived organ like a hair. But then, there is, as you said, an extra layer that’s going to be put on top of that, that this is a thermoregulatory sweat gland that’s going to respond differently than sweat glands in the palms.

IRA FLATOW: So in general, summing up, sweating is a good thing, then.

YANA KAMBEROV: Sweating is a very, very good thing. And it’s really– I’m going to go ahead and plug this. So there are studies that go back to the 1930s and the 1940s by Japanese scientists, which actually demonstrated that where you spend your first two years of life dictates how many of the sweat glands you are actually born with will become activated, which makes it– so sweating is very important, so keeping your children purely in air conditioning is going to affect their ability to thermoregulate later in life.

IRA FLATOW: So it’s important to sweat, to be exposed to heat, to sweat during those first two years of life. Get the kids out of the house in the hot weather.

YANA KAMBEROV: Get the kids out of the house. Exactly.

IRA FLATOW: All right. That’s a great way to sum up. Thank you. That’s all the time we have. Yana Kamberov is Assistant Professor Of Genetics at the Perelman School of Medicine at University of Pennsylvania, and Rory Telemeco is post-doctoral scholar in the Department of Biological Sciences at Auburn University. I think we learned so much about sweat. Didn’t know that. Thank you both for taking time to be with us today.

RORY TELEMECO: Thank you.

YANA KAMBEROV: Thank you.

IRA FLATOW: You’re welcome. Have A good weekend.

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