Alternative mating tactics in brown widow spiders: mating with or without male self-sacrifice does not affect the copulatory mechanism

Abstract

Male self-sacrifice during mating is one of the most extreme forms of male reproductive investment. In two species of widow spiders (genus Latrodectus), males trigger sexual cannibalism by “somersaulting” into the fangs of the female after copulatory coupling is achieved. In this position, sperm are transferred with the secondary sexual organs, the transformed pedipalps of the male, while the female starts feeding on his opisthosoma. In Latrodectus hasselti and L. geometricus, matings also occur with subadult females (i.e. females in their last moulting stage) but during these “immature” matings, males do not perform the somersault. Consequently, mating positions differ dramatically between matings with adult and subadult females. Here, we investigate the copulatory mechanism of adult and immature matings in the brown widow L. geometricus by shock-freezing copulating pairs and 3D X-ray microtomography. We hypothesize differences in the copulatory mechanism and depth of insertion of the sperm transfer structures between the two mating tactics. We found that the copulatory mechanism does not differ between adult and immature mating tactics and do not depend on whether a somersault occurs. Furthermore, the somersault does not improve intromission depth of the male sperm transfer organs into the female sperm storage organs. Our results suggest that the somersault has evolved solely due to the selective advantages of sexual cannibalism. The costs and benefits of both mating tactics need to be further explored using paternity studies in order to understand their relative adaptive value.