Dueling frogs: do male green tree frogs (Hyla cinerea) eavesdrop on and assess nearby calling competitors?

Individuals produce advertisement signals with intended purposes and targets. However, these signals can be received by “eavesdroppers,” who may extract information from them and alter their behavior according to the extracted information. In anuran systems, males congregate at breeding sites to produce advertisement calls to attract receptive females and fend off rival males. Both sexes directly assess these calls in dyadic encounters and make decisions based on the call’s characteristics, e.g., frequency. What is unknown is whether bystander males eavesdrop on these same calls to inform their future competitive decisions. Here, we examined whether male green tree frogs (Hyla cinerea) eavesdrop on competing males, assess their competitor’s call frequency, and respond accordingly. We exposed males to playbacks of two competing males that varied in call frequency—high, average, low—and quantified latency to call, time spent calling, and number of calling bouts. We found that males had reduced latency to call and called more when eavesdropping low-frequency competition, but not average or high-frequency competition. Focal male size also influenced how they responded, with larger males being more responsive than smaller males. Our results indicate that male green tree frogs are capable of eavesdropping on nearby male calls and produce behavioral responses accordingly. Further, it appears males are able to alternate between assessment strategies dependent upon the frequency of the eavesdropped competition. These findings indicate that males not only directly assess an opponent’s call in dyadic encounters, but also indirectly through eavesdropping.