Costs to females and benefits to males from forced copulations in fruit flies

Forced copulation, which is perhaps the most extreme form of sexual conflict, is ubiquitous among many species including humans. To better understand the evolution and maintenance of forced copulations, it is imperative to assess their costs and benefits. We followed up on recent studies indicating frequent forced copulations in two wild populations of fruit flies, Drosophila melanogaster, and quantified their effects on males and females under controlled laboratory settings. Compared to females that mated consensually, females that were forcibly mated had fewer progeny, higher mortality rates and higher frequencies of wing damage that prevented flight. Males that forcibly copulated fathered much fewer progeny than did males that mated consensually. The reasons for the relatively small reproductive gains to males from forced copulations were the lower female fertility and higher female mortality from forced than consensual copulations as well as the higher frequencies of rematings by females that were forcibly copulated. It is likely that males attempt to force-copulate in spite of the low potential reproductive gain because of the scarcity of sexually receptive females and the consequent low probability of attaining the high fitness associated with consensual matings.