The function of the mating plug in the chalcedon checkerspot butterfly

After mating, males of many species of butterflies seal the female's copulatory opening with a 'mating plug'. We examined the proximate function of this mating plug in the chalcedon checkerspot butterfly, Euphydryas chalcedona (Lepidoptera: Nymphalidae). Using interactions between free-flying males and six groups of treated females, we assessed the role of the mating plug in (1) physically preventing copulation, (2) signalling that the female has mated, and (3) inducing female mate-refusal behaviour. Males approached, landed and courted females in the manipulated and control groups with equal frequency in the field. Mating plugs did not reduce courtship persistence. When transplanted to virgin females, mating plugs usually prevented copulation, even though the females were still receptive. Mating plugs did not influence female behaviour. On the other hand, mated females became refractory immediately following mating and rarely copulated with males, even when their mating plugs were removed. Refractory behaviours of females acted in concert with mating plugs within one day of mating. However, the fact that the mating plug impedes copulation when females are receptive suggests that conflict may arise between females and their prior mates. Some females that would otherwise remate after digesting the first male's spermatophore may still be prevented from doing so because of the mating plug.