Frame-by-Frame Lip Tracking Shows Indris Tune Like Sopranos

Researchers say markerless motion tracking of 19 wild lemurs provides the first direct evidence that indris open their mouths wider on higher notes.

According to the study, published in PLOS Computational Biology, researchers captured the first direct evidence that wild indris coordinate their mouth opening with the frequency of their songs. The team, citing albianews.com's report, analyzed 83 video recordings of 19 wild indris in Madagascar, combining field audio with markerless motion-tracking technology. Corresponding author Chiara De Gregorio, an honorary research fellow at the University of Warwick, said that every time the pitch of an indri's song rose, its mouth could be seen opening wider, frame by frame — the kind of coordination usually associated with trained human singers rather than a lemur in the forest canopy.

Human singers are trained to widen their mouths as pitch rises, a technique that aligns the resonant frequencies of the vocal tract with the harmonics of the voice and amplifies the sound. The new study suggests the indri uses a strikingly similar process, known as formant tuning, which could make the difference between a song that carries through thick Madagascan forest and one that does not. First author Filippo Carugati of the University of Turin told outlets that humans and indris last shared a common ancestor around 74 million years ago, meaning the similarity in vocal production is likely the result of convergent evolution — the same solution arrived at independently across deep evolutionary time.

The researchers tracked upper and lower lip position frame by frame, comparing changes in mouth opening with changes in vocal frequency, and found that the indris opened their mouths wider to project higher-pitched vocalizations through the forest. According to The Pryor Information Publication's account of the study, Carugati explained that applying markerless pose estimation to footage recorded in the wild let the team look not just at the sounds indris make but at the physical movements behind them, connecting facial movements directly to changes in pitch — something, he said, that simply cannot be captured by listening to the audio alone.

The indri, found only in the rainforests of Madagascar, is the world's only known singing lemur, producing complex, haunting songs that carry across vast distances. Scientists say the singing strengthens bonds within the animals' social groups and warns neighboring groups to keep their distance. For an animal communicating through dense forest, the outlets report, formant tuning could be the difference between a song that carries and one that does not — which is why this frame-by-frame confirmation matters for understanding how vocal signals survive in challenging acoustic environments, and why singing enthusiasts may find a lemur echoing the training of an opera soprano so compelling.