Data from: Female polyandry and size-assortative mating in isolated local populations of the Japanese common toad Bufo japonicus

In anurans, female polyandry under male harassment is distributed across taxa because of external aquatic fertilisation. According to the sexual selection theory, male–male competition for access to females is affected by the operational sex ratio (OSR) and population density. The Japanese common toad, Bufo japonicus, is widespread in mainland Japan, and like the European common toad, B. bufo, it engages in explosive breeding. We observed the breeding behaviour of B. japonicus in isolated local populations for three years in two breeding ponds with different population sizes and densities: large-low (L-pond) and small-high (S-pond). We analysed the relative polyandry ratio in egg clutches laid by females and estimated the size-assortative mating pattern to be an indicator of male–male competition in the two ponds. Both ponds tended to exhibit a size-assortative mating pattern; however, the frequency of polyandry was different in the two ponds (L-pond = 20% and S-pond = 90%). We also found that polyandry could occur without multiple amplexus with a high population density, i.e. eggs were often fertilised by free-swimming sperm in the small shallow pond. We propose that high female polyandry ratios without continuous male harassment are generated because of a male-biased OSR and a high population density in the small pond.

在无尾两栖类（Anurans）中，受雄性骚扰的一妻多夫制（polyandry）在多个类群中广泛分布，这与其体外水生受精的繁殖方式密切相关。根据性选择理论，雄性间争夺雌性交配权的竞争强度受有效操作性别比（operational sex ratio, OSR）与种群密度的调控。日本蟾蜍（Bufo japonicus）广泛分布于日本本岛，与欧洲普通蟾蜍（B. bufo）一样，均采用爆发式繁殖（explosive breeding）策略。本研究针对两处种群规模与密度存在显著差异的繁殖池塘——大种群低密度池塘（L-pond，简称L塘）与小种群高密度池塘（S-pond，简称S塘）——中的孤立局域种群，开展了为期三年的繁殖行为观测。研究人员分析了雌性所产卵团（egg clutches）中的相对一妻多夫比例，并以体型选型交配（size-assortative mating）模式作为雄性间竞争强度的评估指标，对两处池塘的情况分别进行了量化分析。两处池塘均呈现出体型选型交配的趋势，但一妻多夫制的发生频率存在显著差异：L塘为20%，S塘则高达90%。本研究同时发现，在高种群密度条件下，一妻多夫制可无需多次抱合（multiple amplexus）即可发生——在小型浅水环境中，卵团常由自由游动的精子完成受精。研究人员据此提出：小型池塘中偏雄的操作性别比与高种群密度，共同促成了无持续雄性骚扰背景下的高雌性一妻多夫制比例。