Focusing on the behaviour of common fruit flies (Dropsophila), Tan’s paper, published in Nature, looks at the competition between male fruit flies when looking for a mate. Tan and his co-authors, former Wadham lecturer Stuart Wigby, Pau Carazo, Felicity Allen and Tommaso Pizzari, demonstrate that the competition between related males is less intense than between unrelated males, and that females experience less harm as a result.
Says Cedric: “Sex leads to potential for conflict between the sexes where males outcompete each other over females to a point that often harms the female, a process akin to the tragedy of the commons. Here we show that male-male relatedness can switch competition to cooperation and mitigate the intensity of female harm.
“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,” said Dr Tommaso Pizzari of Oxford University's Department of Zoology, who led the study. “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.”
According to their research findings: “Females exposed to groups of three brothers unrelated to the female had higher lifetime reproductive success and slower reproductive ageing compared to females exposed to groups of three males unrelated to each other. Triplets of brothers also fought less with each other, courted females less intensively and lived longer than triplets of unrelated males.”
However, when two brothers were matched with an unrelated male, the unrelated male sired on average twice as many offspring as either brother, demonstrating that relatedness can profoundly affect fitness through its modulation of male-male competition.
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 do not disperse far away from the place they have hatched from. It is therefore likely that many flies living together will be related, but unrelated males that turn up are likely to father a disproportionately higher number of offspring.
There are two principal reasons for studying these relationships adds Cedric: “First, many species of animal exhibit some degree of limited dispersal in which individuals within groups are on average more related than the population average. Therefore related individuals are often in contact and would interact in the socio-sexual context. Second, studying how relatedness-effects harm females is key to understanding the high variability in female harm observed across and within animal species.”