Post-copulatory sexual selection in two tropical orb-weaving leucauge spiders

Abstract

This chapter focuses on descriptive and experimental studies of the sexual biology of two spider species, Leucauge mariana and Leucauge argyra. We examine general questions related to female effects on paternity by taking advantage of several unusual traits: Direct female participation in forming copulatory plugs and physical clasping by the female rather than the male during copulation in both species; and derived traits in the genitalia of males and females and occasional female cannibalism of conspecific males by trapping the male’s genitalia in adhesive copulatory plugs in L. Argyra. These derived traits, combined with other aspects of sexual interactions that are typical of spiders in general, such as complete, easy female avoidance of unwanted sexual advances of males, copulatory courtship by males, and imperfect male ability to remove copulatory plugs from the female’s genitalia, constitute a rich mixture from which clear lessons can be drawn. Post-copulatory sexual selection probably acts on male genitalia and their behavior in both species. Two derived male genitalic clasping and clamping devices L. Argyra may be associated with the extremely aggressive female behavior. One genital structure in male L. Mariana may have evolved under sexual selection by sperm competition to remove copulatory plugs from female genitalia. In contrast to expectations from theory based on male-female conflict, the female genitalia of L. Argyra has a derived trait that selectively facilitates rather than impedes male genital clamping. In contrast to lock-and-key expectations, another prominent, derived female genital structure in L. Argyra does not mesh with any male structure during copulation; it may instead function in sensing male genital movements. Additional non-genital male traits associated with copulation are also probably under post-copulatory sexual selection in both species. Experimental manipulations of male chelicerae and of possible female receptors of stimuli from the male chelicerae demonstrated that stimulation from secondary sexual modifications of the male chelicerae influences several female reproductive processes in L. Mariana, including receptivity to remating, female interruption of copulation, and copulatory plug formation. Additionally, two male copulatory courtship behavioral patterns correlate with greater plug formation in L. Mariana, and one pre-copulatory courtship behavioral pattern in males of L. Argyra correlates with plug formation, suggesting additional possible male effects on post-copulatory female reproductive processes. Other male sexual behavior correlates with the timing of female emission of plug material that may affect a male’s chances of surviving the encounter. In sum, female Leucauge have powerful effects on whether copulation occurs, when it will end, whether a mating plug will be formed, and whether the male will survive the encounter. Several genital and non-genital male traits likely evolved under a mix of post-copulatory sexual selection (sperm competition and probably cryptic female choice [CFC]) to elicit favorable modulation of these female effects.