Abstract:

This dissertation examines ecological, morphological and behavioral factors underlying multiple mating by females in the orb-web spider Argiope trifasciata . Field studies reveal that females often have access to multiple males, whereas males rarely visit more than one adult female. Females regularly accept multiple mating partners, and are polyandrous under natural conditions. However, an observed increase in post-copulatory mate guarding and remating by males limits polyandry when operational sex ratios (OSRs) are extremely male-biased. In laboratory trials involving twice-mated females, fertilization of eggs is largely limited to the first male to inseminate each of the female's paired spermathecae. When a single male mates twice with a previously virgin female, he inseminates each spermatheca once, and can essentially monopolize egg fertilization. The extent to which females suffer from this monopolization depends on fitness consequences of mating twice with the same male (monandry) as opposed to once with each of two different males (polyandry). Laboratory studies reveal that females suffer no apparent costs of mate diversity, but polyandrous females produce more eggs than monandrous females, resulting in an overall fecundity benefit of polyandry. No genetic benefits of polyandry were detected, but potential benefits can result from genetic diversity of offspring experiencing unpredictable and varying environments. The use of DNA fingerprinting (Amplified Fragment Length Polymorphisms) to determine paternity of offspring of polyandrous females did not reliably permit exclusion of potential sires, and therefore examination of whether offspring survival under different environmental conditions varied with paternity was not possible. Finally, a descriptive model of reproductive payoffs, developed from and applied to data from A. trifasciata and a congener, demonstrated how changes in OSR, mating history, and benefits of multiple mating should affect mating strategies and the extent of sexual conflict. The combination of a benefit of polyandry to females and attempts by males to monopolize mates sets the stage for strong conflict-of-interest between the sexes. Sexual cannibalism is a common female behavior that promotes mate diversity, and therefore fitness benefits of polyandry may lead to selection for increased female aggression toward mates in A. trifasciata and other sexually cannibalistic species.