Torrent frogs emit acoustic signals of a narrower spectral range in habitats with longer-lasting biotic background noise

Animal acoustic communication can be challenging under intense noise. Amphibians that inhabit noisy environments offer a great opportunity to study potential mechanisms that maximise signal transmission. Based on the hypothesis that frogs can adjust their calls to circumvent communication problems, we tested the effect of the duration of biotic sounds (intermittent acoustic signals from two syntopic species) on calling properties of a torrent frog species. We recorded 54 focal males at four localities with varied levels of synchronous noise. Duration of noise for each recording was obtained by summing the duration of calls emitted by two closely related species. Using linear mixed-effect models with air temperature and body size as co-factors, we found that focal males emitted calls at a narrower frequency bandwidth when they were exposed to longer durations of heterospecific signals. We suggest that males reduce the frequency bandwidth of their calls to minimise the masking effect of the background noise and to amplify auditory induction for the receiver individuals. This is the first study reporting that biotic noise can trigger changes in frequency bandwidth in frogs, shedding light on the role of the environment in shaping animal behaviour.