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Laying Odds on Hens, Roosters Gamble their Genes
By Edward R. Winstead

Roosters allocate progressively less sperm to females they mate with over and over.

Evolution has been good to the rooster.

Male chickens can mate with females as often as they like and still hope to spread their genes far and wide. That's because roosters can control how much sperm they expend during a sexual encounter, allowing them to use more or less depending on social and environmental factors.

Two important factors are the competition—whether other roosters are mating with the same hen—and whether a rooster has already mated with a particular hen.

“Roosters distribute their sperm resources strategically according to several factors, including the reproductive value of the female,” says Tommaso Pizzari of the University of Leeds, the United Kingdom. His team recently discovered this phenomenon while studying a group of free-ranging chickens in Sweden.

Roosters judge a female's reproductive value by the size of her comb. Hens with large combs tend to produce large eggs and lots of them. From an evolutionary perspective, fertilizing these eggs is like hitting the jackpot.

The researchers collected the semen of roosters who mated with hens in the presence of other males and alone. The fowl were accustomed to being around humans, and the hens wore harnesses to capture semen. The birds mated over plastic pads that caught any excess semen that dripped out.

A rooster's social status influenced his allocation of sperm. When a dominant rooster faces competition from other males, he will expend a lot of sperm and progressively increase the number of sperm. This is like buying more tickets to increase one's chances of winning the lottery.

Hanne Løvlie of Stockholm University collecting data.

By contrast, subordinate roosters invest less sperm if several dominant males are mating with the same hen. Realizing they have little chance of success, they save their sperm for another occasion.

This makes sense because it takes energy to produce sperm and reserves can be depleted.

Farmers have always known that over time roosters grow less interested in mating repeatedly with same hen. By counting semen, the researchers discovered that roosters also reduce the sperm progressively as they continue to mate with the same female.

But when a new female is introduced, the rooster wants to copulate and allocates considerably more sperm to his new mate.

The physiology of sperm allocation is not yet clear, but the researchers plan to hunt for some of the genes involved. They will have new tools next spring when the chicken genome sequence is likely to be completed.

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Hens eject sperm; roosters are clueless

When it comes to influencing the content of new chicken genomes, females have a trump card. Hens are not large enough to fight off a sexual advance, but they can squirt semen back on a rooster after copulating, which lasts about a second.

This behavior was reported three years ago by Pizzari and his colleagues in the journal Nature. Using some of the same chickens as in the new study, also published in Nature, the researchers found that hens ejected the sperm of all but the dominant roosters in the group, thereby selecting high quality fathers for their offspring.

The researchers are testing whether females grow bored mating with the same male. Early results indicate that they do.

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Pizzari, T. et al. Sophisticated sperm allocation in male fowl. Nature 426, 70-74 (November 6, 2003).


Pizzari, T. & Birkhead, T.R. Female feral fowl eject sperm of subdominant males. Nature 405, 787-789 (June 15, 2000).


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