EMILY KWONG, HOST:
Well, today is Valentine's Day, which means love and connection are on people's minds, and I recently met someone who studies one form of affection, kissing. Matilda Brindle is an evolutionary biologist at the University of Oxford, and she explained to me that scientists aren't fully sure why this behavior evolved.
MATILDA BRINDLE: 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.
KWONG: Whoa.
BRINDLE: 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...
KWONG: I had braces. I understand.
BRINDLE: Well, there you go. Putting your mouth next to someone's face is putting yourself in this vulnerable position, both in terms of pathogen transfer, but also physically, you could be quite hurt by this.
KWONG: What other animals kiss that you think would be surprising to most people?
BRINDLE: Well, ants. Another one is albatrosses. Prairie dogs kiss one another.
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.
BRINDLE: I know. Romantic.
KWONG: Oh. OK, 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.
BRINDLE: Yes.
KWONG: Who was this ancestor?
BRINDLE: 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 could be similar to this ancestor. A safe bet for what they might have been like comes from an ape from the Miocene. It lived around 11.5 million years ago.
KWONG: Wow.
BRINDLE: 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.
KWONG: OK. Now, a lot of primates kiss. You have watched a lot of that footage and describe two kinds of kissing. There's platonic kissing, and then there's sexual kissing, which bonobos do.
BRINDLE: Yeah, basically.
KWONG: What are the leading hypotheses on why sexual kissing evolved in primates?
BRINDLE: There are two leading hypotheses, I would say. The first of which is associated with kind of mate assessment, I suppose.
KWONG: Do I want to have babies with this person?
BRINDLE: Correct. Then the second hypothesis is this idea of pre-copulatory arousal. Having this kind of more comfortable copulation is potentially going to increase the chance of fertilization.
KWONG: Gotcha. Where do you hope this research goes next?
BRINDLE: 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 because I think we're going to find some incredible stuff. We just need to be brave enough to say, OK, 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.
KWONG: Matilda, thank you so much for coming on the show.
BRINDLE: Thank you very much for having me. This was great fun. Transcript provided by NPR, Copyright NPR.
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