A new look at the common fly shows a lowered aggression among male offspring competing for the same females, with brothers benefiting collectively from one another’s mating success. In those instances of harmonic genetic interest, males directed less “violence” toward females. Yet when males live among unrelated flies, competition for mating becomes more frenzied, as more fights break out and females get knocked around, investigators from Oxford University say.

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Among mating Drosophila flies, males living with their brothers cause less harm to females during courting and mating than those living with unrelated flies. Amy Xinyang Hong & Cedric Tan

“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,” zoologist Tommaso Pizzari said in a statement. “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.”

In the study, Pizzari and his colleagues placed trios of virgin males with a single virgin female, allowing them to feed and mate freely in the space. They examined the effect of genetic similarity on mating competition by comparing three types of groupings: those with either three brothers or three unrelated males, as well as those with two brothers and one unrelated male. As some might predict, the most brotherly of trios were most relaxed and spent less time harassing females, the investigators reported Wednesday in the journal Nature.

“Interestingly, this approach worked against them in the [mixed] groups, where the unrelated ... flies typically had as many offspring with the females as both [brother] flies put together,” Pizzari said. “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.”

Although scientists have yet to find evidence linking increased mating to shorter lives for female flies, Pizzari and others say the cause may be both physiological — though this latest study found no such evidence — or behavioral, given that females would then spend less time gathering food and getting rest, in other words, taking care of number one.

That attenuated lifespan, with lowered reproductive opportunity, was most pronounced in the mixed groupings of flies, in which all three competed fiercely for sex with the females. In genetic terms, however, the selfish impulse to mate at all costs backfired on the aggressive males, by reducing overall mating opportunities over the course of their lifespans.

The study allowed investigators the opportunity to examine the “bros before hos” ethos common among some young men, whose complex mating alliances might sometimes yield more altruistic behavior, tradeoffs as currency for future gains. More broadly, the examination validates the role of kin selection in evolution, by which some organisms tend to favor others to the extent of their genetic similarity.

Interestingly, Pizzari said a single “renegade fly” introduced to a grouping would be enough to cause a sexual stir, with males responding behaviorally to greater genetic competition.

Source: Pizzari T, Wigby S, Allen F, Tan DKW, Pau C. Within-Group Male Relatedness Reduces Harm To Females In Drosophila. Nature. 2014.