74 million years apart, same technique: Lemurs use formant tuning like trained sopranos

An international research team has found the first direct evidence that a wild animal independently evolved a vocal technique used by trained human soprano singers — and it is a lemur living in the rainforests of Madagascar.

The study, published in PLOS Computational Biology by researchers at the University of Warwick and the University of Turin, shows that indris (Indri indri) — the world’s only known singing primate other than humans — coordinate their mouth opening with the pitch of their songs in exactly the way human singers are trained to do: the higher the note, the wider the mouth opens.

Formant tuning in the forest canopy

In human vocal performance, the technique is called formant tuning — widening the mouth as pitch rises to align the resonant frequencies of the vocal tract with the harmonics of the voice. The effect amplifies the sound. It is a core technique taught in classical vocal training and one that sopranos deploy specifically to project high notes across a concert hall without a microphone.

For the indri, a large black-and-white lemur found only in Madagascar’s eastern rainforests, the researchers believe it serves an equivalent purpose. These animals produce complex, haunting songs that travel across vast distances through dense forest — strengthening social bonds within family groups and warning neighboring groups to maintain distance.

In that environment, a singing technique that maximizes the carrying power of high-pitched notes could be the difference between a call that reaches its intended audience and one that dies in the undergrowth.

“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 the research team measured it

The findings rest on an analysis of 83 video recordings of 19 wild indris, combining field audio with markerless motion-tracking technology — a computational method that estimates the position of anatomical landmarks from standard video footage without requiring markers to be physically attached to the animal.

The team tracked the position of the upper and lower lip in each recording frame by frame, comparing changes in mouth opening directly to changes in vocal frequency. The correlation held consistently: as pitch went up, the mouth opened wider, and the relationship was measurable and statistically robust.

“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 Dr. Filippo Carugati of the 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 of parallel evolution

The most striking implication of the finding is evolutionary. Humans and indris last shared a common ancestor approximately 74 million years ago. The two lineages have been evolving entirely independently for that entire span of time, meaning neither inherited the formant tuning technique from the other.

The only explanation is convergent evolution — the same functional solution arriving independently in two separate lineages separated by tens of millions of years, because the underlying acoustic physics of projecting high-pitched sound through an obstructed environment pushed both toward the same answer.

That conclusion sits alongside prior findings from the same research group, which established that indris share other rhythmic and melodic features with human music — including beat, tempo, and interval patterns — suggesting that some of the structural properties humans associate with music may have deeper biological roots than previously understood, roots that extend far outside the primate family tree humans occupy.