High mortality and female-biased operational sex ratio result in low encounter rates and moderate polyandry in a spider

Mating systems are shaped by ecological factors such as timing of maturation, spatiotemporal distribution of individuals, and the operational sex ratio. These factors influence male-female encounter rates, which shape the intensity and form of inter- and intrasexual competition. Whereas knowledge of natural encounter rates is vital for understanding the evolution of mating systems, data from the wild are difficult to obtain. We present data on the ecology and natural history of the subsocial spider Stegodyphus bicolor (Eresidae) with the aim of assessing its mating system. We investigated male mate search in relation to female spatial distribution, the timing of maturation of individuals, and the operational sex ratio, which may affect male-male competition and be used to predict mating strategies. We recorded male visits and cohabitation patterns with females of different reproductive state (immature and adults) to investigate evidence for mate choice as an indicator of sperm priority patterns. Finally, we used male visiting rates as a proxy for female natural mating rates. Both sexes matured synchronously: 51% of the females mated and received on average 1.2 visiting males. With increasing female availability male cohabitation time decreased. Preference for adult females, a decline in the occurrence of cohabitation with increasing female availability, and a lack of male protandry suggest that males do not employ strategies to protect paternity. Indeed, female-biased operational sex ratio throughout the mating season and high mortality may result in low male visitation rates. We suggest that ecological constraints shape a moderately polyandrous mating system in this species.