Mating System And Sexual Selection In The Omei Treefrog (Rhacophorus Omeimontis)

Mating behaviors and parental care reproductive individuals define animals' mating systems.By differentially promoting the opportunity and the intensity of sexual selection,the latter being in polyandry and polygyny species,mating systems influence ecological and evolutionary processes in animal.Sexual selection refers to the selection relationship between males and females.It includes intra-sexual and inter-sexual selection and these two aspects of sexual selection largely affect the animal's reproductive success.Therefore,it is important to understand the mechanisms and evolutionary significant of the mating system and sexual selection in animals.Despite the impressive development in studying mating systems and sexual selection,there are still some shortcomings.The type of mating system in many species has not yet been defined,the evolution of mating systems is litter studied in many taxa,and relevant researches have been mostly focused on species with internal fertilization.The studies using in vitro fertilization are still in the description stage.Studies of sexual selection considering both intra-sexual selection and inter-sexual selection are lacking.In this study,we use field observation of breeding behaviors from 2011 to 2013 and molecular parentage analyses of clutches to study the mating system,sexual selection and their evolutionary mechanisms in the Omei treefrog,Rhacophorus omeimontis in Badagongshan nature reserve.A total of 84 clutches have been observed and analyzed over the study period.Our results showed that,in R.omeimontis's mating system involved polygynandry.Polyandry(group spawning)was observed in 65.06%of clutches,and generated 79.07%multiple paternities in polyandry clutches.Multiple paternities ranged from 2 to 5.This suggests R.omeimontis have the mating system of polyandry.Among 81 observed reproducing males,from 2011 to 2013,64.20%obtained 2 or more mates,the maximal mating rate for males was 7,5 and 5,and,parentage results showed that 44.44%of 72 males have had effectively offsprings with more than one female.This suggests R.omeimontis have the mating system of polygynous.Monogamous accounted for 34.94%of clutches.The main driver of mating system in R.omeimontis may come from males.Our results show that the male individuals could improve their reproductive success by getting more spouse(polygyny),while females could not because no improvement of fertilization rate and genetic heterozygosity of offspring were found by getting more spouse(polyandry),though they could obtained a higher genetic diversity to offspring.We have found that ratio of polyandry has been enhanced with OSR in,this result showed that occurrence of polyandry were be enhanced because of more males involved in reproduction because in vitro fertilization in anurans and the lack of physical barriers,females cannot prevent males participating in the amplexus.The increased genetic diversity is likely a byproduct caused by convenience polyandry that female cannot prevent more males involved in mating group,and this does not necessarily bring substantial benefits to offspring.On the other hand,the prolonged breeding model R.omeimontis provides the conditions for males to get more mates in a breeding season(polygyny),and the competitive males could enhance their breeding success by obtaining more mates as the period is relatively long.Males could obtain more breeding benefits than females in this mating system.Thus,we speculate that the formation of mating system in the R.omeimontis is due to the males' pursuit of higher reproductive success rate.There are strong male-male competition in the geographical population of R.omeimontis in Badagongshan Mountain that may be influenced by the mating rate,body size and OSR.The dominant male sired 74.42%eggs in all nests in breeding process,and this high breeding variation indicates a high competitive pressure of males in this species.The number of offspring per male was positively affected by the mating rate,the mating rates of monogamous males groups were more than dominant males in polyandrous.Larger males had no more offsprings,but they had more numerous mating opportunities and larger mates,and had more advantageous amplectant position.In addition,larger males spent more time and allocated more energy in breeding ponds.The number of males who took part in spawning enhanced with OSR.We consider that the high-quality males can allocate more energy in breeding,and can spend more time in the breeding sites to gain higher mating rates,thus leading to the variation of reproductive success among male individuals.In fact,the attending times of males in the breeding sites was significantly positively related to their mating rates.When females appear in the breeding site,dominant male individuals are more likely to monopolize the chance for the mating under the conditions when subordinate males did not attend in the breeding sites.In R.omeimontis,larger males can have stronger competitiveness,allocate more energy to attend the breeding grounds,obtain more and larger mates,and occupy more advantageous position in amplexus.However,larger individuals do not have more offspring.This is due to sperm depletion and the negative effects of age on sperm vitality,the influences of sex ratio and the surrounding environment.The increased OSR may lead to increased number of males attending in the amplexus,which,in turn,may result in high competitive pressure among males.Females have weak choice in the studied population.Our data do not support the following hypothesis:(1)Genetic relatedness selection hypothesis.In breeding process display neither inbreeding nor outbreeding in R.omeimontis,the relatedness between the females and the paired dominant males in polyandrous groups was not significantly greater than that in monandrous groups,indicating that females have no choice about Genetic relatedness of their mates.(2)Good gene hypothesis:The heterozygosity was compared between monogamous males groups and dominant males in polyandrous groups and no significant differences were observed,no significant relationship between males' heterozygosity and females' breeding success(egg number,fertility rate,hatchability).This result indicates that females have no choice about good gene of males.(3)Genetic compatibility hypothesis:there was a weak linear relationship between genetic compatibility(Microsatellite and MHC)and females' breeding success.Genetic compatibility indicators among all of the possible female-male pairs,dominant males from mating groups against their mates,and males from mating pairs against their mates displayed non-significant differences,indicating that females have no choice about genetic compatibility of males.There was a positive relationships between males'heterozygosity and the number of offsprings;and between parental genetic compatibility and offspring genetic heterozygosity showed significant correlation,indicating that breeding success has affected parental genetic compatibility in the R.omeimontis.Actually,females of this species had no choice on mates with above characteristics.We considered that,females may have no chances to compare all possible spouses at the same time;otherwise they may lose the reproduction chances.On the other hand,male biased OSR in the breeding grounds may lead females to be easily intercepted by other males before they access to the high quality males.In conclusion,the Badagongshan Mountain geographic population of the R.omeimontis revealeda mating system with polygynandry.Under this mating system,males' breeding success was improved,while the reproductive success of females was did not.In addition,there was intense male-male competition influenced by males' mating rate,body size and by the OSR.Weak females' choice was observed about males' genetic relatedness,good genes and genetic compatibility.Thus,we consider male-male competition for females and males' pursue of higher reproductive success as the main drivers of this mating system.