During the last decade, it has become clear that communication signals in many taxa are more complex than previously thought (Hebets and Papaj 2005). To explain evolutionary patterns in animal communication, it is critical to understand the mechanisms of signal production, the conditions under which signals are produced, and how signals are perceived by receivers (Bradbury and Vehrencamp 2011 Brumm and Slabbekoorn 2005 Miller and Bee 2012). We suggest that the vocal sac acts as a visual cue and improves detection and discrimination of acoustic signals by making them more salient to receivers amidst complex biotic background noise. Multimodal stimuli elicited greater response from males and triggered significantly more visual signal responses than unimodal stimuli. Our results suggest that abiotic noise of the stream does not constrain signal detection, but males are faced with acoustic interference and masking from conspecific chorus noise. In the present study, we measured sound pressure levels of concurrently calling males of the Small Torrent Frog ( Micrixalus saxicola) and used acoustic playbacks and an inflatable balloon mimicking a vocal sac to investigate male responses to controlled unimodal (acoustic) and multimodal (acoustic and visual) dynamic stimuli in the frogs’ natural habitat. The acoustic communication of anuran amphibians can be masked by the presence of environmental background noise, and multimodal displays may enhance receiver detection in complex acoustic environments. Many animals use multimodal (both visual and acoustic) components in courtship signals.
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