There is more to mating than beating up the competition, according to a new study.

Female fruit flies sometimes choose males who win fights, sometimes choose males who do not fight, and sometimes choose males for no obvious reason, say biologists from the University of Southern California; Cal State University, Sacramento; and the University of California, Davis.

The findings help explain the large variation in aggressiveness in most species, including humans.

"If aggression makes you more likely to father children, all males should be selected to be very aggressive. Male fruit flies (like humans and other animals) show a lot of genetic variation in aggression, and we wanted to find out why," explained study leader Brad Foley, a post-doctoral researcher at USC.

One reason for the variation, according to the study and to previous research on lizards by other groups, may be that no fighting strategy works all the time, just as in the game rock-scissors-paper.

"We showed in fruit flies that even the most genetically aggressive flies can have an Achilles heel, and lose against males who are (for the most part) wimps," Foley wrote.

"There's no single way to win a fight, or win mates," he added. "Females didn't necessarily prefer aggressive males -- some males mated less when they lost fights, but some males mated more if they didn't fight. Moreover, different females preferred different males."

"Unexpected interactions between individuals can define winners and losers (so-called 'chemistry')," Foley concluded. "In order to understand why flies, and humans, and other animals, are so genetically different from each other, we need to stop imagining there's a 'best' kind of strategy."

So while Hamlet was described to have "the courtier's, soldier's, scholar's, eye, tongue, sword," it is not clear which part drove Ophelia mad.

Journal reference: Cabral LG, Foley BR, Nuzhdin SV (2008) Does Sex Trade with Violence among Genotypes in Drosophila melanogaster?. PLoS One 3(4): e1986.doi:10.1371/journal.pone.0001986

The other member of the research team were Larry Cabral of Cal State Sacramento (co-corresponding author with Foley), and Foley's supervisor Sergey Nuzhdin, professor of molecular and computational biology at USC. The paper was written at USC based on experiments conducted at the University of California, Davis, where Foley and Nuzhdin worked previously.

Funding for the study came from the National Science Foundation.

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