Biologists from the American University of Southern California, studying large variation in aggressiveness in most species, including humans, found that there is more to mating than beating up the competition.

The study, funded by the National Science Foundation, concluded: "Unexpected interactions between individuals can define winners and losers (so-called 'chemistry').

The research sought to tackle a paradox in species from fruit flies to humans: If warriors win the spoils, why don't males evolve towards super-aggressiveness?

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.

"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," study leader Brad Foley was quoted as saying by the Science Daily online.

One reason for the variation, according to the study may be that no fighting strategy works all the time. "Females did not necessarily prefer aggressive males - some males mated less when they lost fights, but some males mated more if they did not fight. Moreover, different females preferred different males," Foley noted.