Competitive males but not attractive males reduce female fitness in Gnatocerus cornutus

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General concepts of sexual selection assume that females benefit from mating with more competitive or attractive males. By contrast, sexual conflict theory assumes that females suffer greater costs from mating with such males. To understand how mate choice evolves, it is necessary to examine these different predictions regarding costs and benefits. Moreover, environmental factors may play an important role in the consequences of mate choice benefits. Here, we examined how mate choice benefits and sexual conflict costs are affected by an environmental condition (single or repeated interaction with a mate) in Gnatocerus cornutus (Coleoptera: Tenebrionidae). The males have enlarged mandibles to fight rivals and for courtship display to attract mates, and these characters provide a mating advantage to males. In G. cornutus, compared with a single mating, continuous housing with a male directly enhances female fitness due to increased mating frequency and sperm replenishment. In a single mating situation, there were neither direct benefits nor costs from competitive and attractive males. By contrast, when females were continuously housed with a competitively superior male, female fitness was reduced, probably because of misdirected attacks by the competitive male. Thus, our result suggests that females suffer direct costs from competitive males in G. cornutus but this cost is not incurred when there are few sexual interactions.