The Computative Biology, provides the first direct evidence that wild indri coordinate the amount of time they open their mouths with the frequency of the song they are playing.
The results show that this movement is not random, but changes regularly with increasing pitch, in a pattern consistent with what is known as "vocal canal tuning," a process in which the animal changes the shape of the mouth and throat so that the vocal canal cavities help to enhance certain frequencies.
Lemur sounds detected
Filippo Carogatti, a biological scientist at the University of Turin and the study's lead author, says that using facial tracking technology from wild footage allowed researchers to examine not only the sounds the indri makes, but also the accompanying body movements. The method enabled them to track facial and mouth movements almost frame by frame and then directly correlate them with changes in pitch, something that cannot be gleaned from audio recordings alone.
The researchers collected 83 video clips of 19 wild indri, including 11 females and eight males, belonging to family groups that were tracked during field expeditions that extended from 2008 to 2024 in eastern Madagascar, particularly in the Maromizaha Forest and Mitsengou Reserve.
Filippo explains to Al Jazeera Net that the Andri use long and complex songs to communicate between members of the group and to advertise the area in which the group lives to neighboring groups.
Because measuring the mouth movement of a singing animal among branches and at great heights is not an easy task, the team used a deep learning program to automatically identify different points on the ingre'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 correlated this movement with the fundamental frequency of the sound at that same moment.
After cleaning up the recordings and excluding shots in which leaves or branches obscured parts of the face, the researchers arrived at 3,598 analyzable shots, taken from 23 video clips of 12 animals.
Next, they compared the degree of mouth opening to the fundamental frequency of the sound, which is the physical measure associated with what we perceive as pitch rising or falling.
A vocal trick similar to what singers do.
Filippo believes that this technique used by the indri is similar to what happens with trained singers, especially when performing high notes, when the singer changes the shape of the mouth and vocal tract so that the resonant frequencies better match the tone produced by the larynx.
The researcher says: "But this similarity does not mean that the Indri learned or consciously used the same human singing technique; rather, it suggests that two species separated by tens of millions of years may have independently arrived at a similar vocal solution."
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