Can you hear me now? A review of signal transmission and experimental evidence for the acoustic adaptation hypothesis

ABSTRACT The Acoustic Adaptation Hypothesis (AAH) posits that animal acoustic signals used in long-range communication should be adapted to transmit well within the habitats in which they evolved. However, comparative studies of signal form indicate mixed support for predictions of the AAH. Several studies have employed experimental playback approaches to testing signal transmission which can complement comparative studies. Here, we summarise these experimental playback tests of the AAH in birds, mammals, insects, and anurans, we describe the methodologies used in these tests, and we assess the evidence for habitat-specific signal degradation and species-specific acoustic fidelity (i.e. whether signals propagate best in native versus foreign habitats). Experimental evidence, like comparative evidence, varies across habitats and taxa. Although transmission properties consistently differed by habitat, with closed habitats degrading signals more than open habitats, animal signals were not always adapted to propagate best within their native habitats. Researchers felt they had convincing evidence for species-specific acoustic fidelity in less than half of the 67 reviewed studies, with the most support found for birds and the least for anurans. We discuss potential explanations for differences within and between habitats and taxa and conclude with suggestions for standardised methodology and areas of future research.