Frequency code: orofacial correlates of fundamental frequency.

In the human voice, lip retraction as in smiling can be associated with high fundamental (Fo) and formant frequencies. The aim of this study was to investigate under naturalistic conditions the cross-species generality of the frequency code hypothesis and related orofacial correlates in Fo. Digital spectral, spectrographic, and cinegraphic measurements were made of spontaneous chimpanzee vocalizations and orofacial movements produced and recorded in the field. A significant decline in Fo was found in submissive-scream-to-aggressive-waahbark transitions as predicted by the frequency code. Teeth and lip opening distances during sounds were positively correlated with their duration, Fo, frequency rise and tempo. These positive correlates and a trivariate regression between orofacial opening and Fo give support to the coordination of these appeasing facial and vocal features proposed in the frequency code hypothesis.