The role of temporal cues in rhesus monkey vocal recognition: orienting asymmetries to reversed calls.

An understanding of the acoustic cues that animals use to categorize their vocalizations has important implications for the way we design neuroethological investigations of auditory function. Compared to other species, we know relatively little about the kinds of acoustic features used by nonhuman primates to recognize and categorize vocalizations. To further our understanding, this study explores the role of temporal features in recognition of conspecific vocalizations by rhesus macaques (Macaca mulatta). Experiments were designed to extend an earlier set of findings showing that adult rhesus macaques selectively turn with the right ear leading when a conspecific vocalization is played 180 degrees behind them, but turn left or not at all when a non-conspecific signal is played. Two call types were used as stimuli: shrill barks (alarm call) and harmonic arches (food call). We found that for normal calls, rhesus macaques turned to the right - supporting earlier findings - but for time- reversed shrill barks and harmonic arches, subjects oriented to the left. These results suggest that for at least a subset of calls, rhesus macaques use temporal cues to recognize conspecific vocal signals. The asymmetry of the behavioral response, and the corresponding asymmetry in the time-amplitude waveform, may have important implications for studies of temporal coding in the primate auditory system.