In many anurans, males produce loud advertisement calls that mediate both female choice and male spacing (Ryan 2001 Gerhardt and Huber 2002). However, the selection pressures influencing signaling strategies may differ when environmental noise originates primarily from conspecifics compared with other types of noise because conspecific noise contains a high degree of frequency and temporal overlap between the signals and noise (Gerhardt and Huber 2002).Īnuran amphibians are excellent model systems to study multimodal communication in noisy environments. Hebets and Papaj ( 2005) suggested that multiple signal components may evolve when they increase the signal content (content-based hypothesis), facilitate the perception of each other (inter-signal interaction hypothesis), or enhance signal transmission for instance in noisy environments (efficacy-based hypothesis).Īcoustic signal detection and discrimination can be constrained by abiotic and/or biotic noise sources such as waterfalls or vocalizing hetero- or conspecifics, thus favoring the evolution of complex signaling strategies (Gerhardt and Klump 1988 Schwartz and Gerhardt 1989 Brumm and Slabbekoorn 2005 Gordon and Uetz 2012), which could facilitate faster and more accurate detection by receivers (Rowe 1999 Otovic and Partan 2009). Based on their assumed information content, multimodal signals have been classified as redundant (all signal components elicit an equivalent response in the receiver) or non-redundant (signal components elicit a different response in the receiver). In so-called fixed-composite signals (Smith 1977 Partan and Marler 2005), signal components occur always together. Complex signals can consist of multiple components in a single modality (e.g., acoustic, visual, or chemical) or in multiple sensory modalities (multimodal communication) with components being presented either together or independently (Partan and Marler 1999 Partan and Marler 2005). 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.
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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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