Disentangling the costs of male harassment and the benefits of polyandry for females

Many studies quantify how polyandry affects female fitness by allowing females to either mate with one or several males. But even if the number of matings is standardized, such studies conflate any costs of interacting with males with the potential benefits of receiving sperm from several males, obscuring the benefits of polyandry. We conducted a 2×2 factorial experiment on the mosquitofish, Gambusia holbrooki to quantify the independent effects of male harassment and polyandry. We artificially inseminated virgin females with sperm from either 1 or 5 males (monandry vs. polyandry). Females were then housed in the presence or absence of reproductively ablated males who could harass, but not mate with them. Our design ensured that the number of males inseminating a female was independent of the level of male harassment and the number of matings she received. Females who were not housed with males were instead housed with immature females to maintain densities constant across treatments. Unexpectedly, females that experienced sexual harassment were more likely to give birth, had shorter gestation periods and gave birth to larger broods. Furthermore, polyandrous females were more likely than monandrous females to give birth. Polyandrous females’ sons also reached maturity faster than those of monandrous females. We therefore found no detectable costs to females producing their first brood of being harassed by males when the direct costs of mating were absent. We also showed that, in the absence of mating costs, there are direct and indirect fitness benefits of being inseminated by multiple males. If the costs of the act of mating are small or absent, polyandry will benefit female G. holbrooki producing their first brood.