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Fruit flies with brothers make gentler lovers than those living with unrelated flies, according to a new study.

Scientists found that unrelated male flies compete more for females' attention than related flies. This competition among unrelated flies results 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," lead researcher Dr. Tommaso Pizzari of Oxford University's Department of Zoology, said in a news release. "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."

For the study, researchers placed trios of virgin male flies with single virgin females and allowed them to feed and mate freely.

Researchers compared the behavior and lifespans of the flies in different groups depending on their relatedness: AAA, AAB and ABC. AAA groups contained three full brothers, AAB groups had two brothers and one unrelated male and ABC groups contained three unrelated males.

"Flies in AAA groups were typically more relaxed in their attitude to mating and spent less time harassing the females than males in other groups," said Pizzari. "Interestingly, this approach worked against them in the AAB groups, where the unrelated B flies typically had as many offspring with the females as both A flies put together. This is a classic example of sexual conflict where the selfish interests of individual males can work against the wider interests of the group. In this case, the female flies had shorter reproductive lifetimes and produced fewer offspring overall when unrelated males were constantly harassing them."

The study found that flies in the ABC lived the shortest and had the worst reproductive abilities compared to flies in other groups. Researchers explained that the in the ABC groups, the selfish incentive of each male fathering as many offspring as possible damaged the overall health of the group.

Researchers said the findings highlight the important role of kin selection in evolution, where organisms are more likely to favor others to the extent to which they are genetically related.

'As the AAB studies showed, a renegade fly that gets blown into a group of related flies will probably be more sexually active," said Pizzari. "The related flies in the group will be more complacent about sex since they can be fairly confident that their brothers will be passing along genetic information anyway, meaning less competition for the renegade. The aggressive sexual behavior of this outsider will result in more fights and lower lifespans for the group as a whole, but will benefit the individual as he will father more offspring."

Researchers are not sure why females in groups with unrelated males give birth to fewer viable offspring. They hypothesize that the female harm is caused by the behavioral rather than the physiological effects of courting. Researchers explain that fly courtship involves singing and genital licking. Repeated exposure to courtship may physically damage females and leave them with less time to get the food and rest they need for a healthy life.

The findings are published in the journal Nature.