AYESHA RASCOE, HOST:
You've got to get in the mood for this next story. It's all about kissing. Smooching goes way back - 21 million years, in fact. That's according to new research carried out about the origins of kissing. Matilda Brindle is an evolutionary biologist from Oxford University and was the lead researcher of the study. Welcome to the program.
MATILDA BRINDLE: Thank you for having me.
RASCOE: Why research kissing?
BRINDLE: I'm really interested in the evolution of behaviors, and especially those ones that might seem counterintuitive from an evolutionary perspective. So kissing doesn't seem like it's going to save your life. It might not necessarily be kind of important for survival, and it also doesn't look like it's directly important for reproducing. Animals can reproduce without kissing. So what we were interested in is why this behavior has evolved in the first place.
RASCOE: And you've got a very specific definition of a kiss, and it doesn't sound too romantic.
BRINDLE: No, it is not the most romantic definition ever. So we define kissing as a nonagonistic interaction involving directed, intraspecific oral-oral contact with some movement of the lips or mouth parts and no food transfer.
RASCOE: (Laughter).
BRINDLE: So, I mean, just totally unromantic. A nonagonistic interaction just means that it's not an aggressive interaction. The reason we included that is that some fish will basically fight each other with their mouths wide open and press their mouths into each other. The intraspecific bit just means that it has to occur between the same species. So seems really obvious, but actually, we know from experience that sometimes different species of animal will try and kiss each other. I'm afraid I've been on the receiving end of an attempted snog by a capuchin monkey...
RASCOE: Oh, OK (laughter).
BRINDLE: ...Which - yeah. You know, occupational hazard.
RASCOE: Yes.
BRINDLE: But we wanted to avoid that.
RASCOE: Yeah.
BRINDLE: And then the reason we said that it couldn't include food transfer is, again, there's this behavior called premastication, which is basically where mothers will kind of prechew food...
RASCOE: Yeah.
BRINDLE: ...So it's easier to digest and swallow for their offspring. And then sometimes they'll pass that mouth-to-mouth.
RASCOE: So how do you know, though, that kissing evolved 21 million years ago?
BRINDLE: We ran what's called an evolutionary simulation. What we did was take data on observations of kissing across living species. So we know that most of the large apes kiss, for example. What we also added to that was information on the evolutionary relationships between different species of primate. So when you have those two pieces of information, you can track back across the course of evolution and infer how a trait - in this case, kissing - could have evolved. We found that the ancestor of all of the large apes - so that's humans, chimpanzees, bonobos, orangutans and gorillas - was kissing 21.5 million years ago.
RASCOE: Do we know what's the point of kissing?
BRINDLE: We weren't able to test this, unfortunately, but there are some hypotheses for why kissing might have evolved. So for sexual kissing, we think it might be something to do with mate assessment. Basically, before you get to the stage where you're investing in reproducing with another individual, having a little bit of a kiss could be a way of testing the waters and seeing if they're worth...
RASCOE: And testing the vibes...
BRINDLE: Exactly.
RASCOE: ...To see if the vibing is right. OK.
BRINDLE: Exactly. Yeah.
RASCOE: (Laughter).
BRINDLE: So it's a vibe check. But also, you're checking that they're healthy, you know?
RASCOE: Yes.
BRINDLE: If they smell a bit ill, you might want to...
RASCOE: Yeah.
BRINDLE: ...Swerve away. And then the other idea is that it could be basically a type of foreplay. So individuals are becoming aroused before copulating, and that can actually increase the chance of fertilization during copulation. For nonsexual kissing, so platonic kissing - like, for example, between a mother and offspring or between friends - we think that's to do with bonding. Obviously, really important for mothers and offspring to bond, but there are other ways that kissing could be useful in a kind of friendly way, too. Primates in particular are extremely sociable, and so being able to kind of navigate any tricky social situations with a bit of a kiss could be really helpful.
RASCOE: That's Matilda Brindle from Oxford University. Thank you so much for joining us.
BRINDLE: Thank you very much for having me.
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