Promiscuous birds can blame it on their genes
29 October 2010, by Tamera Jones
Birds are often held up as shining examples of domestic bliss: once they've found an ideal partner, they set up nest and are loyal for the rest of their lives, right?
The song sparrow, Melospiza melodia.
Well, no. While they might stay together, they are in fact serial cheats, having extramarital affairs with their neighbours on a shockingly regular basis.
And it's not just males; the females are at it too. Now in a study published this week, scientists have found that some of this promiscuous behaviour can be squarely blamed on their genes.
Researchers led by Dr Jane Reid from the University of Aberdeen found that up to 18 per cent of female promiscuity is down to genetics. 'If your mother was promiscuous, you're more likely to be too,' she explains.
From a spreading-your-genes point of view, it's easy to understand why male birds might play away from home. By fathering more offspring, there will be more copies of their genes in the world. But it's not so easy to understand what female birds stand to gain from engaging in what appears to be risky behaviour.
Promiscuity is risky for females, because they could catch a sexually-transmitted disease. And their long-term partners somehow know that chicks fathered by another male aren't theirs and won't feed them as often as they'll feed their own chicks.
Not just that, but females can only bear a limited number of offspring, unlike males who can father multiple chicks.
'If your mother was promiscuous, you're more likely to be too.'
Dr Jane Reid, University of Aberdeen
'There are plenty of hypotheses out there to explain why female promiscuity exists. One main idea is that her offspring inherit better genes if she mates with a male that's not her long-term partner,' explains Reid.
Female promiscuity must be beneficial, or it would have died out by now. 'A female's tendency to produce offspring sired by another male must be inherited,' Reid says. 'This is critical. If it's not inherited, you can't get evolution of promiscuous behaviour.'
But until now, the hypothesis has remained unproven. 'It's been incredibly difficult to test this idea, because it relies on someone collecting enough data on wild populations, which is rare,' says Reid.
As luck would have it, a team of researchers led by a group of scientists in Canada has individually marked song sparrows with leg tags since 1975. Mandarte Island in Victoria, British Colombia is home to anywhere from 20 to 50 pairs of song sparrows (Melospiza melodia) at any one time. Since 1993, the same researchers have also taken blood samples from these birds.
The blood samples meant that Reid and colleagues from the universities of British Colombia and Zurich could figure out the paternity of each and every sparrow.
After analysing 16 years of data, they found that 28 per cent were fathered by the wrong male, or not by the mother's long-term mate. This is similar to what other researchers have found. In some birds, up to half of chicks aren't fathered by the mother's nest-mate.
'These birds are socially monogamous. So if a female doesn't get much choice about who to pair up with from the start, it's going to be better for her offspring for her to secretly mate with a better quality male from time to time,' explains Reid.
The team's findings also mean that the big hypothesis about why females are promiscuous still stands. 'One part of the jigsaw puzzle has been solved, but we still need to find out if female promiscuity leads to offspring with better genes,' adds Reid.
Reid specifically wants to know if better genes mean chicks will live longer or will produce more offspring themselves.
Jane M. Reid, Peter Arcese, Rebecca J. Sardell and Lukas F. Keller, Heritability of female extra-pair paternity rate in song sparrows (Melospiza melodia)
Proceedings of the Royal Society B, Published online before print October 27, 2010, doi: 10.1098/rspb.2010.1704
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