A mating plug and male mate choice in Drosophila hibisci Bock

The functional significance of mating plugs and their relation to male mating behaviour in insects are obscure. In Drosophila hibisci, we describe a firm, gelatinous, mating plug that fills the entire uterus at copulation, and evaluate two nonmutually exclusive hypotheses for the plug's function: (1) retention of sperm near the openings of the sperm storage organs and (2) inhibition of further matings. Unlike full-sized plugs, smaller plugs produced by previously mated males failed to retain sperm at the anterior end of the uterus, indicating that full-sized plugs prevent sperm backflow away from female storage organs. Sexually mature males failed to copulate with previously mated, young females, suggesting that the plug may also deter rival matings. In newly emerged females, the plug remains within the uterus for up to 4 days after copulation, but in mature females, the plug and ejaculate are expelled whenever the next mature oocyte descends into the uterus from the common oviduct, which may be soon after copulation. Males remained in copula significantly longer with mature females, perhaps to compensate for this greater likelihood of ejaculate expulsion. Males showed a mating preference for young virgins over older virgin and nonvirgin females. This unusual mating preference may confer fertility benefits made available through the effectiveness of the plug in reducing sperm competition. Copyright 1998 The Association for the Study of Animal Behaviour.