The role of vocal tract filtering in identity cueing in rhesus monkey (Macaca mulatta) vocalizations

Abstract

The importance of individual identity and kinship has been demonstrated in the social behavior of many nonhuman primates, with some evidence suggesting that individually distinctive acoustic features are present in their vocalizations as well. In order to systematically test whether acoustic cues to identity are reliably present across the vocal repertoire of rhesus monkeys (Macaca mulatta), we examined coos, grunts, and noisy screams produced by adult females of two free-ranging groups. First, acoustic analyses were used to characterize spectral patterning, the fundamental frequency, and temporal characteristics of these three distinct call types. Vocalizations were then classified by caller identity, based on discriminant function analyses. Results showed that coos (rich, harmonically structured sounds) were markedly more distinctive by caller than were either grunts or noisy screams, and that spectral-patterning measures related to vocal tract filtering effects were the most reliable markers of individual identity. Grunts (pulsed, noisy calls) were classified at lower, but above-chance rates and spectral patterning cues were again critical in this sorting. Noisy screams (continuous, broadband noise bursts that could include a high-frequency, periodic component) could not be reliably sorted by caller. Playback experiments conducted with the screams showed no response differences when listening animals heard vocalizations produced by kin or nonkin individuals. This result was strikingly different from the corresponding outcome of a previous test with coo calls, but consistent with the acoustic analysis. Implications of these findings for vocal production mechanisms in nonhuman primates and previous studies of rhesus monkey vocalizations are discussed.

Voice processing in human and non-human primates

“…In non-human primates, there is now ample evidence in several species that the laryngeal sound source is also subject to spectral patterning by the supralaryngeal vocal tract, resulting in formants clearly visible on sonograms (Owren et al 1997; figure 1). Thus, monkeys and apes use vocalizations that combine source due to vocal fold movement with filtering by vocal tract-comparable to our vowels (Rendall et al 1998). For example, baboon grunts are very similar to our neutral, central vowel pronounced with a relaxed vocal tract (Owren et al 1997).…”

“…Recent evidence for individual distinctiveness in vocalizations was obtained in the squirrel monkey (Boinski & Mitchell 1997), in the baboon (Owren et al 1997), the rhesus monkey (Rendall et al 1996(Rendall et al , 1998Owren & Rendall 2003), the Japanese macaque (Ceugniet & Izumi 2004a) and the cotton-top tamarin (Weiss et al 2001). Owren et al (1997) showed that the spectral energy peak (formant) patterning varied with caller identity in baboon grunts, and constituted the strongest grouping variable.…”

“…Slight differences in physical morphology between individuals of a same species have the potential to yield consistent acoustic differences. As discussed by Rendall et al (1998), three main sources of individual variation that can lead to acoustic differences in a vocalization are as follows.…”

“…It may constitute an adaptation of extreme importance in facilitating intra-group social cohesion (Rendall et al 1996). Indeed, complex social interactions of most primates call for a good ability to discriminate between other group members from vocal cues alone, to extract kin relations, or even to explicitly recognize each other (Rendall et al 1996(Rendall et al , 1998.…”

Vocal production mechanisms in a non-human primate: morphological data and a model

“…1a. The open ellipses represent ranges for formant data for male (B_m) and female (B_f) chacma baboon grunts (redrawn from Owren et al, 1997;Rendall et al, 2004), and for human laughter (man: H_m, woman: H_f) (redrawn from Bachorowski and Owren, 2001), for rhesus monkey coo calls (R_c) and grunt calls (R_g) (estimated after data from Fitch 1997;Rendall et al, 1998) and for male Red Deer roars (D) (estimated after data from Fitch and Reby, 2001). The straight line, as in Fig.…”