Changes in hormone levels in relation to breeding behavior in male bullfrogs (Rana catesbeiana) at the individual and population levels.

Fluctuations in plasma androgen (testosterone and 5 alpha-dihydrotestosterone), corticosterone (B), and luteinizing hormone (LH) of male bullfrogs in central California were measured during the spring mating season. Androgen and LH levels generally increased in the population prior to the initiation of chorusing and establishment of territories, whereas plasma B peaked in a 2-week period at the start of heavy chorusing; this coincided with a transitory, 1 week decline in androgen. Individual males showed fluctuations in plasma androgen and LH levels throughout the breeding season, often within 1 day, but there was no clear correlation between changes in the two hormones. No time of day effect was apparent on any of the hormones. B and androgen levels were significantly but weakly correlated, (r = 0.35) but LH and androgen were not. Although the seasonal trend of increasing androgen corresponded with the start of intense chorusing (and presumably sexual activity) by the population, behavior of individuals and their circulating androgen levels did not correlate. After acquiring territories, males showed no overall trend of increased plasma androgen. Moreover, males that showed no vocal or territorial behavior had significantly higher androgen and lower B levels than calling males. Increased B levels suggest that territorial behavior and especially direct agonistic encounters represent stresses that could have an inhibitory effect on androgen secretion.