Circumcision and other forms of male genital mutilation have always been a puzzle. The ritual mutilations can leave the man vulnerable to infection and even death. So why do some societies insist on such a risky ritual for their men?

There may be an evolutionary explanation, according to Christopher Wilson, of Cornell University in New York, US. It could function to reduce a young man's potential to father a child with an older man's wife, he says.

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.