Male Drosophila melanogaster adjust ejaculate size based on female mating status, fecundity, and age

In contrast to early predictions, it is now widely accepted that males incur substantive costs from ejaculate production. Hence, males are predicted to allocate their reproductive investments, including ejaculate size, relative to the risk of sperm competition and to female quality. The study of sperm allocation, however, has been technically challenging with nonvirgin females because sperm from different males must be discriminated within the female reproductive tract. To date, such investigations have thus largely been restricted to species that transfer sperm in spermatophores or for which females can be fitted with a harness to capture the incoming ejaculate. In this study, we examined sperm allocation using male Drosophila melanogaster that express a fluorescently labeled protein in sperm heads, allowing us to quantify sperm numbers from different males within the female reproductive tract. We found that male D. melanogaster deliver significantly more sperm to mated, large or young females compared with virgins, small or old females, respectively, whereas copulation duration was only significantly longer with large than with small females. These results provide further evidence for costly ejaculate production and consequent prudent allocation of sperm.