Vocal tract limitations on the vowel repertoires of rhesus monkey and other nonhuman primates.

The vowel repertoire of a rhesus monkey (Macaca mulatta) was explored by means of a computer program that calculated formant frequencies from the area function of the animal's supralaryngeal vocal tract, which was systematically varied within the limits imposed by anatomical constraints. The resulting vowels were compared with those of humans and with recorded vocalizations of nonhuman primates. The computer model indicates that the acoustic "vowel space" of a rhesus monkey is quite restricted compared to that of the human. This limitation results from the lack of a pharyngeal region that can change its cross-sectional area. These animals thus lack the output mechanism necessary for production of human speech. Man's speech output mechanism is apparently species-specific.