SCIENTISTS claim to have discovered the reason that men’s testicles are the size that they are: it’s all to do with the infidelity of our female ancestors.

A study of the evolution of sperm has revealed that the average human female does anything but stand by her man: like many other primates, she is not at all averse to promiscuity if she can get away with it.

Research by a team at the University of Chicago, led by Bruce Lahn, suggests that men are more prone to stray than women, but the notion that human females long for nothing but a stable, monogamous relationship with Mr Right is wrong.

The study, published today in the journal Nature Genetics, examined the evolution of male sperm among 12 species of primate, including human beings. Sperm is useful for the investigation of a species’ sexual practices in the distant past, as its characteristics are largely determined by mating patterns.

In promiscuous species, such as chimpanzees, competition for mating privileges does not end with sex: once inside a partner’s body, sperm must often battle those of a rival to be the one that fertilises the egg. In order to maximise their chances of fatherhood, male chimpanzees have evolved huge testicles and sperm counts for their body size.

Among gorillas, however, females mate only with a dominant male, who collects a harem that will not entertain the idea of sex with a rival until he has beaten the alpha male in a fight. In this polygynous system, a male can be virtually certain that no one else has copulated with his partners — and thus gorillas’ testicles are tiny.

Human testicles are somewhere in between, suggesting that while women are nothing like as promiscuous as female chimps, neither are they as faithful as the female gorilla.

It is likely that our female ancestors cheated on their spouses often enough to leave men with room for doubt about their paternity.

“In species with promiscuous females, there’s more selective pressure for the male to make his semen competitive,” Dr Lahn said. “It’s similar to the pressures of a competitive market place, [where] competitors have to constantly change their products to give them an edge over their rivals — whereas, in a monopoly, there’s no incentive to change.”

Making more sperm is not the only way of boosting a male’s chances of fertilising an egg. If his partner might be cheating on him, he can improve his prospects by producing sticky semen that coagulates in the vagina, creating a “chastity belt” that stops rival sperm from getting past and his own sperm from leaking out. The theory suggests that the more promiscuous the species, the stickier the semen. It seems that human beings have a similar viscosity of semen to orangutans.

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