A new study by researchers at Oxford University has found that flies that live with their brothers are gentler lovers of females, ensuring that they cause less harm to female flies during courting, compared to those that live with unrelated flies.
The study found that unrelated male flies compete more fiercely for females' attention than related flies, resulting in shorter lifespans for males and reduced fecundity for females.
"In large populations brothers don't need to compete so much with each other for female attention since their genes will get passed on if their sibling mates successfully anyway," Dr Tommaso Pizzari of Oxford University's Department of Zoology, who led the study, said.
"Their more relaxed attitude to mating results in fewer fights and they also harm the females less as their courting is not so aggressive. When unrelated flies are together, the females are constantly being pestered for sex, which may leave them little time to eat or rest," Pizzari said.
The study highlights the important role of kin selection in evolution, where organisms are more inclined to favour others to the extent to which they are genetically related.
It is difficult to know exactly how many flies are related in natural groups but as they only live for a few days flies cannot travel too far from their birthplace.
It is therefore likely that many flies living together will be related, but unrelated flies that turn up are likely to father a disproportionate number of offspring.
The study is published in the journal Nature.