Dynamics of jaw kinematics and fundamental frequency suggest vocal tract tuning in the indris’ song

The ability to modify the supralaryngeal vocal tract, particularly by adjusting mouth opening, can align formants and harmonics, thereby enhancing vocal intensity. This phenomenon, known as formant tuning, has been described in human singers as well as in various mammalian and avian species. However, little is known about its role in singing primates. In this study, we investigated whether vertical mouth opening in the indri ( Indri indri ), the only known singing lemur, covaries with the fundamental frequency ( f o ) of its vocalizations, a parameter directly related to the first formant in this species. We analysed 83 video recordings from 19 wild indris, using a markerless pose estimation model based on DeepLabCut. We extracted frame-by-frame measurements of mouth opening, calculated as the pixel distance between predicted landmarks on the upper and lower lips. These measurements were paired with f o values extracted from corresponding audio tracks. A generalized linear mixed model revealed a significant positive relationship between vertical mouth opening and f o , indicating that vocalizations with higher frequency value are associated with greater jaw openings. These results provide the first direct, quantitative evidence that indris modulate their mouth configuration in coordination with f o , supporting the hypothesis that they can tune formants. This ability likely contributes to increased sound intensity and transmission range, enhancing long-distance communication in dense forest environments. Beyond improving our understanding of vocal production in singing primates, this study combines markerless pose estimation with frame-level acoustic analysis to quantify articulatory dynamics from field recordings, providing a multimodal approach for investigating the relationship between vocal tract configuration and acoustic output in animal communication.