Size-assortative mating of Buergeria buergeri (Anura: Rhacophoridae): breeding strategies for balancing fertilisation rate and swimming ability

Buergeria buergeri is a stream-breeding rhacophorid frog. The sexual dimorphism in body size is significant in this species, which might be advantageous for its movement within the stream in an amplected state. However, the fertilisation rate of egg masses decreases as the body size difference between both sexes in the amplectant pair increases because the distance between the cloacae increases. In this study, we examined the mating pattern of B. buergeri and the relationship between female/male SVL ratio in the amplectant pair and fertilisation rate, and the swimming ability within the stream. As a result, when the female/male SVL ratio was less than 1.45, the fertilisation rate reached a plateau (nearly 100%). However, the fertilisation rate decreased as the female/male SVL ratio increased when this ratio was greater than 1.45. Moreover, a significant positive correlation was observed between the female/male SVL ratio and swimming ability. These results suggest that there was a trade-off between fertilisation rate and swimming ability via the female/male SVL ratio. In the field, we observed a significant positive correlation between male and female SVLs in 16 amplectant pairs, with a mean female/male SVL ratio of 1.44. Our experimental results suggest that when the female/male SVL ratio is 1.44, the fertilisation rate is close to 100%, and the swimming ability can be maintained at an intermediate level. Therefore, B. buergeri adopts a size-assortative mating and maintains the female/male SVL ratio of approximately 1.44 to balance both fertilisation rate and swimming ability, thereby increasing reproductive success.