Vocal tract length and formant frequency dispersion correlate with body size in rhesus macaques

Body weight, length, and vocal tract length were measured for 23 rhesus macaques (Macaca mulatta) of various sizes using radiographs and computer graphic techniques. Linear predictive coding analysis of tape-recorded threat vocalizations was used to determine vocal tract resonance frequencies (“formants”) for the same animals. A new acoustic variable is proposed, “formant dispersion,” which should theoretically depend upon vocal tract length. Formant dispersion is the averaged difference between successive formant frequencies, and was found to be closely tied to both vocal tract length and body size. Despite the common claim that voice fundamental frequency ${\mathrm{(F}}_0)$ provides an acoustic indication of body size, repeated investigations have failed to support such a relationship in many vertebrate species including humans. Formant dispersion, unlike voice pitch, is proposed to be a reliable predictor of body size in macaques, and probably many other species.