Page 46 - Spring 2018
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 Anthropocene Soundscape Ecology
Evolutionary Change
Acoustic signals can change over evolutionary time if varia- tion in signal design is genetically determined, and it impacts lifetime reproductive success and, as a consequence, the genetic contribution of the signaler to the next generation (Slabbekoorn, 2013). Birds that sing songs that are detected easily or that convey acoustic details clearly should benefit by surviving longer or attracting more or better mates earlier or with less effort than birds that do not sing as well. This is the interpretation, for example, that helps explain the simi- larities in alarm calls of many songbirds, which are high- pitched long notes that fade in and out smoothly. In this way, callers warn each other about danger in the air. At the same time, the acoustic design of the signals is such that they are hard to hear and localize, making it difficult for predators to find the signaler. Sounds that are easier to detect and localize would obviously affect survival negatively.
The loud crows of roosters (Gallus domesticus), the elaborate songs of nightingales (Luscinia megarhynchos), and the var- ied trills of canaries (Serinus canaria) are all also thought to be the result of evolutionary adaptation (ten Cate, 2004). In the case of these advertisement signals, the benefits are posi- tively correlated with being heard and found. These acous- tic signals have become louder, more complex, and more broadband and rapid as a result of sexual selection. This is both for the perceptual salience of features that signal cer- tain qualities of the sender and for standing out well against the background noise of the species-specific habitat.
The environment can affect signal evolution in various ways (Wiley and Richards, 1982). How well the message within an acoustic signal is transmitted from sender to receiver is a function of the propagation properties of the habitat and the perceptual interference from competing sounds (Figure 2). The competition for acoustic space depends on the tim- ing and spectra of the local sound sources and also propaga- tion properties, which may vary with height and angle of the typical transmission pathway through the vegetation (Slab- bekoorn, 2004). Low frequencies, for example, penetrate better through dense vegetation than do high frequencies, and all sounds do better through relatively open forest lay- ers. Furthermore, absorption and reflection by the ground may cause strong attenuation for low frequencies and sounds generated and received at low perches.
Environmental selection of bird song has led to many ex- amples of habitat-dependent acoustic variation between and within species. Bird species in dense rainforests often
Figure 2. Ambient-noise profiles vary geographically and reflect habi- tat types (Slabbekoorn 2004, 2017). Left: each habitat has a charac- teristic set of sound sources, which can be of abiotic (wind, rain, for- est streams) or biotic (vocal members of the local animal community) origin. Center: each habitat also has a characteristic filtering effect on sounds propagating from source to potential receivers. Top: dense vegetation favors low frequencies and slow, long-drawn-out notes; bot- tom: open urban habitat allows notes of wideband frequencies and favors brief, frequency-modulated notes that are relatively resistant to wind turbulence degradation. Right: power spectra illustrate examples of a diverse spectral pattern of a rain forest (top) and the simple urban pattern biased to low frequencies of traffic noise (bottom).
use relatively low-frequency sounds compared with species living in more open woodland spaces (Morton, 1975; Ryan and Brenowitz, 1985). The dense rainforest attenuates high frequencies more quickly than low frequencies and typically harbors rich and vocal animal communities that provide competition at high frequencies. Birds that favor breeding habitats close to noisy streams, with relatively high levels of low-frequency ambient noise, have been reported to sing relatively high-frequency songs. The spectral divergence between signal and noise renders the songs audible in such acoustically challenging environments (Figure 1).
A particularly nice and well-replicated example of noise-de- pendent song variation can be found in gray-breasted wood wrens (Henicorhina leucophrys) of South America (Dingle et al., 2008). These birds use low- and high-frequency notes when there are relatively low levels of acoustic interference across frequencies but refrain from using high frequencies when there is more high-frequency energy in the ambient noise (Figure 3). Consequently, wood wren populations living at high altitude in the Andes use a wide frequency range for singing because there is little competition from other sound-producing animals. Populations of low-altitude wood wrens, however, face severe competition for acoustic space due to the presence of a more diverse and abundant
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