Human music often has a natural rhythm to it, and the roots of that rhythm might stretch back to the ancestors we shared with indris, a type of lemur
25 October 2021
Eerie wails pierce the morning calm of lowland rainforest in eastern Madagascar and are soon joined by more. The haunting cries are the song of the indri – a critically endangered, metre-tall lemur. Now research suggests the primate’s calls have a great deal in common with human music.
Indris (Indri indri) sing to communicate with other family groups, or to locate and reunite with family members, says Chiara De Gregorio at the University of Turin in Italy. But the degree of rhythm in this soulful keening and the calls of other primates isn’t well understood. So De Gregorio and her colleagues started dissecting the indri’s song.
The researchers recorded songs from 20 different indri groups over 12 years in Madagascar’s rainforests and analysed the timing of the notes.
They found that the indri used two distinct rhythm categories: 1:1, where the notes are evenly spaced like a metronome, and 1:2, where the gap between one note is twice as long as the previous one. Such rhythm categories – or “categorical rhythms” – are universal in human music.
“This is the first evidence of the presence of a typical trait of human music in another mammal,” says De Gregorio. She adds that just two bird species – thrush nightingales (Luscinia luscinia) and zebra finches (Taeniopygia guttata) – are known to show this trait when they sing, but each displays just one categorical rhythm.
“Instead, indris share with human music two different rhythms, which makes their songs quite complex and articulated,” she says.
Finding these universal musical characteristics in indris may indicate that “intrinsic musical properties are more deeply rooted in the primate lineage than previously thought”, says De Gregorio.
Alternatively, considering that lemurs and humans last shared a common ancestor about 77 million years ago, categorical rhythms could have evolved independently twice among primates.
Since the study focused purely on the timing properties of the calls, the rhythms’ significance for communication is unclear. But these rhythms may generally play a role in song coordination and social bonding, write the authors.
Simon Townsend at the University of Zurich in Switzerland, who wasn’t not involved with this study, says the study “beautifully illustrates” the value of using comparisons with other species to find out what features of music and rhythm are, and are not, unique to humans.
Alexandre Celma-Miralles at Aarhus University in Denmark would like to see similar work on gibbons, which also sing and are – being apes – much closer relations to humans.
De Gregorio and her team plan to investigate whether indris are born using the rhythm categories or if they learn them. Though the primates “still have so much to teach us”, they are facing a bleak future.
“Every attempt to build captive populations has failed and their habitat is vanishing at a very fast rate,” she says.
Journal reference: Current Biology, DOI: 10.1016/j.cub.2021.09.032
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