In this paper we present the results of an interspecific study on the androgen levels of several species of Old World tropical frogs. These data are used to test three hypotheses: (1) in unpredictable, aseasonal environments with opportunistic breeding, gonadal hormones have a permissive rather than an activating role in the expression of mating behavior (Crews and Moore, 1986), (2) a behaviorally induced androgen response is present in opportunistic aseasonal species as well as in temperate seasonal breeders (Crews and Moore, 1986; Wingfield et al., 1990), and (3) because maintenance of high plasma testosterone levels has an associated energetic and/or fitness cost (Moore and Marler, 1987; Wingfield et al., 1990), continuously breeding frogs have lower testosterone levels than seasonal, discontinuous breeders. Androgen and corticosterone levels were found to vary significantly among species exhibiting the same mating behavior, providing support for the hypothesis that hormones have a permissive role. Differences in androgen level were correlated with (1) variation in level of male-male aggression, (2) relative density of male frogs, and (3) call rate and power output of male vocalization. These results indicate that a behaviorally induced androgen response is present in opportunistic, aseasonal breeders. Furthermore, tropical, aseasonal, opportunistic breeding frogs appeared to have lower androgen levels, generally, than confamilial temperate, seasonal breeders. This difference is consistent with the hypothesis that maintenance of high plasma testosterone levels is energetically expensive.
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