Why do we kiss? It's an evolutionary conundrum

The evolutionary purpose of kissing has long eluded scientists. Smooching is risky, given things like pointy teeth, and inherently gross, given an estimated 80 million bacteria are transferred in a 10 second kiss. And yet, from polar bears to humans, albatrosses and prairie dogs, many animals kiss. So, what gives? Evolutionary biologist Matilda Brindle https://matildabrindle.weebly.com/ tells us the sordid details driving this behavior, what distinguishes different kinds of kissing and whether culture has anything to do with why people kiss. Interested in more of the science behind love and connection? Email us your question at [email protected] mailto:[email protected]. Listen to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave http://plus.npr.org/shortwave. See pcm.adswizz.com https://pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences. NPR Privacy Policy https://www.npr.org/about-npr/179878450/privacy-policy

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Speaker 00

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• 0:18Emily Kwong?

  You're listening to Short Wave from NPR. Happy early Valentine's Day, Short Wavers, and Singles Awareness Day. And if you love Parks and Rec like I do, Galentine's Day.

• 0:30Speaker 02

  Every February 13th, my lady friends and I leave our husbands and our boyfriends at home, and we just come and kick it breakfast style. It's like Lilith Fair, minus the angst.

• 0:40Emily Kwong?

  Seriously, that scene changed my life. But yeah, this time of year, love and connection are on a lot of people's minds. And I recently met someone who studies a particular behavior often used to show each other how we feel. I'm talking, of course, about a kiss.

• 0:55Speaker 03

  You know, most people have had a kiss, whether that's romantic or more platonic. So I think people are really interested in that. And then also surprised, you know, the first question is, what other animals kiss? Oh, god, which ones? What do they do? What does it look like?

• 1:09Emily Kwong?

  Matilda Brindle is an evolutionary biologist at the University of Oxford, who has studied the evolution of kissing.

• 1:16Speaker 03

  So, you know, I get wheeled out at parties quite often.

• 1:19Emily Kwong?

  And by the way, she says that not all human cultures kiss.

• 1:22Speaker 03

  In Western society, at least, it just does seem like this huge, huge cultural phenomenon. You know, movies end with a kiss. We have the kiss of life or the kiss of Judas. All of these really important kind of cultural things.

• 1:37Emily Kwong?

  She is awoken with a kiss. She literally can't wake up until someone puts lips on her.

• 1:43Speaker 03

  Exactly. It's this amazing weird thing we do that looks totally pointless and kind of a bit gross and, you know, counterintuitive. We're probably sharing loads of germs when we do it.

• 1:55Emily Kwong?

  Oh, yes, we are. 80 million bacteria are estimated to be transferred, on average, in a 10-second kiss.

• 2:01Speaker 03

  Yeah, I mean, that's just bonkers, isn't it?

• 2:04Emily Kwong?

  So today on the show, the evolutionary conundrum of kissing. Why do we do it? What other animals lock lips? And is kissing a biological behavior or a cultural one? I'm Emily Kwong, and you're listening to Short Wave, the Kiss and Tell science podcast from NPR.

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• 3:38Emily Kwong?

  Okay, Matilda, you came on our show a few months ago to talk about the evolutionary history of kissing.

• 3:44Speaker 03

  I did.

• 3:45Emily Kwong?

  Specifically in primates. I haven't stopped thinking about it, so you're back.

• 3:50Speaker 03

  Well, I'm very happy to be back.

• 3:52Emily Kwong?

  Why is kissing such an evolutionary conundrum?

• 3:55Speaker 03

  It's an interesting one. I think the key here is that kissing seems like a very unhygienic and vulnerable thing to do, particularly if it's a kind of sloppy kiss and there's a lot of tongue going on. We know that there are loads of bacteria exchanged.

• 4:10Emily Kwong?

  Whoa.

• 4:11Speaker 03

  And that is dangerous. You know, we can get all sorts of diseases from kissing. And then the other aspect of it being vulnerable is that we have teeth, you know, which sounds really obvious, but actually...

• 4:23Emily Kwong?

  I had braces. I understand.

• 4:25Speaker 03

  Well, there you go. Putting your mouth next to someone's face is potentially a dangerous thing to do if you're not sure they like you, particularly, you know, if you're a polar... Bear, that's some big teeth you're coming up against. And that is potentially problematic. So you're putting yourself in this vulnerable position, both in terms of pathogen transfer, but also physically, you could be quite hurt by this.

• 4:48Emily Kwong?

  Well, wait, can you go back to the part where polar bears kiss?

• 4:50Speaker 03

  They kiss, yeah, and we're not quite sure why. I would recommend looking this up on, you know, YouTube or another video sharing platform. Not while you're eating. I cannot stress that enough. There is a lot of froth and foam going on, and we're not sure why this is.

• 5:09Emily Kwong?

  Oh, my gosh. Oh, my gosh. I'm sorry, I'm watching this video of polar bears from the Budapest Zoo, and it is like their mouths are foaming. It's like a car wash in between their mouths.

• 5:22Speaker 03

  Yeah, it's a frosty nightmare.

• 5:24Emily Kwong?

  So you you consider this kissing. How do you define kissing?

• 5:27Speaker 03

  So we define kissing as a non-agonistic interaction. So that just means it's not aggressive. Intra-specific, which means between the same species. Oral contact, which is just mouth to mouth, with some movement of the lips or mouth parts and no food transfer.

• 5:47Emily Kwong?

  OK, so non-aggressive, same species, oral contact.

• 5:51Speaker 03

  Yeah.

• 5:52Emily Kwong?

  That's possibly the least romantic definition of kissing imaginable.

• 5:56Speaker 03

  Well, we needed this very clunky definition because we want to understand the evolutionary pathways that this has taken, which means that we've got to have a wide enough definition that it's applicable to polar bears and humans and ants and albatrosses and all of these different animals, you know, that might not even have mouths or lips, sorry. So that's why we had this kind of wide definition, and we fully acknowledge that it is not a sexy one.

• 6:22Emily Kwong?

  I mean, but check, check, check, check, check, check for these polar bears. They look like they're having a great time.

• 6:28Speaker 03

  Yeah.

• 6:28Emily Kwong?

  It was tender in its own aggressive way.

• 6:31Speaker 03

  Yeah.

• 6:31Emily Kwong?

  What other animals kiss that you think would be surprising to most people?

• 6:36Speaker 03

  Well, ants. Another one is albatrosses.

• 6:39Emily Kwong?

  The birds.

• 6:39Speaker 03

  The birds, yeah. And obviously, again, they don't have lips. I think there's a David Attenborough clip somewhere on the internet of this.

• 6:46Emily Kwong?

  Okay, albatrosses, ants, and what's the third one?

• 6:49Speaker 03

  Um, prairie dogs kiss one another.

• 6:52Emily Kwong?

  Oh, really?

• 6:53Speaker 03

  Yeah. There's some very sweet, nothing like the polar bears. There's definitely videos of them out there as well.

• 6:58Emily Kwong?

  Oh, my gosh. Yes, a tiny bit of oxytocin was released as I looked at this photo of these two prairie dogs. One of them is caressing the other's face.

• 7:07Speaker 03

  I know. Romantic.

• 7:10Emily Kwong?

  Okay, so you found out in your research, by tracing the phylogenetic tree of kissing through primates, you found out that kissing was likely present in the ancestor of all apes who lived 21 million years ago.

• 7:22Speaker 03

  Yes.

• 7:22Emily Kwong?

  Who was this ancestor?

• 7:24Speaker 03

  Yeah, I mean, this is a great question. So unfortunately, we don't know exactly who the ancestor was. There's kind of fossils floating around that time that that could be similar to this ancestor. A safe bet for what they might have been like comes from an ape from the Mayas scene. I think it's probably pronounced Pleiades Catalonae. I think it was found in Catalonia in Spain. It lived around 11.5 million years ago.

• 7:56Emily Kwong?

  Wow.

• 7:56Speaker 03

  And they look relatively similar to to modern gibbons, but they won't have been so good at swinging through the trees. They seem like they might have been a lot slower.

• 8:07Emily Kwong?

  Okay. Now, a lot of primates kiss. You have watched a lot of that footage and described two kinds of kissing. There's platonic kissing, so that's when, say, chimpanzees kiss after an argument and like patch things up with each other. And then there's sexual kissing, which bonobos do.

• 8:24Speaker 03

  Yeah, basically. You know, using a lot of tongue. It's described as very sensual.

• 8:29Emily Kwong?

  Wow. Yeah, but like, when you are watching footage of primates kissing to catalog who kisses and how, how do you distinguish between the two, between platonic and sexual kissing?

• 8:40Speaker 03

  Hmm. I think the difference is mainly the environmental context that we're looking at. So we would know that sexual kissing is sexual kissing because it tends to be surrounded by other sexual behavior, whether that is, you know, kind of full intercourse or some other thing that, you know, primates do all sorts of sexual behaviors. When I'm thinking about these things, generally I'm trying to understand them from the perspective of animals that don't talk. As someone studying animal behavior, we cannot ask them this. And so all we can do is observe them. And so for me, the kind of the shorthand for assessing

• 9:22Speaker 03

  that very roughly at the moment is, okay, well, was this occurring in a sexual context? Or was it occurring in a sort of affiliative, more platonic context? You know, was it parent offspring kissing or two friends, that sort of thing?

• 9:36Emily Kwong?

  Mm hmm. Well, looking at the data that you have, what are the leading hypotheses on why sexual kissing evolved in primates?

• 9:45Speaker 03

  Yeah, that's a great question. So there are, there are two leading hypotheses, I would say. The first of which is associated with kind of mate assessment, I suppose.

• 9:57Emily Kwong?

  Do I want to have babies with this person?

• 9:59Speaker 03

  Yes. Do I want to have babies with this person? That is exactly correct.

• 10:03Emily Kwong?

  Or this, this gibbon?

• 10:06Emily Kwong?

  This bonobo.

• 10:07Speaker 03

  So you've got this idea of kind of mate selection. Now, that could be something as simple as, they don't smile very well. You know, they, they smell a bit ill. I actually don't think that I'm gonna copulate with this individual. Or it could be something a little bit more complicated, where they could be testing to see whether they're more genetically compatible. There's some evidence to suggest that we can use kind of orphactory clues to tell whether someone's major histocompatibility complex is similar to our own, or how similar that is. And it makes sense to mate with individuals that have more dissimilar major

• 10:50Speaker 03

  histocompatibility complexes, because that might kind of improve our immune function by having this kind of greater diversity.

• 10:59Emily Kwong?

  You want someone whose genes will lend themselves to an immune system that's different than yours.

• 11:04Speaker 03

  Yes, exactly. That's one hypothesis. Then the second hypothesis is this idea of pre-copulatory arousal. So coming back to the idea that romantic or sexual kissing occurs within the context of sex, having this kind of more comfortable copulation is potentially going to increase the chance of fertilization. So if you've got to the stage where you think, okay, I do want to make with this individual, I'm going to make this investment, then it just might be tipping the scales in favor of being fertilized by that individual.

• 11:39Emily Kwong?

  Gotcha. Let's talk about platonic kissing for a little bit, too. Apes do this. They kiss and make up after they fight.

• 11:47Speaker 03

  Yeah.

• 11:47Emily Kwong?

  There's plenty of cultures, human cultures, of greeting. You kiss people on the cheeks or even the lips. Why do we think platonic kissing evolved?

• 11:54Speaker 03

  Yeah, so again, I think there's a release of oxytocin associated with this, which helps us to bond, makes us kind of want to do this more. And that's really important between parents and their kids. That's a super useful thing to be able to do. And then the other element of this is, again, sharing the microbiome. We know that that's very useful for babies, particularly new babies. It's healthy for their mothers to kiss them a lot because they're sharing their microbiome and, again, increasing that immunity. It's potentially useful for mitigating social tension and kind of just smoothing over social relationships in general.

• 12:36Speaker 03

  So it's not just about bonding or kind of potentially passing on helpful microbes. It can also have this social role in lots of different species. And I think, really, in different species, it's going to be a patchwork of these different functions. So these are useful reasons to kiss.

• 12:56Emily Kwong?

  Is platonic kissing in human cultures now more prevalent than sexual kissing? Do we know?

• 13:01Speaker 03

  I think that depends on the species, to be honest. So platonic kissing is certainly more prevalent in chimpanzees than in bonobos, where it's predominantly, if not all, sexual kissing. And then, you know, in humans, it differs between individuals, it differs within societies. People forget the flexibility of behavior and the fact that...

• 13:27Speaker 03

  Cultural variation is absolutely the rule, not the exception.

• 13:31Emily Kwong?

  Yeah. Well, I mean, does that suggest to you that kissing is something we teach each other to do, even though it has an evolutionary origin? Is this a learned behavior?

• 13:39Speaker 03

  Yeah, I think I think we do teach each other to kiss. We kind of see other people do it. And, you know, nobody's first kiss is good, is it? Mine certainly wasn't.

• 13:48Emily Kwong?

  Mine was awful.

• 13:49Speaker 03

  Yeah, exactly. This is something you learn. I think that there's a strong element of teaching and culture in this. And, you know, like, if you look at over here in Europe, the way we kiss one another on the cheek, however many cheeks that is, that changes wherever you go. You know, humans are a primate. We're extremely flexible.

• 14:08Emily Kwong?

  Yeah.

• 14:08Speaker 03

  Just sexuality in general is extremely fluid in primates. And so I don't see why kissing should be any different. But it certainly doesn't have to just be cultural or evolved. There's, there's an interplay between the two.

• 14:22Emily Kwong?

  Where do you hope this research goes next? What do you want to look into?

• 14:27Speaker 03

  I would just like to see people routinely collecting data on other sexual behaviors as well, particularly non reproductive sexual behaviors. I think we need to take these behaviors seriously. We need to look at them. We need to collect data because I think we're going to find some incredible stuff. We just need to be brave enough to say, okay, maybe this is societally a little bit uncomfortable to look at, but actually, it doesn't matter because there's so much to be discovered. There's so many important things out there that we can learn from this. And so that's where I'd like this research to go.

• 15:04Emily Kwong?

  Matilda, thank you so much for coming on the show.

• 15:06Speaker 03

  Thank you very much for having me. This was great fun.

• 15:10Emily Kwong?

  This episode was produced by Hannah Chin. It was edited by Rebecca Ramirez and fact checked by Tyler Jones. Robert Rodriguez was the audio engineer. I'm Emily Kwong. Thanks for listening to Short Wave, the science podcast from NPR.