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Costs and Benefits of Copulatory Silk Wrapping in the Nursery Web Spider, Pisaurina mira Walckenaer, 1837 (Araneae, Pisauridae)
Males and females share the same goal in sexual reproduction—increased offspring production and viability—yet specialized traits can evolve in one or both of the sexes that serve to increase individual reproductive fitness, sometimes at a cost to a mating partner. Exploring the costs and benefits of such traits in both sexes can provide insights into the role that male-female conflict play in the evolution of mating systems. Under this framework we have explored the evolution of the unique mating behavior observed in the cannibalistic nursery-web spider, Pisaurina mira, where males restrain females by wrapping them with silk prior to and during copulation. First, we tested the function of copulatory silk wrapping by pairing females with males that were experimentally manipulated to either be capable or not capable of silk wrapping. We found that males capable of wrapping females were (a) more likely to obtain two verses one opportunity to transfer sperm (termed insertions) and were (b) less likely to be cannibalized after or during copulation. Following from our initial study, we explored male benefits of increased insertion numbers, which revealed that increased insertion number corresponds to increased sperm transfer and higher fertilization success. Specifically, males transfer roughly two times the amount of sperm when achieving two insertions compared to only one, and this increase in sperm transfer results in males having approximately 25% more offspring. Two additional studies exploring the costs and benefits of males avoiding sexual cannibalism revealed that copulatory silk wrapping results in (a) females losing out on fitness benefits of sexual cannibalism while (b) allowing males to mate multiply. Specifically, we found that consumption of a male mating partner leads to females producing heavier and longer-lived offspring, however, silk wrapping keeps females from obtaining these benefits. Further, regardless of this shared fitness benefit, we also found that cannibalism avoidance by males allows them to mate multiply and achieve higher offspring production. Overall, our results highlight how male and female reproductive strategies can differ, and can ultimately lead to adaptations that clearly benefit one sex while inflicting costs on the opposite sex.
Anderson, Alissa G, "Costs and Benefits of Copulatory Silk Wrapping in the Nursery Web Spider, Pisaurina mira Walckenaer, 1837 (Araneae, Pisauridae)" (2018). ETD collection for University of Nebraska - Lincoln. AAI10788541.