Frequency code: orofacial correlates of fundamental frequency

Area of Science:

• Primatology
• Bioacoustics
• Ethology

Background:

• Human smiling involves lip retraction, correlating with higher vocal frequencies.
• The frequency code hypothesis proposes a link between vocalizations and facial expressions.

Purpose of the Study:

• To test the frequency code hypothesis in chimpanzees under naturalistic conditions.
• To examine the relationship between chimpanzee vocalizations and orofacial movements.

Main Methods:

• Field recordings of spontaneous chimpanzee vocalizations.
• Digital spectral, spectrographic, and cinegraphic analysis of vocalizations and orofacial movements.
• Measurement of fundamental frequency (Fo), formant frequencies, and facial opening.

Main Results:

• A significant decrease in Fo was observed during transitions from submissive screams to aggressive barks.
• Positive correlations were found between teeth and lip opening distances and sound duration, Fo, frequency rise, and tempo.
• Trivariate regression supported a link between orofacial opening and Fo.

Conclusions:

• Chimpanzee vocal and orofacial behaviors align with the frequency code hypothesis.
• Facial expressions and vocalizations appear coordinated, potentially for appeasement signals.
• Findings suggest the frequency code hypothesis has cross-species generality.