Paris (AFP) - A protein in sperm may cause females to come to blows over food -- at least in fruit flies -- according to an unusual study published on Monday.
After mating, female fruit flies in lab tests were significantly more aggressive towards one another, "often headbutting and swatting at each other" in fights over a meal of yeast paste, said a research paper.
They were involved in much longer battles than their virgin sisters, the team found, and fights were more likely to escalate.
Though the topic of the paper may appear quirky at first, it challenges a basic assumption widely shared by biologists: that heightened female-on-female aggression must be driven by competition for food for their young.
The new work found that fruit fly females grew violent after sex regardless of whether coupling yielded offspring.
This implies that "reproduction was not a contributing factor in their obvious change of temperament," the researchers concluded.
In turn, this suggested it was the sex act itself that made the females angry -- or something transferred in sperm.
In subsequent experiments, the researchers mated females with spermless males, and found this did not increase their aggression levels compared to sex with fertile mates.
Adding a protein found in fly sperm -- called a sex peptide, SP for short -- further boosted violence levels, the team discovered.
Many studies in several animal species have shown a link between elevated female aggression on the one hand, and egg laying and offspring rearing on the other.
But it had been assumed that increased competition for food was the cause -- a theory that now looks doubtful, said the team.
Further research will try and determine how long the angering effects of the sex peptide last, and what the benefits are.
"We're trying to work out if perhaps female aggression... is actually a behaviour that is being manipulated by males for their benefit, not the females," co-author Eleanor Bath from the University of Oxford told AFP.
It was "highly likely" that other insects may display the same type of behaviour, the authors added.
Better understanding the phenomenon could be useful for pest control, for instance, with aggressive females let loose to limit insect populations in the wild.
The study was published in the journal Nature Ecology & Evolution.