For males, every mating can mean new offspring, for females, a single mating often provides enough sperm to fertilise all the eggs she can produce, so the big question is: Why do females tend to mate with several males? One possibility is that females are coerced into matings by males, another is that it is actually good for females, perhaps because mating with several males allows them to choose to use sperm from just the best one.
We have been able to answer this question for the first time in a wild invertebrate. Our video monitoring shows us who mates with whom, and our DNA fingerprinting allows us to count how many offspring each individual leaves in the next generation. This reveals that both males and females have more offspring when they have more mating partners, so males don’t need to bully females into matings – females also gain from being promiscuous.