The reluctant fly: what makes Sepsis cynipsea females willing to copulate?

A female's willingness to copulate (i.e. her receptivity) can depend on several endogenous, environmental and social factors. The black scavenger or dung fly, Sepsis cynipsea (Diptera: Sepsidae), is a classic example of strong female reluctance to mate and sexual conflict over mating. Laboratory studies have found high variability in female receptivity and even virgins unwilling to mate. The aim of this study was to determine the proximate factors influencing female receptivity. Fresh dung was necessary for egg production. Female receptivity strongly depended on a female's egg-laying cycle rather than age: receptivity was highest when females had no ripe eggs. The absence of eggs in the female's reproductive tract is probably required for spermatophore transfer, leading to the unusual precopulatory guarding and postoviposition mating of sepsids. Nonvirgins were less receptive than virgins, except when they were in need of sperm, and more receptive when they were larger and had previously laid more eggs. Only when not in need of sperm did females copulate with males larger than their previous mate. This suggests sequential female choice, but females typically copulated with the first of up to 10 presented males or not at all, provided that they were at the appropriate stage of their laying cycle. Female unwillingness to mate in this and probably other species therefore has various, sometimes physiological (intrinsic) causes. These must be controlled in experiments assessing mate choice, costs and benefits of mating, or sexual conflict. Not taking into account female egg-laying state can mask female choice, bias data and suggest wrong associations.