A new study has found that the indri, one of Madagascar’s best-known singing lemurs, opens its mouth wider as its pitch rises. The behaviour resembles a technique used by human singers, particularly opera singers, when performing high notes. The movement may help the indri strengthen its voice and carry it farther through dense forests.
Indris coordinate mouth opening with pitch
Published on 21 September in the journal PLOS Computational Biology, the study provides the first direct evidence that wild indris coordinate the extent of their mouth opening with the frequency of the songs they produce. The results showed that mouth movement is not random, but changes regularly as pitch rises, consistent with a process known as vocal-tract tuning.
In this process, the animal changes the shape of its mouth and throat in ways that allow cavities in the vocal tract to amplify certain frequencies. This means that sound depends not only on the note produced by the larynx, but is also affected by the shape taken by the vocal tract during singing and the resulting changes in resonance.
Facial-motion tracking reveals the singing mechanism
Filippo Carugati, a biological sciences researcher at the University of Turin and the study’s first author, said the team’s technique for tracking facial movement in video footage enabled it to study the sounds produced by indris alongside the physical movements accompanying them. The method allowed the researchers to track the face and mouth almost frame by frame, then directly link movement to changes in pitch—information that cannot be obtained from audio recordings alone.
Indri songs used for communication and territorial signalling
Carugati explained that indris use long, complex songs to communicate among members of the same group and to announce the territory occupied by the group to neighbouring groups. Analysing these songs in the wild requires monitoring animals that move and sing among branches high above the ground, making direct measurement of mouth movement difficult.
To overcome this difficulty, the team used a deep-learning programme to automatically identify a set of points on the indri’s face, including the lips, eyes, nose and corners of the mouth. The researchers then measured the distance between the upper and lower lips at each moment and linked the extent of mouth opening to the fundamental frequency of the sound recorded at the same moment.
After cleaning the recordings and excluding shots in which leaves or branches obscured parts of the face, the team obtained 3,598 frames suitable for analysis, taken from 23 video clips involving 12 animals. The researchers compared mouth opening with the fundamental frequency of the sound, the physical measure linked to what humans perceive as a higher or lower pitch.
Clear link between frequency and mouth opening?
The comparison showed a clear relationship between the two variables: as the fundamental frequency rose, the indri’s mouth opened wider. The researchers also found no significant difference between males and females in this relationship, suggesting that the pattern is not limited to one sex within the species.
Carugati said the indri’s behaviour resembles the technique used by trained singers, particularly when performing high notes. Singers change the shape of the mouth and vocal tract so that resonance frequencies align more closely with the note produced by the larynx.
But this resemblance does not mean that indris learned the same human singing technique or use it consciously. Rather, it suggests that two species separated by tens of millions of years may have independently arrived at a similar vocal solution.
The team recorded a link between wider mouth opening and higher sound frequency. The researchers’ simulation models also showed that this change can affect resonance in the vocal tract, but the findings do not prove that the animal deliberately alters the shape of its mouth to achieve a specific vocal outcome.
The study included technical limitations, as the measurements were based on two-dimensional recordings of movement occurring in three dimensions. This means that estimates of mouth opening may be affected by the angle of the head and the distance between the animal and the camera. Leaves, branches and changes in lighting also caused errors in some frames, requiring the researchers to inspect the data and exclude unreliable sections.
The team stressed that jaw opening is not the only factor capable of changing sound, as the lips, pharynx and other parts of the vocal tract may also contribute to shaping resonance. Some of these factors are difficult to test directly, particularly because the indri is critically endangered and cannot be subjected to extensive anatomical experiments.
According to Carugati, the study received no dedicated funding, and its authors declared that they had no conflicts of interest.
