A large number of symbiotes are transmitted via kissing. Kissing transmits mononucleosis, influenza, coronaviruses, herpes, syphilis, HPV - and many other diseases. Practically eveyone has at least one of these. It is in the interests of these parasites and symbiotes to manipulate their human hosts into kissing each other. In this picture, human genes take more of a back seat, and the story of kissing hs more to do with how various parasite and symbiont visitors manipulate their human hosts into kissing each other. Just as coughing and sneezing are partly human and partly parasite caused, so it could be with kissing. Kissing would also be a case of a behavior favored by many different sets of parasite genes.
The more sociable a species the more symbionts it has. There's a "spiral of sociality", where symbionts manipulate their hosts into ever closer contact, which in turn provides more opportunities for other symbionts to spread. This is an important theory in memetics - since memes act as social symbionts that induce ultrasocial behavior as part of their reproductive cycle. In other species, this "spiral of sociality" can lead to food sharing, grooming, coprophagia and many other social behaviors. Humans are quite sociable - and kissing seems to be part of the mix.
How might parasites manipulate human behavior to influence kissing frequency? Since parasites are inside their hosts, they are in a good position to influence behavior. Drugs are one possibility. Another possibility is sterilizing the host. Resources spent raising children are resources that could have been spent kissing people. HPV is a famous example of human parasite-induced sterility. It blocks up the tubes of women with cancerous cells, preventing future births. Less well known, but HHV-6A is another common cause of female infertility. It infects saliviary glands, is spread via saliva and it causes infertility. The more partners a woman has, the higher her chance of infertility. Parasites are responsible for this. Men are probably harder to sterilize, but erectile dysfunction is one method. Various STDs induce this. Erection issues make it harder for a man to keep hold of any one partner, but they may make it easier for a man to kiss multiple prospective partners - and that's a situation that parasites might prefer.
What evidence is there for symbiote hypothesis of kissing? Alas, I don't really know much about that yet. The hypothesis makes some predictions. It predicts that kissing frequency is likely correlated with infection with some specific parasites. It predicts that kissing behavior is more likely to be initiated by older individuals - since they will be carriers of a wider set of kissing-inducing symbionts. It predicts parasite-induced infertility - and that at least is known to be fairly common.
The symbiote hypothesis of kissing is much the same as the symbiote hypothesis of sexual intercourse. As with kissing most people assume that sexual intercourse is there to benefit human genes. However a large number of diseases are spread by sexual intercourse, and a good fraction of observed sexual behavior could actually be there to benefit them.
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If true, then we are all technically face eating zombies!
ReplyDeleteFace sucking zombies, maybe. There is a kissing scene in David Cronenberg's "Shivers" movie. It involves two women. A bit like that, perhaps.
DeleteI didn't mention it in the article above, but there's a hypothesis with one foot in the "human mating behavior" camp and one foot in the "parasite" camp - that kissing is a signal of commitment that is costly and difficult to fake. The cost comes through the sharing of kissing parasites. A kiss signals that you are interested in someone enough to risk taking on their load of kissing parasites. That is a step not to be taken lightly.
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