Density-dependent mating tactic expression is linked to stress hormone in Woodhouse's toad

The social environment can alter mating tactic expression through modulatory effects on the endocrine system. In anuran amphibians (frogs and toads), males often adopt noncalling mating tactics in response to increased chorus density, but the underlying physiological factors mediating this response are poorly understood. Recent models suggest that density-dependent expression of alternative mating tactics in anurans is associated with a cascade of hormonal events: high chorus density is expected to promote an increase in circulating androgen levels that elicits an increase in vocal effort, a reduction in energy reserves, and elevations in circulating corticosterone (CORT) levels that, in turn, suppress vocalization. Here, we show that chorus density was positively correlated with the proportion of males adopting a noncalling “satellite” mating tactic in explosive-breeding Woodhouse's toad (Bufo woodhousii). We also show that chorus density was positively correlated with circulating CORT levels in calling males, thereby linking density-dependent mating tactic expression to a hormone that has previously been shown to elicit noncalling mating tactics in this species. Chorus density, however, was not correlated with circulating androgen levels, vocal effort, or body condition. Our results suggest that satellite tactic expression may not be readily traceable to energetic constraints associated with calling behavior—a leading prediction of current models.