Facultative male investment in prolonged mate-guarding in a butterfly

Sexual conflict over mating may induce male mate-guarding prolonged in time in the absence of males, via devices such as mating plugs, widespread in insects. In most Lepidoptera, internal plugs are common, while in some butterfly families large external devices, called sphragides, evolved independently. Lack of, or incomplete sphragis in a few individuals were reported in sphragis-bearing species. Previous studies focused on typifying species-specific devices in a few specimens of many species. In contrast, we investigated alternative mate-guarding devices in detail of a sphragis-bearing butterfly. We conducted a six-year observational field study in a Clouded Apollo Parnassius mnemosyne population. Inspecting 492 females, we identified 3 different devices, filament, stopple and shield (i.e. sphragis) increasing in size and structure complexity, implying differential male investment and effectiveness in securing paternity. Shield dimensions, colour and all devices’ shapes varied considerably. Shields were far more frequent than other devices. Some devices were lost, and a few were video-recorded when removed by males, showing the role of different parts of male external genitalia. We discuss potential causes of device variation and the role of removal attempts, and assess potential costs and benefits for both sexes.