Male circumcision is a weapon in the sperm warsHave at it
Sperm competition theory predicts that males will evolve ways to ensure that their sperm, and not another male's, fertilises a female's eggs. Genital mutilation, in this view, is just another way to win the sperm war.
In some forms of mutilation, the handicap to sperm competition is obvious. There is subincision, for example, where cuts are made to the base of the penis. This causes sperm to be ejaculated from the base rather than the end, and is performed in several Aboriginal Australian societies, says Wilson.
In some African and Micronesian cultures, young men have one of their testicles crushed.
Male genital mutilation makes it less likely that a male will manage to father a child with another man's wife, Wilson says.
Circumcision is one of the less painful forms of mutilation, but it is also less effective at reducing sperm competition. Wilson suggests, however, that the lack of a foreskin could make insertion or ejaculation slower, meaning brief, illicit sex is less likely to come to fruition and lead to a pregnancy.
Younger men, he says, willingly submit to having their reproductive ability reduced because they benefit socially from the older men, by forming alliances, and by gaining access to weapons or tribal lore.
The older men have also gone through the ritual, and seen their own reproductive effectiveness reduced. But if a man with, say, four wives wants to ensure that any children his wives produce are his, there is pressure to make sure other men can't successfully impregnate them.
The husband's own reproductive ability is impaired, but continuous and repeated access to his wives makes up for it, while any genital mutilation is a greater handicap to an interloper trying to sneak brief occasional sex with his wives.
Price of alliance
"An older married man must form alliances, or associate with younger or unmarried men at some point, and it would be better to associate with and invest preferentially in those who are least likely to threaten his paternity, especially in societies where cuckoldry is rife," says Wilson.
"Men who demand genital mutilations as part of the price for alliance and investment would be less vulnerable to exploitation of such relationships and loss of paternity to peers."
Wilson has now tested the idea. If the sperm competition theory is correct, he reasoned, then male genital mutilation should be more common in societies where men tend to have multiple wives, especially those in which the wives live apart from the husband.
The mutilation would also probably be carried out in a public setting, witnessed mostly by other men, and performed by a non-relative. Men who refused would face social sanctions.
Who's the daddy?
Wilson searched anthropological databases and found that his predictions were borne out: 48% of highly polygynous societies practice some form of male genital mutilation, and in societies in which wives live in separate households that increases to 63%.
Only 14% of the monogamous societies in the database practice male genital mutilation.
It might also be the case that selection works at a group level, so that societies that enforce mutilation are more stable because of less conflict over paternity, Wilson says.
David Barash, an evolutionary biologist at the University of Washington in Seattle, US, says that the paper makes a convincing case. "Wilson has tackled a perplexing question and come up with a persuasive preliminary answer to an evolutionary enigma: why do men submit to procedures that seem to reduce their fitness?" he says.
Journal reference: Evolution and Human Behavior (vol 29 p 149)
Friday, June 6
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