FROM an evolutionary perspective, monogamy looks good for females and bad for males. For mothers, it means devotion as dad is going to be around to help look after the kids. For fathers it is more of a prison sentence, because it restricts a male's ability to inseminate lots of females at relatively low cost. In most circumstances, unless the young are likely to die without paternal support, a male has little incentive to stay with an individual female when he has so many other females to breed with.

That, at least, was the conventional wisdom until fairly recently. But modern genetic techniques have shown that in many species females in apparently monogamous relationships often produce families that have more than one father. To explain this, biologists have theorised that these females are mating with males who are genetically superior to their regular mates, thus getting the benefit of parental assistance from a cuckold and good genes from a Lothario.

Proving that, though, is a lengthy process. And it is only now that Aurélie Cohas and her colleagues at the University of Lyon, in France, seem to have done so. Their paper in July's edition of the Journal of Animal Ecology shows that for marmots, at least, a bit on the side can help a female's evolutionary chances no end.

The marmots in question live in the Grande Sassière Nature Reserve in the French Alps. For 12 years, Dr Cohas's team trapped them, sexed them, aged them, tagged them with radio transponders and then tracked them to see what they got up to, and with whom. She noted all breeding behaviour and recorded all births. Most pertinently, she identified which offspring were the result of monogamous relationships and which the result of cheating. She then studied those offspring, to see how they fared.

What she found was that young born as a result of adultery were 30% more likely to survive for at least two years, and thus have a decent chance of breeding themselves, than those fathered by a female's permanent male partner. That does, however, raise the question of what the correspondent males are providing to generate this advantage.

Marmots live in family groups, with breeding restricted to a dominant male and a dominant female. Young marmots—males in particular—have a choice when they reach adulthood. They can hang around and hope to take over, or they can leave and become wanderers. Dr Cohas's genetic analyses have shown that it is wanderers, rather than nest mates or neighbours, that females play away with. Wanderers have one clear advantage. Nest mates and neighbours are likely to be relatives. Indeed, marmot social structure actively encourages inbreeding. Wanderers, by contrast, bring fresh genes.

The benefit of outbreeding might be explanation enough for the vigour of their offspring. But it is also possible that wanderers have other advantages. To survive as a lone marmot, you have to be tough. The mere fact of wandering and surviving might therefore be a good indication of genetic fitness.

Whichever is the case, in marmots the idea that a little of what you fancy does you good clearly applies to both sexes. Whether there are any lessons in this for human adulterers remains to be seen.

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