Transition from monogyny to polygyny in Nephila senegalensis (Araneae: Nephilidae) is not accompanied by increased investment in sperm

Abstract

A common male adaptation to prevent sperm competition is the placement of a mating plug. Such plugs are considered as an extreme investment if they comprise parts of the genital systems and render the male sterile. Genital mutilation occurs in monogynous spiders of several families and may co-occur with permanent sperm depletion, meaning that sperm production is terminated once males become mature. Within the orb-web spider genus Nephila, monogynous mating strategies are considered ancestral, although some species have reverted to a polygynous mating strategy. Although genital mutilation does not occur in these species, permanent sperm depletion (PSD) remained. We compared investment in sperm between an effectively plugging (Nephila fenestrata Thorell, 1859) and a closely-related nonplugging species [Nephila senegalensis (Walckenaer, 1841)]. Sperm investment should be higher in N. senegalensis because males are able to mate with several females, whereas N. fenestrata males can only achieve a maximum of two copulations, generally performed with the same female. The absence of a plugging mechanism in N. senegalensis and the inability to monopolize females by means of mating plugs results in a higher risk of sperm competition. Thus, we predicted higher investment in sperm producing tissue and larger sperm storage organs in males of N. senegalensis compared to N. fenestrata. We examined the testes and deferent ducts of both species for size and cell-quality differences using light and transmission electron microscopy and analyzed the volume of the sperm reservoir in the male copulatory organ (i.e. spermophor) using X-ray microcomputed tomography. In contrast to our prediction, the lumen of testes, deferent ducts, and spermophor of N. senegalensis males were significantly smaller than in N. fenestrata.