Hormonal and social correlates of courtship signal quality and behaviour in male green treefrogs

Abstract

Circulating hormone levels often vary considerably among males competing for mates, but how this variation affects the signalling phenotype is poorly understood. In the green treefrog, Hyla cinerea, males competing for mates engage in vocal contests that increase circulating levels of the adrenal glucocorticoid corticosterone (CORT) and decrease androgens in contest losers, who characteristically adopt a noncalling ‘satellite’ mating tactic. We previously showed that CORT administration to calling males reduces vocal effort and increases the probability of satellite behaviour during simulated territorial intrusions, suggesting that elevated CORT compromises vocal quality and decreases the propensity to call in a context-dependent manner. However, the extent to which these effects reflect variation in vocal behaviour of males in natural choruses is unclear. Here, we examined how hormone levels, social context and other potential determinants of vocalization, including body size and condition, relate to vocal attributes and behaviour in calling males and in associated satellites after removal of calling ‘host’ males. Results revealed that satellites had higher CORT and lower androgens and were smaller and in poorer condition than host males. Host removal resulted in vocalization in 74% of satellites. These males invested less effort in vocalization than hosts, and CORT level best explained these differences. The remaining 26% of satellites did not vocalize after host removal and had lower androgens than satellites that vocalized. Results provide support for context-dependent effects (i.e. host presence/absence) of elevated CORT on the probability of vocalization and link high CORT to reduced vocal quality. In contrast, low androgens were associated with a low probability of vocalization regardless of social context, and androgen level was a poor predictor of vocal attributes. Our results link high CORT and low androgens to variation in signalling quality and behaviour that potentially reduces the probability of attracting females.