Reproductive fitness in monarch butterflies, Danaus plexippus

Sexual coercion may increase the intensity of post-copulatory sexual selection. Male monarch butterflies (Danaus plexippus) can sometimes force females to mate, and there is substantial variation in male mating frequency and fertilization success. I investigated both pre- and post-copulatory processes that influence male and female reproductive fitness in this coercive mating system.; Using captive monarchs, I found significant additive genetic variance associated with male mating success. Successful males not only mated more often (by definition), but also were more likely to copulate with a female during any given mating attempt. However, successful males were not better able to discriminate between males and females, nor did they attempt to mate more frequently. Observations of wild monarchs at a CA overwintering site revealed substantial mating activity both on the ground and in the tree canopy. Mating attempts in the tree canopy were more likely to involve two males, but among heterosexual mating attempts, canopy and ground attempts did not differ in the likelihood of ending in copulation. Ground mating attempts were more likely to end in copulation when both butterflies were facing down at the start of an attempt and when attempts occurred on less complex substrates.; I investigated post-copulatory processes in captive monarchs. Sperm precedence was highly variable, and ranged from complete first to complete second male precedence. Males that were hand-paired to their mate transferred fewer sperm and fertilized fewer eggs than males who mated on their own. This pattern may result from lack of sufficient stimuli to the female or male, or interference with spermatophore placement among hand-paired males. The genetic markers we used to assign paternity, genotype at the phosophoglucose isomerase locus, affected sperm precedence in each year of the study, although different genotypes were favored in each of the two years. We suggest that this enzyme may be important in sperm function or transport, and that genotype at the PGI locus may interact with temperature. Male mating history, which determines the size of the spermatophore delivered, had a marginal effect on sperm transfer, but no effect on sperm precedence.