Relationships among steroid hormone levels, vocal effort and body condition in an explosive-breeding toad

Mutually reinforcing hormone–behaviour relationships may drive temporal changes in steroid hormone levels and vocal effort in anuran amphibians. Recent models propose that chorus activity stimulates the production of androgens in signallers and receivers, thereby mediating an increase in vocal effort. The energetic demands associated with high vocal effort should, in turn, elevate circulating corticosterone levels to promote the mobilization of energy reserves. High threshold corticosterone levels, however, are expected to negatively affect androgen levels and, hence, vocal effort. Steroid hormone levels and vocal effort are thus expected to be low at the onset of chorus activity, increase over sequential nights of calling until peak levels are reached, and subsequently decline; energy reserves should show inverse temporal trends. To test these predictions, we examined temporal changes in androgen (dihydrotestosterone and testosterone) levels, corticosterone levels, vocal effort and body condition in Woodhouse's toads, Bufo woodhousii. Contrary to model predictions, androgen levels, vocal effort and body condition remained relatively unchanged and circulating corticosterone levels tended to decrease within and across nights of chorus activity. Furthermore, although vocal effort was not correlated with circulating androgen levels, it was positively correlated with circulating corticosterone levels, supporting the prediction that high vocal effort promotes the production of corticosterone. Yet despite this correlation, poor body condition was not associated with high corticosterone levels, suggesting that factors other than energetic state influence corticosterone levels in B. woodhousii. We discuss alternative hypotheses regarding relationships among the measured parameters in this explosive-breeding species.