Broken genitals function as mating plugs and affect sex ratios in the orb-web spider Argiope aurantia

Background: To curtail competition with their own sperm, males of several spider and insect species mutilate their genitals or sacrifice themselves entirely using their genital parts or their whole dead bodies as mating plugs. The orb-web spider, Argiope aurantia, is a species characterized by both male self-sacrifice and extreme female-biased sexual size dimorphism. Plugs may reduce female mating frequency and hence the availability of females to males as mates. Questions: Do male genital tips that break off during copulation function as mating plugs in A. aurantia? Does plugging of females affect sex ratios? Methods: A series of mating trials to test the mating plug hypothesis and to determine the mating frequencies. I compare experimental results with data from field-collected individuals and estimate the effect on sex ratios. Results: Genital tips that are broken off do function as mating plugs. However, although males plug females, re-mating by females does occur and females mate on average with 1.6 males. Partly as a consequence of plugging, the operational sex ratio (sexually active males to females) is strongly male-biased in A. aurantia. Because males typically insert both pedipalps into the same female, the effective sex ratio (males to females that mate at least once) is also male biased, which supports a recent model for the evolution of monogynous mating systems (males mating with only one female).