The costs of copulating in the dung fly Sepsis cynipsea

Finding, assessing, rejecting, and copulating with a mate is assumed to carry fitness costs, particularly for females, that have to be traded off against fitness benefits of mating such as increased fecundity, fertility, longevity, or better quality offspring. Female dung flies, Sepsis cynipsea (Diptera: Sepsidae), typically attempt to dislodge mounted males harassing them by vigorous shaking. Shaking duration has been shown to reflect both direct and indirect female choice in this species. The latter is an expression of the females' general reluctance to mate due to presumed costs of mating. We investigated the costs of copulation in the laboratory. Females were randomly assigned to one of three treatment groups and allowed to copulate either not at all, once, or twice. The males' armored genitalia injured females internally during copula. Injuries were visible as sclerotized scars in the female ovipositor, and their occurrence increased with mating frequency. Presumably due to these injuries, mated females showed higher mortality. This effect was statistically independent from additional costs of reproduction related to oviposition, as copulation also increased lifetime egg production and tended to augment oviposition rate (eggs per day), while fertility (proportion of offspring emerged) was unaffected. We thus found high mortality costs of copulating, indicating substantial sexual conflict, which helps explain female reluctance to mate in this species.