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Vol. 343 ,
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Roosters Share the Love (But Not Evenly)
5 November 2003 (All day)
Males of the species Gallus gallus are hardly gallant. A male wild chicken scatters his seed among females according to his own sense of the competition, according to new research. The results indicate that the males have evolved a sophisticated way to choose where to put their resources.
Hens who mate the rounds end up with sperm from more than one competing rooster. For a rooster to win at this game, he has to contribute more sperm than the other guys. But willy-nilly inseminating is costly and time consuming--a rooster might run out of sperm before the best hen waltzes his way. Evolutionary biologist Tommaso Pizzari at the University of Leeds, U.K., and colleagues wondered if the wild roosters hedge their bets by inseminating some females with more sperm than others.
Pizzari and company fitted the hens with films on their cloacas to collect the sperm deposited by roosters. Then the team set up a mating ring in full view of an adjacent pen. The researchers allowed one male at a time to mate with the hen, watched by up to three other males to simulate competition for the mate. When a dominant rooster competed with just one voyeur, he ejaculated less sperm than when competing with three males, indicating that roosters consider how many suitors a female has. Subdominant males, on the other hand, apparently forfeit the most competitive situations and send out the most swimmers when they're competing against just one other male, the team reports in the 6 November issue of Nature.
In a second test, roosters were allowed to copulate as long as they wanted with one hen. At some point between 10 minutes and an hour, they stopped. When presented with a new female, the spent rooster resumed mating and had reserved some sperm. Lastly, the team found that if given a choice of two hens, roosters would spout more sperm for the one with the larger comb, which other research has linked to higher quality eggs.
"Typically, choice is considered a female trait," says evolutionary biologist Don Levitan of Florida State University in Tallahassee. "This study indicates that males also chose where to allocate their limited resources in order to maximize the number and quality of the offspring they produce." Even if roosters are cads, this finding may generate a bit of respect for their shrewd stratagems.
Pizzari's lab at the University of Leeds