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.

Evolutionary responses that rescue populations from extinction when drastic environmental changes occur can be friend or foe. The field of conservation biology is concerned with the survival of species in deteriorating global habitats. In medicine, in contrast, infected patients are treated with chemotherapeutic interventions, but drug resistance can compromise eradication of pathogens. These contrasting biological systems and goals have created two quite separate research communities, despite addressing the same central question of whether populations will decline to extinction or be rescued through evolution. We argue that closer integration of the two fields, especially of theoretical understanding, would yield new insights and accelerate progress on these applied problems. Here, we overview and link mathematical modelling approaches in these fields, suggest specific areas with potential for fruitful exchange, and discuss common ideas and issues for empirical testing and prediction.