Discrete or graded variation within rhesus monkey screams? Psychophysical experiments on classification
Publication Type: |
Journal Article |
Year of Publication: |
2002 |
Authors: |
C. G. Le Prell, M. D. Hauser, D. B. Moody |
Publication/Journal: |
Animal Behaviour |
Keywords: |
discrete variation, graded variation, rhesus monkey, screams, varation, vocalization |
ISBN: |
0003-3472 |
Abstract:
Gouzoules et al. (1984, Animal Behaviour,32, 182-193) presented evidence that semifree-ranging rhesus monkeys, Macaca mulatta, produce acoustically distinctive classes of scream vocalizations that carry different functional messages. To determine the perceptual validity of these vocal classes, we conducted psychophysical experiments on captive rhesus monkeys. We trained two monkeys to maintain contact with a metal response cylinder during presentation of nontarget stimuli, and to release the cylinder to report detection of target stimuli. For one subject, tonal screams served as nontarget stimuli and arched screams served as targets. These conditions were reversed for a second subject. Once natural exemplars were correctly discriminated, both subjects correctly generalized to synthetic targets. Variability in responses to nontarget stimuli, however, suggested that scream categories were not well defined following training. This result suggests that rhesus monkeys do not perceive categorical distinctions between arched and tonal screams, at least under the testing conditions implemented. Rather, our results provide evidence for a graded category. To explore which acoustic features are most important for classifying novel exemplars as tonal or arched screams, we ran several follow-up experiments with novel scream exemplars. Generalization trials suggested that variation in rate of frequency change, maximum frequency of the fundamental and harmonic structure may be important to the discrimination of screams.