Mate-guarding success depends on male investment in a butterfly

Males of many insects, including butterflies, produce mate-guarding devices, such as mating plugs, to prolong guarding and prevent future female matings in the male’s absence. In a few butterflies, large external mate-guarding devices, i.e. sphragides, occur. Gór et al. (2023) found conspicuously large size and morphological variation of mate-guarding devices within a single population of the polyandrous Clouded Apollo (Parnassius mnemosyne, L.) butterfly. They termed the externally visible male-produced devices as Copulatory opening APpendices (CAP) consisting of small devices, termed small CAPs, and the much larger shield (i.e. sphragis). Our aim was to reveal CAP replacement dynamics within females during their lifetime and to understand how male investment into small CAPs or shields is (i) related to CAP persistence on the female, i.e. securing paternity, (ii) associated with female quality, measured as size, and (iii) with actual adult sex ratio. We investigated a univoltine Clouded Apollo population to estimate CAP replacement risks, using multistate survival models, in an extensive observational study through six years based on mark-recapture. Shields are the most frequent mate-guarding devices and are more persistent than small CAPs, often lasting for life, excluding future matings. Thus, most females bearing a shield are deprived of postcopulatory female choice, and the genetic variance in their offspring may be reduced compared to those bearing small CAPs, thus mating more often. The ratio of shields to all CAPs gradually decreases towards the end of the flight period. Males are more prone to produce a shield when mating females with wider thoraces and when the ratio of males (i.e. competition) is higher in the population. To our best knowledge, this is the first quantitative study to investigate potential factors on which male investment in mate-guarding devices may depend, and how the variation in these devices impacts CAP persistence on females