Physical aspects of anurans constrain sound production, and noisy habitats pose a challenge to signal recognition and detection. Habitat acoustics impose selection on anuran calls within the phylogenetic and morphological constraints of the vocal apparatus of senders and the auditory system of receivers. Visual displays and alerting calls can be used as alternative or additional signal strategies to overcome these problems. In this study, we investigated sound pressure levels and spectral features of calls of the ranid rock-skipper frog Staurois latopalmatus, exclusively found at waterfalls of Bornean streams. A total of 176 calls and waterfall recordings were analyzed to characterize acoustic signals and environmental noise. To obtain information on possible signal adaptations, dominant frequency and snout–vent length of 75 ranid species were collected from the literature and compared to our findings. Distributions along acoustically characterized rapids and waterfalls within a 1-km long river transect showed that S. latopalmatus exclusively occurs in noisy habitats. Two different call types could be distinguished in S. latopalmatus: a short, single-note call and a long, multi-note call. Both calls had a lower sound pressure than the noise produced by waterfalls. The dominant frequency analyses revealed that the signal-to-noise ratio can be maximized within high frequency bands around 5 kHz. Correlations of frequency versus body size in ranids indicated that S. latopalmatus has higher call frequencies than predicted by body size, suggesting acoustic adaptation to environmental noise. We conclude that acoustic signal efficiency in environments with low-frequency dominated noise can only be attained through high frequency calls. The single-note call is interpreted as an alerting signal directing the receiver's attention to a subsequent visual signal. The multi-note call is interpreted as a graded aggressive call. We suggest that microhabitat characteristics represent strong selective pressures on the form of acoustic signals. Short calls with a narrow frequency band could reflect a trade-off among detectabilty, sound propagation and discrimination between individuals.
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Vol. 65 • No. 2