IRA FLATOW: This is Science Friday. I’m Ira Flatow. A bit later in the hour, the bird flu. This highly pathogenic strain is ripping through flocks, especially poultry, and now it’s making its way into mammals. So just how worried should we be? We’ll be talking to scientists monitoring the current outbreak.

But first, we are quickly approaching summer, and I’m thinking about how to keep my body cool. I do the linen shirt, the wide-brimmed hat, wear some shorts, but you know what I wonder about, my hair. Does hair play a part in keeping you cool? Turns out, it does.

According to a fascinating new study, it depends on your type of hair. Curly hair, curly locks are better than straight hair at keeping your scalp cool, but why? Well, we’ll be answering that and taking your questions. Give us a call. Our number is 844-724-8255, 844-SCI-TALK or tweet us @SciFri.

Do you have curly hair? Do you notice your head staying cooler in the sun than your straight-haired friends? Are you curious about the evolutionary benefits of curly hair? We want to hear from you. 844-724-8255, 844-SCI-TALK or tweet us @SciFri.

Joining me now to help answer these questions and more is my guest Dr. Tina Lasisi, incoming assistant professor in anthropology at the University of Michigan. She joins us from Los Angeles. Welcome to Science Friday.

TINA LASISI: Hi, Ira. How are you doing?

IRA FLATOW: Nice to have you– fine I am. Thank you. I’m doing well.

To start off, I want to talk a little bit about the experimental setup you use to come to this conclusion. You used a mannequin named Newton, three different wigs, right?

TINA LASISI: That’s right.

IRA FLATOW: Tell me about a little more how you designed this experiment.

TINA LASISI: Well, I have to give credit where credit is due, and I was working with some great people, some thermal engineers and physiologists at Loughborough University who work in the field of environmental ergonomics. So they’re very used to asking questions about how different fabrics affect our ability to cool down or to stay warm, and they have these climate-controlled chambers where they have a wind tunnel where they put Newton and usually put different clothes on him. But what I decided to do was put a bunch of different wigs on Newton and see what that did in terms of heat transfer.

IRA FLATOW: And what made you think about doing that? Did you wonder about that yourself?

TINA LASISI: Absolutely. So I’ve been wondering about curly hair since I first started learning about human evolution in undergrad, and I always wondered why does it exist, why do I have it, and what would the evolutionary benefit be of it. Because if you think of most mammals, most mammals have straight hair, and there’s a couple of them that have crimped hair if you’re thinking about sheep. And then we have a lot of very special looking dogs that we have bred to look all kinds of ways including poodles– and I personally have a poodle– and they have hair that is kind of curly but it’s nowhere near that texture that you see in tightly-coiled hair like a lot of people of African descent have and some Southeast Asian people have. So I was very curious about why evolution decided now is the time for me to come up with something new.

IRA FLATOW: And we have photos of Newton the mannequin at sciencefriday.com/curls if our listeners want to see it. Now as I mentioned at the top, the curliest hair kept the mannequin’s head to coolest. Did you figure out why that was?

TINA LASISI: So we haven’t understood– or we have a guess about why that should be happening, and a lot of that has to do with work that’s been done on mammals before. So a lot of research on mammalian coats shows that if you have a deep coat, like a very long coat, counterintuitively that can keep you cooler in solar radiation. And that’s because the amount of radiation that reaches the skin is much less than if you had a shorter coat, because it basically bounces off the different hair fibers.

But the added benefit that we have with curly human hair is that you’re also able to lose much more heat. So usually with a thick fur coat, you have a trade off. You minimize how much. You’re overheating from the sun, but you also can’t really get rid of as much heat. And with human curly hair, we have this one way mirror effect, where basically we’re able to minimize how much heat is coming in without minimizing how much heat we can lose, which is perfect if you are out in the dry savanna and a biped who is just trying to evolve with their big brain.

IRA FLATOW: You also looked at the role of sweat in helping people cool down, and found something interesting.

TINA LASISI: Yes. So Newton, unfortunately, can’t sweat yet. But what we did– what we did is we made his scalp wet and then put the wigs on top of it to simulate what it would be like to evaporate sweat from the scalp. And we basically wanted to understand how that would be influenced by different hair. And, unsurprisingly, if you have no hair on your head, it’s really easy to evaporate all of that sweat because there’s no barrier. And so you get to that hotter temperature sooner, that water or sweat evaporates, and you cool down.

But the thing is, you need to cool down because a naked scalp or a bald scalp– however you want to call it– it overheats a lot more. So what we know is that with curly hair, basically the hair is keeping you so cool that it’s kind of getting in the way of that sweat evaporating optimally.

IRA FLATOW: Wow, so people with– a lot of people shaved their heads now, right? They have to watch out?

TINA LASISI: That’s true. I mean, I myself decided to cut my hair very short. And, yes, I would say now that I don’t have all of that protection anymore– the main thing I learned from my experiments is, please, people out there who do not have any hair, by choice or by nature, wear a hat. Wear a hat, please.

IRA FLATOW: Wear a hat. That’s good. There’s also a study to test how much hair affects sweating. They had men go outside in the sun with hair, then shave it off.

TINA LASISI: Yes, there’s this really cool study that was done in Brazil, I believe, where they had a bunch of men do exercise out in the sun with their hair, and then they repeated those same exercises after shaving their hair off. And what they found is that all of their physiological markers stayed the same, with the exception of how much they were sweating.

So the men after they shaved their heads were sweating a lot more. And what you can infer from that is, if the body temperature is the same but you need to sweat more once your hair is shaved off, that is telling you that the body is probably compensating by sweating more, which kind of adds to our results and says, OK, well, if you’re able to keep your head cool, you don’t need to sweat as much, which means you save water.

IRA FLATOW: I’m just going to throw this out because I just thought about it. If you have a bald head and you’re sweating, are you sweating more to cool yourself off? Does your body no there’s a place it could release the heat through your head?

TINA LASISI: I don’t know if it’s so much about knowing. And that’s where future research is going to be really interesting and exciting. But my suspicion is that it just has to do with all your physiological responses are because of what your brain is able to tell your body. And if your head is overheating, then your brain is going to say, OK, guys, we’re overheating, we need to cool down.

And so it makes sense to me that, if the head is overheating, that is going to give that signal very quickly of, we need to start sweating.

IRA FLATOW: Yeah. Let’s talk about the curly versus straight hair study. What surprised you most about that, about the results?

TINA LASISI: So what surprised me most was the extent of the effects. I didn’t expect it to be so dramatic. I had some personal anecdotal experience of hanging out with a straight haired friend who happened to touch my hair while we were out in the sun– entirely with consent– and almost burned their hand on it. And they said, ow, it’s like so hot. Are you not overheating? And I was like, I don’t feel anything.

Then, on the other hand, with their super straight hair, they were suffering. They were struggling. So I kind of knew that there would be some difference. But just to illustrate how much the difference was, we ran all the experiments– and we ran the curliest wig first, and then the naked scalp, and then we ran the moderately curled wig, and then we ran it with straight hair– and we actually had to redo all of our experiments at a lower temperature because the straight haired wig made the scalp overheat so much that we couldn’t measure the temperature differences that we needed for our results.

IRA FLATOW: Wow. Off the needle. Pin the needle, as we used to say. Here’s a tweet from Rachel from Rochester. Says her curly hair keeps her hot, not cool. But then Stasi from Lake Mary, Florida, says, I want to know, will curly hair keep me hotter in cold weather? I mean, the good news.

TINA LASISI: That is a great question. And the answer is no, which also gives us an interesting possible hypothesis to explore in terms of humans who lived in very cold places. So our current working hypothesis is that our ancestors– last common ancestors of all humans– had tightly curled hair and dark skin. And then once they started leaving the continent, adapting to different places, it may have been the case that, in addition to adapting their skin color, they had to adapt their hair because they actually had to stay warm instead of staying cool.

IRA FLATOW: Well, it seems like if curly hair has lots of benefits, why did some people evolve to have straight hair?

TINA LASISI: Well, because you’re going to lose a lot of heat, right? So straight hair is very good at keeping that heat in. And also just from a genetics perspective, if there’s no pressure to keep curly hair, then you can have all kinds of variation evolve. And what we think happened is that this was really important as humans were growing their bigger brains, but once they had those big brains, they could use them and each other’s support to come up with ways of avoiding overheating that didn’t require them to be out in the sun.

IRA FLATOW: Now, I know that researchers have documented that darker skin color is related to the amount of UV radiation they soak in. People from hotter, sunnier places tend to have darker skin tones. But curly hair seems not to evenly match up here.

TINA LASISI: That is correct.

IRA FLATOW: What’s going on?

TINA LASISI: So that’s something that I noticed from the beginning, because my advisor, Nina Jablonski, she’d done all of this work on skin color, which inspired me. And I noticed that with hair, we don’t have the same patterns. You have people in really hot places, like the Amazon, who have stick straight hair, and in some places in Asia as well.

And basically that supports this idea of, there isn’t a pressure. There may have been a pressure early on, and natural selection acted on it once it was our common ancestor that needed to grow that bigger brain. But afterwards, it just wasn’t a selective pressure.

And, on top of that, the curly hair only works if the source of heat is solar radiation. If you’re talking about ambient temperature, having no hair is the best, because any barrier between you and the environment, especially if it’s humid– yeah, you’re out of luck.

IRA FLATOW: That’s it. Well, we’re out of time, Dr. Lasisi. Thank you for taking time to be with us today.

TINA LASISI: Thanks for having me.

IRA FLATOW: Dr. Tina Lasisi, incoming assistant professor of anthropology at the University of Michigan.

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