Ejaculate feeding and female fitness in the sexually dimorphic fly Prochyliza xanthostoma (Diptera: Piophilidae)

Recent theory and empirical evidence suggest a role for sexual conflict in the evolution of male ejaculates, including 'nuptial gifts'. According to the sexual conflict hypothesis, the greater the probability of female remating, the stronger the selection on males to induce an elevated reproductive rate in their mates, even if this reduces some components of female fitness, such as survivorship. Piophilid flies show an unusual form of nuptial gift: following copulation, females expel and ingest much of the sperm and accessory fluids received from the male. We investigated the effects of ejaculate ingestion on female fitness in the piophilid carrion fly Prochyliza xanthostoma by manipulating females' opportunity to ingest the ejaculate under two background food levels (fed ad libitum versus starved). We also estimated females' probability of remating. Few P. xanthostoma females mated more than once in repeated pairings with males. Females permitted to ingest the ejaculate oviposited sooner and laid more eggs than females prevented from ingesting the ejaculate, but ejaculate ingestion did not affect female survival. Hence, ejaculate feeding appeared to increase females' net fitness. However, these effects were evident only in fed females, suggesting that ejaculate meals provide mainly stimulation rather than nutrition. The benign effects of ejaculate feeding on female fitness, and the lack of evidence of sexual conflict, are consistent with the low female remating rate in this species. None the less, our results suggest that sexual conflict has played a role in the evolution of these traits.