Modes of vocal variation in Sykes's monkey (Cercopithecus albogularis) squeals.

Acoustic and laryngographic measurements of Sykes's monkey (Cercopithecus albogularis) squeals showed that acoustic variation between exemplars was principally due to 4 underlying modes of modification: (Class I) variation in the transfer function produced by articulation, (Class II) variation in F0 produced by inflection, (Class III) variation in turbulence, and (Class IV) variation in the pattern of vibration of laryngeal tissue with large or small surface areas (polyphonic variation). These findings suggest that some modes of variation (Class I, II, and III calls) were due to phonatory and articulatory acts resembling, at a very general level, sound production mechanics used in speech. Acoustic variation in the polyphonic group (Class IV calls) is not evident in speech and was attributed to the possibility of independent and simultaneous vibrations of the vocal lip and the principal component of the vocal folds.