Social Mating System Of O’ Mei Treefrog(Rhacophorus Omeimontis)

Social mating system is different mating patterns in various populations based on association among individuals.These different patterns are formed through long evolution and have unique evolutionary mechanism. In this study, we selected the O’mei treefrog in Baoxing County of Sichuan province for our example. Observation and field data were collected through two breeding seasons in2010and2011. Through the observation and statistical analysis, we found that males in breeding season would stayed in ponds for at most25days, and more than four times mating happened. The number of males in clutches with a single female are ranged from one to eight. Indicated that the social mating system of O’mei treefrog is promiscuity. In order to explore the evolutionary mechanism of this mating system, we analyzed from four aspects. First, we tried to find the association among nightly number of males and females at the breeding site and operational sex ratios(OSR, number of receptive females/number of males) and number of males in clutch. The results showed that both the sex ratios in breeding site and the OSR were male-biased, and we found that with the increase of OSR, the number of males in clutches significant increased. Second, through analyzed the data of males’ body size in mate, we found that there’s no relationship between body size and mating frequency or clutch position, imply that mating is random. Third, when we tried to find whether the number of males in clutches have any impact on reproductive success (include clutch size, fertilization success and hatching success) by using linear regression analysis and discovered that there’s no significant correlation, indicate that female frogs mate with more males can not increase their reproductive success. Fourth, when we tried to find whether the number of males in clutches have any impact on offspring adaptability and discovered that along with the increase in males number in clutches, the survival of offspring decreased, but the average head length of tadpoles at the first metamorphosis increased. Because those datas about tadpole were got from samples in clutches for training, whether it is a valid information remains to be further validation. From those analysis we believe that the reason for this promiscuity in O’mei treefrog is the extreme male-biased OSR. Another possible reason is "trading-up hypothesis",which suggested that the females didn’t make a choice with multi-mating or single-mating, speculated that female frogs would trade-up:choose polyandry can increase the opportunity to encounter males with high quality that improve the adaptability of offspring, but also increase the opportunity to clutch with more males with poor quality.