Environmental Influences on the Structure of Primate Vocalizations

Abstract

Under natural conditions vocal signals are embedded in background noises, and are degraded by the action of diffraction, reflection, frequency-dependent absorption, and refraction. A fuller appreciation of the evolution of communication systems may be achieved by developing an understanding of the impediments to communication which occur in the natural environment, and investigating possible adaptations in the structure of either productive and receptive systems which may act to counter these impediments. The idea that characteristics of vocal signals have evolved a form acting to counter ecological impediments to communication has received increasing attention over the past 2 decades. Although most investigators have addressed questions pertinent to the structure of bird song (Ryan and Brenowitz 1985), other investigators have focused on the vocalizations of diverse groups of vertebrates including bats (Griffm 1971), frogs (Ryan 1986), whales (payne and Webb 1971; Payne and Guinee 1983), ground squirrels (Brown and Schwagmeyer 1984), and nonhuman primates (p.M. Waser and M.S. Waser 1977; P.M. Waser and Brown 1984, 1986). Selection may shape not only signal form (where more reliable or longer-distance transmission of information is advantageous), but also perceptual capabilities (where more complete or accurate reception of information is beneficial). In addition, selection may shape other aspects of communicative behavior, such as the timing of vocaliza-tions or the choice of broadcast sites. In this chapter we briefly review some examples of possible environmental effects on perceptual capabilities and calling behavior; we then summarize the results of our studies on the acoustic ecology of equatorial African primate habitats, and fmally describe in more detail our ongoing work focused on how habitat acoustics influence the fidelity of calls broadcast in natural environments. 1 Habitat Effects on Perceptual Abilities and Calling Behavior Brown and P.M. Waser (1984) found that arboreal, forest-living blue monkeys are up to 20 dB more sensitive to low-frequency sounds (signals 500 Hz and lower) than are semiterrestrial, more open-country rhesus monkeys. This difference in sensitivity paral