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

Keywords: Drosophila melanogaster forced copulation fruit fly sexual behaviour sexual conflict 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. In most animals, females invest more resources in reproduction than males. This generates divergent mating strategies between the sexes such that males often fight amongst themselves for access to females, females are more selective than males in choosing mates, and males may harm females or coerce them to mate The last strategy constitutes conflict between the sexes, which can lead to an evolutionary arms race where males evolve better armament and females evolve improved defences There have been excellent research programmes devoted to studying particular types of sexual conflict and their evolutionary consequences (e.g. however, one of the most extreme manifestations of sexual conflict, forced copulation, has received relatively little attention in spite of its prevalence among many animals, including humans to understand the evolutionary dynamics resulting from forced copulation, we need to quantify how its exact costs and benefits translate into male and female reproductive success. As far as we know, no study has quantified reproductive benefits from forced copulations in males. In females, we know of only a single study by Thornhill (1984), in which female scorpion flies (Panorpa latipennis) provided with nuptial gifts in consensual matings laid more eggs than females that were forcibly copulated with males offering …