Monogamy when there is potential for polygyny: tests of multiple hypotheses in a group-living fish

Monogamy within social groups where there exists a high potentialfor polygyny poses a challenge to our understanding of matingsystem evolution. Specifically, the traditional explanationthat monogamy evolves due to wide female dispersion, affordingmales little opportunity to defend multiple females, cannotapply. Instead, monogamy in groups potentially arises becausefemales compete for breeding resources such as breeding sites,food, and paternal care. We conducted manipulative experimentsto determine whether females compete over limiting resourceswithin groups of the obligate coral-dwelling goby, Paragobiodonxanthosomus (Gobiidae). Breeding females behaved aggressivelytoward individuals of their own sex and evicted subordinatefemales that were large and mature from the group. Experimentalremoval of nest sites caused breeding partners to breed in alternativenest sites, demonstrating that nest site limitation was notthe cause of female competition. Supplemental feeding resultedin an increase in the fecundity of breeding females but no maturationof subordinate females, demonstrating that food-limited femalefecundity was a likely cause of female competition. Finally,supplemental feeding of breeding pairs demonstrated that thedifference in eggs hatched by fed versus unfed males was lessthan the difference in eggs laid by fed versus unfed females,suggesting that paternal care limitation might also drive femalecompetition. These results suggest that competition over foodand possibly paternal care selects for dominant, breeding femalesto suppress the maturation of subordinate females to minimizecompetition. Monogamy in association with group living is thereforelikely to have evolved because female competition prevents malesfrom utilizing the potential for polygyny.