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.