THE ROLE OF VOCAL-TRACT FILTERING IN IDENTITY CUEING IN RHESUS-MONKEY(MACACA-MULATTA) VOCALIZATIONS

The importance of individual identity and kinship has been demonstrate d in the social behavior of many nonhuman primates, with some evidence suggesting that individually distinctive acoustic features are presen t in their vocalizations as well. In order to systematically test whet her acoustic cues to identity are reliably present across the vocal re pertoire of rhesus monkeys (Macaca mulatta), we examined coos, grunts, and noisy screams produced by adult females of two free-ranging group s. First, acoustic analyses were used to characterize spectral pattern ing, the fundamental frequency, and temporal characteristics of these three distinct call types. Vocalizations were then classified by calle r identity, based on discriminant function analyses. Results showed th at coos (rich, harmonically structured sounds) were markedly more dist inctive by caller than were either grunts or noisy screams, and that s pectral-patterning measures related to vocal tract filtering effects w ere the most reliable markers of individual identity. Grunts (pulsed, noisy calls) were classified at lower, but above-chance rates and spec tral patterning cues were again critical in this sorting. Noisy scream s (continuous, broadband noise bursts that could include a high-freque ncy, periodic component) could not be reliably sorted by caller. Playb ack experiments conducted with the screams showed no response differen ces when listening animals heard vocalizations produced by kin or nonk in individuals. This result was strikingly different from the correspo nding outcome of a previous test with coo calls, but consistent with t he acoustic analysis. Implications of these findings for vocal product ion mechanisms in nonhuman primates and previous studies of rhesus mon key vocalizations are discussed. (C) 1998 Acoustical Society of Americ a.