Scientists have discovered that the indri lemur of Madagascar opens its mouth wider when producing high notes, a vocal technique used by opera singers to make their voices travel farther.
In the dense forests of Madagascar, where vegetation obscures vision beyond a few meters, the indri rely on expressive songs to communicate.
Scientists found that these primates open their mouths wider when emitting high notes, which is consistent with an audio technique called "formant tuning" or "formant shifting," a technique in audio engineering used to modify the natural resonance of the human voice without changing the basic pitch.
“Imagine your mouth and throat as a tube,” says Filippo Carugati of the University of Turin, lead author of the study published in the journal PLOS Computational Biology . “When you open your mouth wider, you change the shape of the tube, and this changes the frequencies that resonate.” This modification enhances the power and range of the sound.
Scientists analyzed 83 video recordings of 19 Madagascan indri-brea lemurs, tracking facial features frame by frame with synchronized sound, and confirmed that mouth opening strongly predicts pitch.
Carogatti describes their songs as "between wailing and whistling: long, high notes, moving and almost sad to the human ear, but very structured and sung in duets with masculine and feminine phrases intertwined rather than simply overlapping."
These songs play a crucial territorial role, as groups announce their presence and avoid dangerous confrontations. “It’s a beautiful example of communication replacing conflict,” says Carogatti. The indri are also sometimes sung to signal reunions after separation. “They remind me of how humans find each other when they’re lost: one calls out ‘Marco!’ and the other replies ‘Polo!’” says study co-author Chiara De Gregorio.
The indri joins other animals that use similar mechanisms, such as howling wolves. But these animals are threatened with extinction due to deforestation, and De Gregorio says: "Their survival ultimately depends on protecting them in the wild."
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