{ "abstract": "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 (fo) 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 fo values extracted from corresponding audio tracks. A generalized linear mixed model revealed a significant positive relationship between vertical mouth opening and fo, 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 fo, 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. Author summary: In many animals, including humans, the shape of the vocal tract, and particularly how wide the mouth is opened, can influence the resonance properties of the voice, potentially amplifying sound and increasing its transmission range. This phenomenon, known as formant tuning, is well-documented in professional singers, but evidence in non-human primates remains scarce. Here, we investigated whether indris, a species of lemur known for their complex songs, adjust their mouth movements to influence the acoustic properties of their calls. Using video recordings of wild individuals and a pose estimation approach to track facial movements, we measured how widely indris open their mouths while singing and compared these movements with the fundamental frequency of their vocalizations. We found that indris open their mouths more when producing higher-pitched sounds. This coordinated adjustment suggests that they may actively modify their vocal tract to enhance sound transmission. Our study provides new evidence that singing primates can fine-tune their vocal production and introduces a method to study these mechanisms in wild animals using combined video and audio data.", "abstract_shown": true, "affiliations": [ "Department of Life Sciences and Systems Biology, Università di Torino, Turin, Italy", "Department of Psychology, University of Warwick, Coventry, United Kingdom", "Fondazione Zoom, Cumiana, Turin, Italy", "Groupe d’Étude et de la Recherche sur les Primates de Madagascar (GERP), Antananarivo, Madagascar", "Institut Supérieur de Technologie Régionale (ISTR), Manakara, Madagascar" ], "article_type": "Research Article", "arxiv": null, "author_total": 13, "authors": [ { "affs": [ 0 ], "name": "Filippo Carugati" }, { "affs": [ 1 ], "name": "Olivier Friard" }, { "affs": [ 2 ], "name": "Chiara De Gregorio" }, { "affs": [ 3 ], "name": "Daria Valente" }, { "affs": [ 4 ], "name": "Anna Zanoli" }, { "name": "Valeria Ferrario" }, { "name": "Lia Laffi" }, { "name": "Silvia Leonetti" }, { "name": "Valeria Torti" }, { "name": "Elisa Protopapa" }, { "name": "Longondraza Miaretsoa" }, { "name": "Cristina Giacoma" }, { "name": "Marco Gamba" } ], "bibtex": "https://research.draconitic.com/paper/31mqf75dv6pc5.bib", "cited_by": 0, "doi": "10.1371/journal.pcbi.1014781", "fulltext_url": "https://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.1014781", "id": "31mqf75dv6pc5", "issns": [ "1553-7358" ], "landing_url": "https://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.1014781", "language": null, "licence": "CC-BY-4.0", "licence_label": "CC BY 4.0", "licence_source": "journal", "licence_url": "https://creativecommons.org/licenses/by/4.0/", "metadata_licence": null, "native_subjects": [ [ "openalex", "1309|Animal Vocal Communication and Behavior" ], [ "plos", "/Biology and life sciences/Anatomy/Digestive system/Mouth" ], [ "plos", "/Biology and life sciences/Anatomy/Digestive system/Mouth/Lips" ], [ "plos", "/Biology and life sciences/Anatomy/Head/Face" ], [ "plos", "/Biology and life sciences/Anatomy/Head/Face/Lips" ], [ "plos", "/Biology and life sciences/Anatomy/Head/Jaw" ], [ "plos", "/Biology and life sciences/Organisms/Eukaryota/Animals/Vertebrates/Amniotes/Mammals/Primates" ], [ "plos", "/Biology and life sciences/Zoology/Animals/Vertebrates/Amniotes/Mammals/Primates" ], [ "plos", "/Medicine and health sciences/Anatomy/Digestive system/Mouth" ], [ "plos", "/Medicine and health sciences/Anatomy/Digestive system/Mouth/Lips" ], [ "plos", "/Medicine and health sciences/Anatomy/Head/Face" ], [ "plos", "/Medicine and health sciences/Anatomy/Head/Face/Lips" ], [ "plos", "/Medicine and health sciences/Anatomy/Head/Jaw" ], [ "plos", "/Physical sciences/Physics/Acoustics" ], [ "plos", "/Physical sciences/Physics/Resonance" ], [ "plos", "/Physical sciences/Physics/Resonance/Resonance frequency" ] ], "openalex": "W7213921422", "pdf_url": "https://journals.plos.org/ploscompbiol/article/file?id=10.1371/journal.pcbi.1014781&type=printable", "pmcid": null, "pmid": null, "published": "2026-09-21", "publisher": "Public Library of Science", "ris": "https://research.draconitic.com/paper/31mqf75dv6pc5.ris", "sources": [ { "first_seen": "2026-10-30T12:17:43Z", "native_id": "10.1371/journal.pcbi.1014781", "source": "plos", "url": "https://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.1014781" } ], "subjects": [ { "field": "medicine", "origin": "plos:anatomy", "subfield": "physiology" } ], "title": "Dynamics of jaw kinematics and fundamental frequency suggest vocal tract tuning in the indris’ song", "title_text": "Dynamics of jaw kinematics and fundamental frequency suggest vocal tract tuning in the indris’ song", "type": "peer-reviewed", "updated_at": "2026-10-30T12:17:43Z", "url": "https://research.draconitic.com/paper/31mqf75dv6pc5", "venue": "PLOS Computational Biology", "versions": [] }