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Twin vocal folds as a novel evolutionary adaptation for vocal communications in lemurs.

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Nakamura, Kanta 
Kanaya, Mayuka 
Matsushima, Daisuke 
Dunn, Jacob C 
Hirabayashi, Hideki 


Primates have varied vocal repertoires to communicate with conspecifics and sometimes other species. The larynx has a central role in vocal source generation, where a pair of vocal folds vibrates to modify the air flow. Here, we show that Madagascan lemurs have a unique additional pair of folds in the vestibular region, parallel to the vocal folds. The additional fold has a rigid body of a vocal muscle branch and it is covered by a stratified squamous epithelium, equal to those of the vocal fold. Such anatomical features support the hypothesis that it also vibrates in a manner like the vibrations that occur in the vocal folds. To examine the acoustic function of the two pairs of folds, we made a silicone compound model to demonstrate that they can simultaneously vibrate to lower the fundamental frequency and increase vocal efficiency. Similar acoustic effects are achieved using different features of the larynx for the other primates, e.g., by vibrating multiple sets of ventricular folds in several species and further by an evolutionary modification of enlarged larynx in howler monkeys. Our multidisciplinary approaches found that these functions were acquired through a unique evolutionary adaptation of the twin vocal folds in Madagascan lemurs.


Acknowledgements: We thank Yuta Shintaku, Toshifumi Udono, Sumiko Tsubouchi and Atsuko Kataoka for preparing samples. We thank JMC (#2018017), FCZ, WRC and EHUB for providing samples. Some specimens were provided through the Great Ape Information Network, Japan. We appreciate Naoto Toyoda for his help in statistical analysis.


Animals, Vocal Cords, Lemur, Laryngeal Muscles, Vibration, Acoustics

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Springer Science and Business Media LLC
Japan Society for the Promotion of Science (20K11875, 19H01002)
Mitsubishi Foundation (Research Grant in the Natural Sciences (202310032))