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Avoiding Brotherly Love
23 September 2004 (All day)
Mating with close relatives often leads to no good, so most animals try to avoid it. So pity the female red jungle fowl. With randy and aggressive brethren, they don't have much choice when it comes to mates. But the hens can avoid the ill effects of inbreeding by picking which sperm fertilize their eggs, scientists have discovered.
Among promiscuous animals, males and females may have conflicting strategies when it comes to inbreeding. Because males can produce many sperm fairly cheaply, it's no great loss if mating with their mother or sister happens to yield a few bad eggs. But for females, producing eggs requires more effort and they ought to strenuously avoid inbreeding.
In the promiscuous red jungle fowl Gallus gallus, the wild progenitor of domestic chickens, the risk of inbreeding is high because hens and cocks stay close to their home turf. Smaller female jungle fowl can do little to resist incest. But because they can store sperm, females may be able to choose whose sperm wins their egg.
Evolutionary biologist Tommaso Pizzari at the University of Leeds, U.K., and colleagues investigated this possibility by mating female jungle fowl with brothers and unrelated males, then counting how many sperm reached each egg. Females stored fewer sperm from brothers and fewer of those sperm reached their eggs, even though males pumped more sperm into their sisters in an apparent attempt to compensate. Males and females may be locked into a kind of escalating sperm warfare, the researchers suggest online this week in Proceedings of the Royal Society B.
The results also suggest that males and, maybe, females can recognize kin, but the researchers aren't sure how they do this or how the hens discriminate against brotherly sperm. "One possibility is that they may be able to do this behaviorally through sperm ejection straight after copulation," says Pizzari. But even if females can't recognize kin, there may be some physiological mechanism that rejects relatives' sperm, he adds.
The study is "very exciting because it's a very clear demonstration of cryptic female choice," and the first direct evidence that relatives' sperm is selected against in vertebrates, says Ben Sheldon, an evolutionary ecologist at the University of Oxford. He's also impressed that the researchers were able to show that the birds were responding to relatedness rather than familiarity.