Matilda Brindle (2:03)

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

Emily Kwong (2:06)

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 Foreign.

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Emily Kwong (4:01)

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

Matilda Brindle (4:07)

I did.

Emily Kwong (4:08)

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

Matilda Brindle (4:12)

Well, I'm very happy to be back.

Emily Kwong (4:14)

Why is kissing such an evolutionary conundrum?

Matilda Brindle (4:18)

It's an interesting one. I think the key here is that kissing seems. 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 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 I had braces.

Emily Kwong (4:46)

I understand.

Matilda Brindle (4:47)

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.

Emily Kwong (5:10)

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

Matilda Brindle (5:13)

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.

Emily Kwong (5:31)

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.

Matilda Brindle (5:45)

Yeah, it's a frothy nightmare.

Emily Kwong (5:46)

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

Matilda Brindle (5:50)

So we define kissing as a non agonistic interaction. So that just means it's not aggressive, intraspecific, 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.

Emily Kwong (6:09)

Okay, so non aggressive, same species, oral contact.

Matilda Brindle (6:13)

Yeah.

Emily Kwong (6:14)

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

Matilda Brindle (6:19)

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, they 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.

Emily Kwong (6:45)

I mean, but check, check, check, check, check, check for these polar bears. They looked like they were having a great time. Yeah, it was tender in its own aggressive way. What other animals kiss that you think would be surprising to most people?

Matilda Brindle (6:58)

Well, ants. Another one is albatrosses.

Emily Kwong (7:01)

The birds.

Matilda Brindle (7:02)

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.

Emily Kwong (7:09)

Okay. Albatrosses, ants, and. What's the third one?

Matilda Brindle (7:12)

Prairie dogs kiss one another.

Emily Kwong (7:14)

Oh, really?

Matilda Brindle (7:15)

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

Emily Kwong (7:21)

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.

Matilda Brindle (7:29)

I know, romantic.

Emily Kwong (7:32)

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. Yes. Who was this ancestor?

Matilda Brindle (7:47)

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

Emily Kwong (8:18)

Wow.

Matilda Brindle (8:19)

And they look relatively similar 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.

Emily Kwong (8:29)

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.

Matilda Brindle (8:46)

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

Emily Kwong (8:52)

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?

Matilda Brindle (9:04)

I think the difference is mainly the, the environmental context that we're looking at. So we would know that sexual kissing is sexual kissing because it tends to be surround 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 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?

Emily Kwong (9:59)

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

Matilda Brindle (10:08)

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

Emily Kwong (10:19)

Do I want to have babies with this person?

Matilda Brindle (10:22)

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

Emily Kwong (10:25)

Or this gibbon, this is bonobo.

Matilda Brindle (10:29)

So you've got this idea of kind of mate selection. Now, that could be something as simple as they don't smell very well. You know, 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 olfactory 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 histocompatibility complexes, because that might kind of improve our immune function by having this kind of greater diversity.

Emily Kwong (11:21)

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

Matilda Brindle (11:26)

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 mate 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. Gotcha.

Emily Kwong (12:02)

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

Matilda Brindle (12:09)

Yeah.

Emily Kwong (12:10)

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?

Matilda Brindle (12:17)

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. 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.

Emily Kwong (13:18)

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

Matilda Brindle (13:24)

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 humours it differs between individuals, it differs within societies. People forget the flexibility of behavior and the fact that cultural variation is absolutely the rule, not the exception.

Emily Kwong (13:53)

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?

Matilda Brindle (14:01)

Yeah, 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? Or mine certainly wasn't. Mine's awful. 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. Just sexuality in general is extremely, 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 an interplay between the two.

Emily Kwong (14:44)

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

Matilda Brindle (14:50)

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.

Emily Kwong (15:26)

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

Matilda Brindle (15:29)

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

Emily Kwong (15:32)

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.

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