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Short‐term elevations in circulating stress hormones (glucocorticoids) often mediate reduced investment in reproductive behaviour in favour of self‐maintenance and survival, but the significance of these effects in sexual selection and communication remain largely unexplored. Here we examine the acute stress response in the context of intra‐ and intersexual selection in the green treefrog Hyla cinerea. Males of this species produce agonistic acoustic signals that stimulate elevations in the stress hormone corticosterone (CORT) in rival males, suggesting a central role for CORT in male–male competition. We hypothesized that elevated CORT levels in losers of vocal contests mediate an increase in the energy allocated towards self‐maintenance and survival that compromises investment in courtship signalling, resulting in reduced attractiveness to females. Consistent with this hypothesis, we show that CORT administration to calling males, simulating CORT production in vocal contest losers, caused a rapid reduction in the duration of advertisement calls and vocal effort. Elevated CORT also increased the propensity to produce aggressive calls during simulated territorial intrusions that was followed by an increased probability of non‐calling “satellite” mating behaviour. This suite of CORT effects on vocal behaviour are known to decrease attractiveness to females and were not related to a decrease in androgen level or differences in body condition in CORT‐ vs. saline‐injected males. Our results suggest that CORT production during intraspecific vocal contests compromises male attractiveness. Male vocalizations may therefore reliably indicate recent contest outcomes and/or the stress status of males, implicating CORT as a mediator of honest signalling. We propose that the effects of agonistic acoustic signals on CORT production in this species are maintained by CORT‐mediated life‐history trade‐offs; decreased conspicuous signalling in high stress‐responsive males potentially increases survival by reducing the likelihood of escalated contests with dominant males and detection by predators. Adoption of alternative non‐calling satellite mating tactics may contribute to maintaining individual variation in stress responsiveness by increasing the chances that competitively inferior males acquire mates. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13054/suppinfo is available for this article.

Amphibians inhabiting montane riparian zones in the Neotropics are particularly vulnerable to decline, but the reasons are poorly understood. Because environmental contaminants, endocrine disruption, and pathogens often figure prominently in amphibian declines it is imperative that we understand how these factors are potentially interrelated to affect montane populations. One possibility is that increased precipitation associated with global warming promotes the deposition of contaminants in montane regions. Increased exposure to contaminants, in turn, potentially elicits chronic elevations in circulating stress hormones that could contribute to montane population declines by compromising resistance to pathogens and/or production of sex steroids regulating reproduction. Here, we test this hypothesis by examining contaminant levels, stress and sex steroid levels, and nematode abundances in male drab treefrogs, Smilisca sordida, from lowland and montane populations in Costa Rica. We found no evidence that montane populations were more likely to possess contaminants (i.e., organochlorine, organophosphate and carbamate pesticides or benzidine and chlorophenoxy herbicides) than lowland populations. We also found no evidence of elevational differences in circulating levels of the stress hormone corticosterone, estradiol or progesterone. However, montane populations possessed lower androgen levels, hosted more nematode species, and had higher nematode abundances than lowland populations. Although these results suggested that nematodes contributed to lower androgens in montane populations, we were unable to detect a significant inverse relationship between nematode abundance and androgen level. Our results suggest that montane populations of this species are not at greater risk of exposure to contaminants or chronic stress, but implicate nematodes and compromised sex steroid levels as potential threats to montane populations.

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