In the human voice, lip retraction as in smiling can be associated with high fundamental (F₀) 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 F₀. 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 F₀ 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, F₀, frequency rise and tempo. These positive correlates and a trivariate regression between orofacial opening and F₀ give support to the coordination of these appeasing facial and vocal features proposed in the frequency code hypothesis.

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