Ask a ScienceBlogger took a vacation a little while back, but it's back now in a new form. The last installment was hosted at Cognitive Daily, and now Thoughts from Kansas gets to answer this pressing query: "How did kissing evolve? Are humans the only primates who kiss? Why do we?"
Setting aside the rather obvious proximate answer to the final question, it turns out that these questions are largely unresolved. The origin of kissing isn't clear, in part because kissing encompasses a range of behaviors. Kissing a baby, pecking your friend's cheek in greeting and locking lips with a loved one are all very different acts. Kissing is not a sexual or social act in all human societies. Such kissing appears to have been uncommon in the West until the Greeks, and ancient writings from India talk about the so-called Eskimo kiss, or nose-rubbing, but not kissing on the lips.
The most commonly cited theory about kissing goes to an observation by ethologist Irenäus Eibl-Eibesfeldt, who noticed that in certain human cultures, mothers will chew food up, then kiss the baby, transferring the food into its mouth. This motherly kiss could have carried over to adult relations, producing the other forms of kissing. Alternatively, different cultures could have acquired and lost the practice of kissing independently. Or premastication may be a red herring.
Non-human primates premasticate too, but they also use kisses for other reasons. This photograph by primatologist Frans de Waal, the author of Peacemaking Among Primates, demonstrates that chimpanzees and other primates will kiss either after a fight to make peace, or to prevent a fight.
Based on his studies, de Waal links kisses with biting, with a shift in intentions along a continuum of behaviors. De Waal sees a kiss as a controlled bite, which by its gentleness demonstrates good will. Kissing on the lips creates a reciprocity and stimulates a lot of nerves in nice ways. Indeed, it's hard to ignore pleasure-seeking as a component in kissing behavior, especially when you consider the nature of kisses in bonobos.
Bonobos, our closest primate relatives, use pleasure as a conflict resolution tool far more extensively than other primates do. Bonobos use sex to resolve conflicts over anything from food to the right to walk along a particular branch. Kisses between chimpanzees are fairly chaste, but kisses between bonobos, who share with humans sexual behaviors including face-to-face copulation and French kissing, are not. The tongue has at least as many nerves as the lips, so kissing with the tongue is one more sign of friendship and a pleasurable reward for ending the fight or avoiding it in the first place.
A third explanation for kissing goes to the biochemistry of attraction. There are glands in the cheeks which emit pheromones, and glands in the nose which detect them. Experiments in mice and in humans have shown that we can detect deviation in a highly variable part of the genome associated with the immune system, the major histocompatibility complex or MHC, through olfaction, and that females (humans and mice) tend to prefer the odor of men with MHCs different from their own. This may be a way to avoid inbreeding, or an adaptation to strengthen the immune system. Some people have suggested that the close approach involved in a kiss on the lips, or the Eskimo kiss allows the two people to sample one another's pheromones. However, I suspect that there are easier ways to catch a whiff of a stranger, and once people are on intimate enough terms to be kissing, they've probably already figured out what their partner smells like.
I hope no one thinks I've only given lip service to the question posed. Until there's a more definitive answer to these questions, I guess we'll all just have to continue our experiments with kissing.
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In the interests of Science, I am willing to experiment with Paulina, and maybe with a couple of her friends...
http://www.imdb.com/name/nm0005320/
And yes, the link is safe for work. Unfortunately.
Have you considered it may be a trust issue. Many species, including us humans have a personal area, our 3 ft of space. So for many species, and in dogs it is considered a matter of trust to let another that close into their space as to have contact with the face.
Personal space certainly explains the general phenomenon of hugging, kissing, shaking hands, etc. Allowing someone into that space is a demonstration of trust, and asking to enter it is a request and an offer of trust.
That doesn't explain kissing specifically. The extension of an open hand, either for a handshake or a hug, is a sign that you are not armed, so it makes the invitation to trust clear. All puckering up does is, perhaps, trigger some maternal instinct related to premastication. That weakness may be a sign of trust, like a dog showing its belly, or it may be something else entirely.
Of course, once you discover it, it makes total sense. The question is just how that first kiss happened.
I vote for the last one (the biochemistry part).
I love kissing.
I think the biochemistry hypothesis on why we kiss is a good idea. But I wanted to share with you a similar post, on why we kiss, where I bring up the fact that there are some cultures that don't kiss. How does that change things?
I hadn't seen the 10% figure you offered, but earlier drafts of this post did mention that some cultures with premastication don't use kissing in its sexual context. I had trouble running down original citations in the literature, so I ultimately cut that line.
What I had said was that I don't think it changes much. It demonstrates that kissing isn't some sort of genetically coded instinct, but I doubt many people thought it was to begin with. Kissing is probably closer to a meme than a gene, and I expect that it would be easier to lose a meme or to redevelop it than to gain or lose a novel gene.
If anything, this weakens the biochemistry argument, since that's the one least dependent on cultural transmission. If it's rooted deeply in our biochemistry, it ought to be uniform. If it's based on a tenuous link between childcare and adult sexuality, you'd expect it to be variable between cultures.
Where did you get the 10% figure?