Ecological Studies of Bufo bufo formosus BOULENGER : (VI) Differences of Postmetamorphic Growth Rate and the Sexual Maturity between Sexes in the Natural Population

1. In the National Park for Nature Study, differences of the postmetamorphic growth rate and the sexual maturity between sexes in the natural population of the Japanese common toad, Bufo bufo jormosus, were analysed by based on the data obtained from May in 1973 to March in 1980. 2. Very small toads metamorphosed are observed around ponds in late May to early June every year. Young male toads grew faster than young females, but also reached a large size during the first full year after the transformation. In August of the second year, the body size of males reached as large as toads which emerged in the coming spring. Females, however, came to grow rapidly from summer of the second year and became large than males in autumn of the year. Difference of the mouth size between sexes was very significant after October of the second year, and of the body weight was significant after July of the third year, too. The body weight of females increased till autumn of the fourth year, whereas the growth rates of males were very low after summer of the third year. The season of growth, as indicated by an increase in the body size measurements of metamorphased toads, is primarily from mid May to early October, with most of it during June to August. 3. There is evidence that male toads reach sexual maturity a year earlier than females, that is, some males come to emerge in spring of the third year (about 21 months after transformation), while some females do the fourth year (about 33 months after transformation). 4. It is concluded that the difference of the body size between sexes in the breeding populations results from the differential growth rates of the sexes. 5. The observed sex ratios of two to four males to one female in breeding populations. Are probably explained by females breeding less frequently than males as the expenditure on egg production is possibly more affected by environmental conditions than is sperm production, and females may not breed as often as males in consequence.