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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