Michael J. Owren
Dept. of Psych., Reed College, 3203 SE Woodstock Blvd., Portland, OR 97202
Robert M. Seyfarth
Dorothy L. Cheney
Univ. of Pennsylvania, Philadelphia, PA 19104
Free-ranging baboons (Papio cynocephalus ursinus) produce brief, tonal ``grunt'' calls in a variety of social circumstances. These calls bear a striking resemblance to human vowels, being composed of regular, apparently harmonic energy forming several prominent energy peaks below 5 kHz. This resemblance was tested by examining 216 grunt calls from nine adult female baboons for clues to the mechanisms apparently involved in their production. Comparisons of spacing of purported harmonics in grunt frequency spectra to the results of other pitch-extraction methods strongly supported the supposition that these calls are produced using regular vocal fold vibration at approximately the same rates found in adult humans. Examination of individual grunt waveforms showed variation in apparent modes of vibration corresponding to human phonation in the modal and pulse registers. LPC analyses revealed an overall spectral pattern approximating that of the vowel (schwa). However, formant positions were found to vary to a greater degree among calls produced by different individuals than between communication contexts. This outcome suggests a low degree of flexibility in articulator positioning. Overall, baboon grunts acoustic features appear to reflect the action of a source-filter production system in which vocal tract filtering mainly provides cues to caller identity.