Locatability of vocal signals in Old World monkeys: Design features for the communication of position.

Abstract

The ability of Old World monkeys (Macaca) to resolve the spatial origin of monkey vocalizations was psychophysically determined. Monkeys were trained through positive reinforcement operant conditioning procedures to report the detection of a change in the horizontal coordinate (azimuth) of the signal''s location. Each monkey was tested with an array of signals, including natural calls, low-pass filtered components of calls and simulated elements of calls. The acuity of localization of different macaque vocalizations exceeded a 4-fold range (from less than 4.degree.-15.degree.). Both harsh cells and frequency-modulated clear calls were readily localized. The acuity of localization was unaffected by the harmonic content of clear calls. Signal locatability is 1 of the dimensions that have influenced the structure of macaque vocal signals.