Changes in plasma levels of gonadotropins and sex steroids in the toad, Bufo japonicus, in association with behavior during the breeding season.

Plasma levels of gonadotropins and sex steroids in Bufo japonicus were surveyed at different behavioral stages during the breeding period in March. All hormone levels were lower in torpid toads, which were found underground 1 week before the start of the breeding migration, than in active toads in the breeding season, although the levels were higher than those in the other months. LH and FSH levels in toads migrating toward a breeding pond were still low or only slightly elevated. Gonadotropin levels increased markedly when toads arrived at the pond and were highest for individuals actually in the pond. These LH and FSH surges occurred in both sexes, although peak levels were higher in females. Gonadotropin levels decreased in individuals of both sexes when they left the pond after breeding ("landed" individuals). LH and FSH levels in solitary males were significantly lower than those in amplexing males. In females, there was no significant difference in the level of any hormone between amplexing and solitary individuals. Circulating testosterone increased to the highest level in migrating males; this elevated level was maintained after arrival in the pond. DHT in males showed a pattern similar to that of testosterone. Both androgens decreased to the lowest level in landed males. In females, estradiol was dominant during migration, while progesterone became dominant shortly after arriving near or in the pond. This progesterone surge coincided with the LH surge. Androgen may stimulate amplexus behavior in male toads, and then amplexus seems to induce the LH surge that is necessary for spermiation. In females, however, amplexus seems to have no direct correlation with the gonadotropin surge, and the LH surge may induce the progesterone surge which causes ovulation.