Type of Resources
The Acoustic Adaptation Hypothesis (AAH) posits that animal acoustic signals meant for long-range communication should be adapted to transmit well within the habitats in which they evolved. However, comparative studies investigating predictions of the AAH relating to signal form indicate that support for the hypothesis is not universal. Several studies have employed experimental playback approaches to testing signal transmission which can provide complimentary analyses to comparative studies of signal form in the field. Here, I first summarize these experimental playback tests of the AAH in birds, mammals, insects, and anurans, describing the methodologies used in these tests, and assessing the evidence for 1) habitat-specific signal degradation and 2) species-specific acoustic fidelity (i.e. whether signals propagate best in native versus foreign habitats). Experimental evidence matched comparative evidence for the AAH in that it varies across habitats and taxa. Although transmission properties were frequently shown to vary across habitats with closed habitats degrading signals more than open habitats, animal signals were not always adapted to propagate best within their native habitats. I discuss potential explanations for differences in support within and between habitats and taxa and conclude with suggestions for standardized methodology along with areas of future research. To further clarify the relationships between acoustic signal propagation and habitat structure, I then quantified the songs and preferred habitats of seven species of Troglytidae wrens, hypothesizing that components describing the most variability in song form and habitat structure correlate in accordance with predictions of the AAH. I found that songs and habitats are generally unique by species, although overlap exists between Rock Wrens (Salpinctes obsoletus), Canyon Wrens (Catherpes mexicanus), and Cactus Wrens (Campylorhynchus brunneicapillus) as well as Bewick’s Wrens (Thryomanes bewickii), House Wrens (Troglodytes aedon), and Marsh Wrens (Cistothorus palustris), while Pacific Wren (Troglodytes pacificus) songs and habitats were entirely unique. Songs could be described primarily by frequency structure and repetition rate and habitats by humidity and horizontal density as well as ground structure, and I found that a wren’s song frequency structure significantly negatively correlates with the aridity and openness of its habitat. Finally, I tested the degradation of each song in each habitat, assessing blur ratio (BR), a measure of energy loss, signal-to-noise ratio (SNR), a measure of signal separation from noise, and excess attenuation (EA), representing clarity loss. I predicted that (i) the songs of each species will degrade less in native habitat when compared to several other foreign habitats; (ii) within species, the songs sourced near to the study sites will transmit better than those sourced from geographically distant locations, and (iii) within a species in its native habitat, locally-sourced song will transmit better in the spring rather than the summer. I found highly species-dependent support for these predictions, with some species having improved propagation by at least one measure compared to several others, while others had worse. Further, each measure of signal degradation was significantly correlated with habitat structure metrics in opposing ways. Together, these results indicate that it may be physically impossible for a species to maximize transmission by all measures in any given habitat, and that adherence to the predictions of the AAH likely depends on ecological factors such as territory size, nesting ecology, mating system, and functional communication needs. My study is the first to test the AAH by reciprocal playback in more than four habitat types, and the first to examine seasonal and geographic components in more than one habitat type. Further, it is the first to assess signal structure and propagation using several quantitative metrics of habitat structure, making it one of the most robust tests of the AAH to date. Determining whether and which songbirds adapt to the pressures of their acoustic environments can help us understand the evolution of signal diversity and species differentiation, and may aid in determination of the behavioral consequences of both natural and anthropogenic habitat alteration.