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The perils of promiscuity

Promiscuous Birds Promiscuous birds hatch more dud chicks, according to new findings, which counter existing ideas about the evolution of promiscuity.

Dr Jane Reid and Dr Rebecca Sardell from the University of Aberdeen report in the Proceedings of the Royal Society B that when promiscuous song sparrows mate with birds other than their partner, these couplings produce worse quality offspring.

Many female birds are promiscuous and it's generally been assumed that this is because they can get better quality genes from a stranger than from their partner.

However, Reid and Sardell found that song sparrow chicks sired by non-partner males actually had slightly less chance of surviving to breeding age (about 2 per cent less) than chicks fathered by their mother's regular partner.

They also used a method called quantitative genetics to determine that non-partner males were less genetically fit than the female's regular partner.

The study looked at breeding behaviour in an isolated population of song sparrows living in grasslands on the island of Mandarte, off the coast of Canada.

Although these birds team up with a single mate for one or even several years, previous genetic studies have shown that about 26 per cent of chicks are sired by a male other than the female's partner.

Unanswered questions

So why would females bother being promiscuous if their chicks aren't benefiting?

"That's a really good question to which I do not yet know the answer," says Reid.

"That's one reason why I find my current results quite exciting - I think they will force us to reconsider the situation and rethink our understanding of the evolution of promiscuity."

"One possibility might be that genes that cause promiscuity in females also cause promiscuity in males. If promiscuity is beneficial for males (in terms of the total number of offspring they leave) then the genes causing promiscuity could be maintained in a population, even if promiscuity is costly for females."

Commenting on the study, avian behavioural biologist Dr Simon Griffith from Macquarie University says the approach Reid has taken is 'quite neat'.

"There's always been an argument that females must gain some benefit from being promiscuous, because the risk of being found cheating is that the partner won't care for the young as much or will desert her," he says.

"But increasingly people aren't finding clear support for that idea. So we've had to start investigating why else might females be doing this."

Griffith says one reason promiscuous females are selected for may be to avoid inbreeding, or in case their partner is sterile. This would be especially important in an island environment, where these birds are found.

He says the costs of promiscuity found by Reid might seem small (a 2 per cent less change of chicks surviving to breeding age).

"But," says Griffith, "they are important, because evolution works incredibly slowly, on the basis of very small incremental changes from generation to generation".

Reid says her findings potentially have implications for human behaviour.

"In terms of their mating system, humans are not really so different from socially monogamous birds and other mammals. In time, it may well be that studies of these wild populations do give us some insights into the evolution of our own social (and non-social!) reproductive systems," she says.