University of Warwick and University of Turin researchers find the world's only singing lemur opens its mouth wider on higher notes, just like a trained soprano.
We are often left open-mouthed in awe as an opera singer hits a soaring top note. Now, researchers at the University of Warwick have found that the indri, the world's only known singing lemur, appears to use a strikingly similar vocal technique to hit high notes.
The new study, published in PLOS Computational Biology, shows the first direct evidence that wild indris (Indri indri) coordinate their mouth opening with the frequency of their songs. In short: finding that the higher the pitch of the song, the wider the indris open their mouth to sing it.
The Opera Singer of the Forest
Found only in the rainforests of Madagascar, indris produce complex, haunting songs that carry across vast distances, strengthening bonds within their social group and warning neighbouring groups to keep their distance.
Human singers are trained to widen their mouths as pitch rises, aligning the resonant frequencies of the vocal tract with the harmonics of the voice and amplifying the sound. This process, known as formant tuning, may serve a similar purpose for the indri. For an animal communicating through thick forest, it could make the difference between a song that carries and one that doesn't.
"Every time the pitch of an indri's song rose, we could see its mouth opening wider, frame by frame," said Dr Chiara De Gregorio, corresponding author and Honorary Research Fellow at the University of Warwick. "It's the kind of fine-tuned coordination between voice and body that we usually associate with trained human singers, not with a lemur singing from the forest canopy."
How They Found It
The research team analysed 83 video recordings of 19 wild indris, combining field audio with markerless motion-tracking technology. They tracked upper and lower lip position frame by frame, comparing changes in mouth opening with changes in vocal frequency, finding the indris opened their mouths wider to project higher-pitched vocalisation through the forest.
"Applying markerless pose estimation to footage recorded in the wild allowed us to look not just at the sounds indris make, but at the physical movements behind them," said first author Filippo Carugati, Researcher, University of Turin. "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. That's something you simply can't capture by listening to the audio alone."
74 million Years Apart, Yet Remarkably Similar
Humans and indris last shared a common ancestor approximately 74 million years ago, meaning the similarities in vocal production between the two species are likely the result of convergent evolution, the same solution arrived at independently across deep evolutionary time.
