Close-range vocal signals elicit a stress response in male green treefrogs: resolution of an androgen-based conflict

Abstract

Male courtship signals often stimulate the production of sex steroids in both female and male receivers. Such effects benefit signallers by increasing receptivity in females, but impose costs on signallers by promoting sexual behaviour and aggression in male competitors. To resolve this androgen-based conflict, males should use strategies that suppress sex steroid production in rival males. In green treefrogs, Hyla cinerea, chorus sounds (i.e. advertisement calls from aggregates of males) are known to stimulate androgen production in receiver males. Here, I examined whether males of this species counter these effects by eliciting an endocrine stress response in male conspecifics during close-range vocal interactions. I show that corticosterone (CORT) levels were higher in males that lost vocal contests in natural choruses compared to contest winners and nonaggressive males. Testosterone levels were also lower in contest losers compared to nonaggressive males, but not contest winners; dihydrotesterone levels did not differ among the three groups. Aggressive and advertisement calls were then broadcast to males in an experiment that simulated close-range vocal communication. Aggressive calls rapidly (45 min) elicited an increase in CORT and a reduction in androgens in receivers. Advertisement calls did not elicit an increase in CORT, but CORT levels were sustained relative to controls exposed to silence and were accompanied by a reduction in androgens in small males. Endocrine responses to acoustic signals in this species thus vary depending upon context, call type and size of signal receivers. Signallers benefit from eliciting CORT production in competitors because elevated CORT suppresses vocalization.