LOS ANGELES, April 18 (UPI) -- U.S. researchers say warriors may win the spoils, but don't evolve toward super-aggressiveness because the fittest males don't always get the girl.

Biologists at the University of Southern California; California State University, Sacramento; and the University of California, Davis; said female fruit flies sometimes choose males who win fights, sometimes choose males who do not fight and sometimes choose males for no obvious reason.

The findings, published in the journal PLoS ONE, 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," study leader Brad Foley, a post-doctoral researcher at USC said in a statement. "Male fruit flies -- like humans and other animals -- show a lot of genetic variation in aggression, and we wanted to find out why."

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, Foley said.

When it comes to mating, flies, humans and other animals are so genetically different from each other there is no "best" kind of strategy to winning a mate, Foley said.