Sexual conflict over mating system: The case of a pair-territorial filefish without parental care

We studied the optimal mating system from the male's and female's point of view in the longnose filefish, Oxymonacanthus longirostris, which is predominantly monogamous but can also be polygynous, and lacks parental care. This fish typically moves about in closely coordinated heterosexual pairs, which share a feeding territory throughout the year. We conducted two types of male removal experiment involving manipulation of male and female mating status to examine whether the fitness consequences for each sex are influenced by mating status, and we estimated the fitness components for naturally monogamous and polygynous individuals. Polygyny was advantageous to males because of higher reproductive success, but monogamy was advantageous to females because they produced fewer eggs when their mates mated polygynously. This was due to polygynous males spending less time on territorial defence with their mates than monogamous males, resulting in a decrease in female feeding frequency. Thus, there was a sexual difference in the optimal mating system. The cost of polygyny to females led to intense female-female competition over the male in a polygynous group. However, two factors, a slightly female-biased sex ratio in the population and interference by males in the aggression between females, made it difficult for some females to form the optimal mating system. Females in a polygynous group, therefore, make the best of a bad situation. Thus, the resulting mating system of O. longirostris seems to be an outcome of sexual conflict over the mating system: males gain from polygyny, whereas monogamy increases female fitness.