The costs of avoiding matings in the dung fly Sepsis cynipsea

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Abstract
Mating is generally assumed to carry costs, particularly for females, which have to be traded off against each other and against the fitness benefits of mating. To understand any particular mating system and the evolution of sexual conflict, these costs have to be evaluated. Female dung flies, Sepsis cynipsea (Diptera: Sepsidae), typically attempt to dislodge mounted males by vigorous shaking. Such female reluctance to mate can only evolve if the cost of avoiding matings does not exceed the cost of copulation. We investigated female precopulatory costs of assessing and rejecting males in terms of increased predation, wing injuries, and (indirectly) energetics, all ultimately affecting mortality, and compared them to the costs of copulation assessed in this and a companion study. Females housed with a male had lower survivorship than females housed with another female. This was largely due to the costs of copulation rather than presumed energetic costs of avoiding males, which were minor. Male harassment augmented female wing injuries, which accumulate with age in the field and laboratory, but in laboratory experiments using one common predator, wing injuries did not increase the susceptibility of S. cynipsea to predation, nor did their mating behavior per se. Instead, predation was highest and survivorship lowest in all-male groups, probably because males are more active in search of females and harass each other. Overall, the precopulatory costs of mate assessment and rejection were low relative to the costs of copulating, explaining female reluctance behavior in this and possibly other species.

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