Researchers from University of Warwick and University of Turin studied the world’s only known singing lemur species and found that, in a remarkable feat of convergent evolution, the animal uses the same tricks to project its voice as a trained human soprano.
The findings are published in the journal PLOS Computational Biology.
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The indri (Indri indri) is endemic to the rainforests of Madagascar and listed as Critically Endangered on the IUCN Red List, threatened by habitat destruction, fragmentation and increasing levels of illegal hunting.
This rare lemur is known for producing complex, haunting songs that carry far through the thick layers of these unique ecosystems. The melodies are not just there to sound pretty – they are used to strengthen bonds within the lemurs’ social groups and warn others to keep their distance.
And, as it turns out, the way the indris project their vocalisations is not random at all.
Studying the indri
By analysing 83 video recordings of 19 wild indris and combining field audio with markerless motion-tracking technology (a technique used to record and analyse movement without attaching physical markers to the subject's body) the scientists found that the indris open their mouths wider to project higher-pitched vocalisations through the forest. Human singers use an almost identical technique.
If you know anything about singing, the process of formant tuning will ring a bell – human singers are trained to widen their mouth as pitch rises, aligning the frequencies of the vocal tract with the harmonics of the voice and amplifying the sound.
This phenomenon has also been described in various mammalian and avian species, but its role hasn't been closely investigated in primates before. It is particularly interesting in the context of singing, because singing is a rare behaviour in primates – it evolved independently in four unrelated taxa, and accounts for approximately 16 per cent of all primate species.
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The scientists tracked upper and lower lip position of the singing lemurs frame by frame and compared changes in mouth opening with changes in vocal frequency. Based on these findings, they think the indri use the process of formant tuning for a similar purpose humans do. And for an animal communicating through a thick and vast forest, this could be 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.”
Humans and indris last shared a common ancestor approximately 74 million years ago, so any similarities in vocal production between the two species are likely the result of convergent evolution. Put simply, this means it's likely humans and indris developed the technique independently.
Read the full findings here.
Top image: An indri (Indri indri) sitting on a branch. Maromizaha Forest, Madagascar. Critically Endangered. September 2018. Credit: Filippo Carugati
