Opera singers and shrieking lemurs share vocalization strategies

With piercing, emergency siren-like calls, the indri (Indri indri) is the world’s only known singing lemur. The primates found in Madagascar’s rainforest swing through the trees, making these sounds that contain a surprisingly human element. 

To hit those high notes, indris appear to deploy a technique similar to human opera singers. The lemurs coordinate their mouth opening with their songs—the higher the pitch of the song, the wider they open their mouth to sing it. A study published today in the journal PLOS Computational Biology details the findings.

The song of the indri

Hitting those notes

The indri’s complex, haunting songs can carry across vast distances. They can strengthen the bonds within their social group and warn neighbouring groups to keep their distance. 

By comparison, human singers can also make noises that travel. Singers are trained to widen their mouths as a song’s pitch rises, to align with the resonant frequencies of the vocal tract, with the harmonics of the voice and amplifying the sound. This process is called formant tuning, and it may serve a similar purpose for the indri. It could mean the difference between a song that carries and one that doesn’t, which is crucial for animals that need to communicate signs of danger through a thick rainforest. 

“Every time the pitch of an indri’s song rose, we could see its mouth opening wider, frame by frame,” primatologist Chiara De Gregorio, a study co-author and research fellow at the University of Warwick in England, said in a statement. “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.”

Analyzing technique

In the study, the team analyzed 83 video recordings of 19 wild indris, combining field audio with markerless motion-tracking technology. They followed the upper and lower lip position frame by frame, comparing the changes in mouth opening with changes in vocal frequency. They saw that the indris opened their mouths wider to project higher-pitched vocalizations 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,” added co-author Filippo Carugati, a naturalist at the University of Turin in Italy. “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.”

Humans and indris last shared a common ancestor who lived roughly 74 million years ago. According to the team, this means the similarities in vocal production between both species likely occurred as the result of convergent evolution, where the same solution arrived independently across deep evolutionary time.