A test of the Energetics-Hormone Vocalization model in the green treefrog

Abstract

Male courtship displays may be regulated by, and affect the production of, circulating hormones. The Energetics-Hormone Vocalization (EHV) model, for example, posits that interactions among chorusing male anuran amphibians stimulate androgen production that then mediates an increase in vocal effort. Increased vocal effort is expected to deplete energy reserves and increase glucocorticoid levels that, in turn, negatively affect androgen levels and vocalization. Androgen levels, glucocorticoid levels, and vocal effort are thus expected to increase across and within nights of chorus activity and should be positively correlated in calling males; energy reserves should decline temporally and be inversely related to glucocorticoid levels. We tested predictions of the EHV model in the green treefrog, Hyla cinerea. Consistent with the model, both testosterone and dihydrotestosterone levels increased across the breeding season in calling males. However, testosterone levels decreased and dihydrotestosterone levels did not change within nights of chorus activity, suggesting that chorusing behavior did not drive the seasonal elevation in androgens. Corticosterone (CORT) level remained relatively stable across the breeding season and decreased within nights of chorus activity, contrary to model predictions. Body condition, the proxy for energetic state, was inversely correlated with CORT level but discrepancies between model predictions and temporal patterns of CORT production arose because there was no evidence of a temporal decrease in body condition or increase in vocal effort. Moreover, androgen and CORT levels were not positively correlated with vocal effort. Additional ecological and physiological measures may be needed to support predictions of the EHV model.