Title: The acoustic adaptation hypothesis across terrestrial vertebrates: a meta-analysis
Abstract: ABSTRACT Acoustic communication plays a prominent role in various ecological and evolutionary processes involving social interactions. The shape of acoustic signals is thought to be influenced not only by the interaction between a signaller and a receiver but also by the acoustic characteristics of the environment through which the signal is transmitted, which is the essence of the so-called “acoustic adaptation hypothesis” (AAH). Specifically, the AAH posits that habitat structure affects frequency and temporal parameters of acoustic signals emitted by a signaller depending on their acoustic degradation properties. To date, this hypothesis has received mixed results, with the level of support depending on the taxonomic group under study and the methodology used. We conducted a systematic literature search of empirical studies testing for an effect of vegetation structure on acoustic signalling and assessed the generality of the AAH using a meta-analytic approach based on 371 effect sizes from 75 studies and 57 taxa encompassing birds, mammals and amphibians. Overall, our results do not provide consistent evidence for the AAH, neither in within-species comparisons (suggesting no overall phenotypically plastic response of acoustic signalling to habitat structure) nor in among-species comparisons (suggesting no overall evolutionary response). However, when considering birds only, we found a weak support for the AAH in within-species comparisons, which was mainly driven by studies that measured frequency bandwidth, suggesting that this variable may exhibit a phenotypically plastic response related to habitat. For among-species comparisons in birds, we also found support for the AAH, but this effect was not significant after excluding comparative studies that did not account for phylogenetic non-independence. Collectively, our synthesis does not support a universal role of vegetation structure in the evolution of acoustic communication. We highlight the need for more empirical work on currently under-studied taxa such as amphibians, mammals, and insects. Furthermore, we outline a framework for forthcoming research on the AAH. Specifically, we advocate for a more detailed and quantitative characterization of habitats allowing to identify frequencies with the highest detection probability and to determine if frequencies with greater detection distance are preferentially used. Finally, we stress that research efforts should focus on signals in which an increased distance of transmission is beneficial to senders and/or receivers.