Wild indri lemurs widen mouths as song pitch rises

Wild indri lemurs open their mouths more widely as the pitch of their songs rises, according to an observational study of 83 recordings of 19 animals in Madagascar. Researchers from the University of Warwick and the University of Turin combined field audio with markerless motion tracking to document the relationship between vocal frequency and jaw movement. The findings provide the first direct evidence that wild indris coordinate mouth opening with song frequency, showing a vocal-control pattern that resembles the widening of the mouth used by trained human singers to produce high notes.

The indri, or Indri indri, is the world’s only known singing lemur. Found only in Madagascar’s rainforests, it produces complex songs that carry over long distances and are associated with strengthening bonds within social groups and warning neighboring groups. The researchers compared changes in the positions of the upper and lower lips with changes in the fundamental frequency of the animals’ vocalizations. Across the recordings, higher-pitched songs were accompanied by wider mouth openings, revealing frame-by-frame coordination between an indri’s voice and jaw movement.

“Every time the pitch of an indri’s song rose, we could see its mouth opening wider, frame by frame,” said Chiara De Gregorio, corresponding author and an honorary research fellow at the University of Warwick. In human singing, widening the mouth as pitch rises can align the resonant frequencies of the vocal tract with harmonics of the voice and amplify the sound; researchers described this process as formant tuning. The indri results suggest that a comparable form of vocal-tract tuning may occur during the animals’ songs, although the observational analysis does not establish the precise acoustic mechanism or its benefit to the animals.

First author Filippo Carugati of the University of Turin said applying markerless pose estimation to footage recorded in the wild allowed the researchers to connect visible movements of the face and mouth directly to changes in pitch. “It let us study how the face and mouth move while an animal is singing, almost frame by frame, and connect those movements directly to changes in pitch,” he said. The method was necessary because audio alone records how high a call sounds but does not capture whether the singer is changing its jaw and lip position at the same moment. The study examined naturally occurring songs in the wild rather than experimentally manipulating mouth shape or vocal frequency.

The similarity between indri and human singing is evolutionarily notable because the lineages last shared a common ancestor approximately 74 million years ago. The release presents the resemblance as an example of convergent evolution, meaning that similar solutions can arise independently in distantly related lineages. That evolutionary interpretation is not demonstrated by the video analysis alone, because the study did not reconstruct the history of coordinated singing behavior. Instead, its direct result is the measured association between rising vocal pitch and wider mouth opening in 19 wild indris. The findings were published in PLOS Computational Biology in the paper “Dynamics of jaw kinematics and fundamental frequency suggest vocal tract tuning in the indris’ song.”

Subject of Research: Coordination between mouth opening and song pitch in wild indri lemurs

Article Title: Lemur uses opera singer's trick to belt out forest songs

Article References: Lemur uses opera singer's trick to belt out forest songs. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

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Keywords: indri lemur, song pitch, mouth opening, vocal-tract tuning, animal communication, markerless motion tracking, wildlife bioacoustics, convergent evolution, Madagascar, Lemur, uses, opera