In the animal world, incest helps put an end to sexual violence

That's the conclusion of a modelling study of species in which males harm their mating partners. In some species, competition between males can mean that females are harmed during copulation. Male cowpea weevils, for instance, have spiky penises that puncture the female's insides and are thought to help anchor the male inside her, ensuring that he delivers his sperm and outcompetes other males. But another evolutionary pressure comes into play: closely related individuals share many of the same genes, so if a male harms a relative, fewer copies of its genes survive. Daniel Rankin of the University of Zurich, Switzerland, wondered what happened when the competitive and family forces conflict. He set up a computer model in which males were more or less aggressive towards females, and males and females were more or less related to each other. The experiment found that in populations of closely related individuals, males that harmed females less were more successful at spreading their genes. In the long run, this could influence the evolution of the community. In a group where animals are closely related, evolution will push males to be less aggressive towards females, says Rankin.