Stress hormone is implicated in satellite-caller associations and sexual selection in the Great Plains toad.

The effects of androgens on male-typical traits suggest that variation among males in circulating levels can play a major role in sexual selection. We examined whether variation in vocal attractiveness is attributable to differences in androgen levels among Great Plains toads (Bufo cognatus). We found that noncalling "satellite" males practicing an alternative mating tactic were more likely to associate with males producing long calls. However, callers with satellites did not have higher androgen levels than callers without satellites. Rather, callers with satellites had significantly lower corticosterone (CORT) levels than callers without satellites. A CORT manipulation experiment suggested that differences in calls for males with and without satellites were related to differences in CORT levels. Furthermore, there was a negative correlation between CORT level and call duration within most nights of chorus activity. However, the correlation was weak for the pooled data (across all nights), suggesting that local environmental and/or social factors also affect call duration. Last, we show that females preferred broadcast calls of longer duration, characteristic of males with satellites and low CORT. These results imply that satellites optimize their reproductive success by associating with males producing long calls. However, this association should negatively affect the fitness of attractive callers.

Elevated corticosterone levels elicit non-calling mating tactics in male toads independently of changes in circulating androgens.

Competition among males for a limited number of females may result in the expression of condition-dependent alternative mating tactics. In such cases, decision rules mediating mating tactic expression are likely to be influenced by a male's external as well as internal environment. For example, experimental studies with anuran amphibians (frogs and toads) indicate that changes in the social-acoustic environment alter the probability that an individual adopts a calling versus non-calling "satellite" mating tactic. However, there is considerable variation in the behavioral responses of individuals in such studies, suggesting that physiological differences among individuals play an important role in tactic expression. For instance, recent models predict that natural elevation in adrenal glucocorticoids during vocal production alters androgen production and/or neural activity to mediate transitions between reproductive tactic expression in anuran amphibians. Using corticosteroid injections, we show that elevation in circulating corticosterone levels significantly increase the probability that free-ranging male Great Plains toads (Bufo cognatus) and Woodhouse's toads (B. woodhousii) adopt a non-calling satellite tactic. Corticosterone-induced behavioral transitions occurred rapidly (<1 h) and independently of fluctuations in circulating androgen levels, suggesting a direct effect of glucocorticoids on brain centers controlling vocal production. We discuss our findings in the context of behavioral studies that alter the social-acoustic environment to examine its influence on tactic expression.