Family break-ups lead to domestic abuse in fruit flies

Male fruit flies with strong family ties are less likely to become abusive during mating than others, according to new Oxford research.

Drosophila melanogaster (fruit flies) courtship is known to be a violent affair. Males compete aggressively for a female’s attention, attacking each other with their front legs - often harming the object of their affection in the cross fire.

Recent studies have suggested that when two males are related, this fighting tends to be less intense. As a result, females are much less likely to be harmed and can instead enjoy a longer reproductive lifespan.

However, the University of Oxford study, published this week in the journal Proceedings of the Royal Society B, reveals that family bonds are not the only factor that affects the ferocity of fly courtship: the rivals have to like each other too.

The study tested the role of relatedness and familiarity in the fly mating process. By placing trios of virgin male flies with individual virgin females together and allowing them to feed and mate freely, the team were able to compare the behaviour and lifespans of the flies in each group. Four fly groups were assessed; brothers that grew up together, brothers that had been separated at birth, unrelated males that grew up together and unrelated males that never knew each other.

Sally Le Page, lead author and a DPhil student in Oxford's Department of Zoology, said: ‘Theory suggests that if males are related, then they should be nicer to each other and harm females less as a result, but this is not strictly true. Females are harmed least by males that are both related and familiar, i.e. brothers that grew up together.’

The findings show that familiarity is as important as relatedness in fly courtship. Brothers that had grown up together were significantly nicer to the female during courtship than the brothers that had grown up apart. By reducing sexual conflict, the research showed that this union had clear benefits for females, who enjoy a longer lifespan and reproductive period as a result.

The research has the potential to shed light on the role that familial bonds have on species’ social relationships - including humans, who show the effects of inclusive fitness and developing certain social behaviours. However, Le Page stresses: 'We are not saying that, based on these findings, a woman should sleep with the housemate brother of your previous partner – that is a bit of a stretch. There are species caveats that do not cross over. However, it is testing the same framework that can apply to everything.'

With further research there is scope to use the study’s lessons around the factors that make females produce more or less offspring, in the field of conversation and wider pest controller.

Stuart Wigby, co-author and Research Fellow at Oxford’s Department of Zoology, said: ‘Any situation where animals are breeding and you care about how much or how little they breed, it is good to know the fundamental theory of what will affect that success. Whether rearing animals on a farm, a small number of tigers in a Zoo, or a batch of fly larvae, you want to know whether relatedness affects the offspring.’