Male pursuit of higher reproductive success drives female polyandry in the Omei treefrog

Female polyandry and corresponding multiple paternity have been detected in many species. Several hypotheses, such as the fertility guarantee hypothesis, the good gene hypothesis, the genetic compatibility hypothesis and the genetic diversity hypothesis, have been proposed to explain the evolution of female polyandry. Anuran amphibians are widely observed to breed in polyandrous spawning groups; however, few studies have evaluated multiple paternity in these species or the evolutionary mechanisms underlying polyandry. In this study, we addressed these questions pertaining to polyandry in the Omei treefrog, Rhacophorus omeimontis. Eight microsatellite markers of R. omeimontis were used to determine the patterns of paternity. Paternity analyses revealed that offspring were multiply sired in a majority (79.07%) of the polyandrous spawning groups. A male's reproductive output during a breeding season was greatly affected by the mating rate, revealing that male fitness benefited from multiple copulations. By contrast, no evidence supported the fertility guarantee hypothesis that polyandry increases the fertility rate of females or the compatible/good gene hypotheses that polyandrous females produce more compatible or superior offspring. The increased allelic diversity observed in polyandrous clutches is probably a nonadaptive by-product of multiple mating. In conclusion, males join as many copulations as possible to increase their reproductive success, regardless of whether their mate has already been in amplexus with other males. Females may gain no benefits from polyandry but are incapable of preventing superfluous males from joining the mating group due to the physical characteristics and breeding behaviour of anurans. Thus, in R. omeimontis, male pursuit of higher reproductive success is the primary explanation for female polyandry.