A lemur species called Indri indri displays, in its coordinated songs, the isochronous and 1:2 rhythm categories seen in human music, showing that such categories are not, among mammals, unique to our species, according to new research led by scientists from the Max Planck Institute for Psycholinguistics.
Indri indri. Image credit: Filippo Carugati.
The indri, also known as the babakoto or singing lemur, is one of the largest living lemurs.
The species has a length of about 64-72 cm and a weight of between 6 and 9.5 kg. It has a black and white coat and maintains an upright posture when climbing or clinging.
Like all lemurs, the indri is native to Madagascar, island country lying off the southeastern coast of Africa.
It makes loud, distinctive songs, which can last from 45 seconds to more than three minutes.
“There is longstanding interest in understanding how human musicality evolved, but musicality is not restricted to humans,” said Dr. Andrea Ravignani, a researcher in the Comparative Bioacoustics Group at the Max Planck Institute for Psycholinguistics.
“Looking for musical features in other species allows us to build an ‘evolutionary tree’ of musical traits, and understand how rhythm capacities originated and evolved in humans.”
Dr. Ravignani and colleagues wanted to know whether indri songs have categorical rhythm, a ‘rhythmic universal’ found across human musical cultures.
“Rhythm is categorical when intervals between sounds have exactly the same duration (1:1 rhythm) or doubled duration (1:2 rhythm),” they explained.
“This type of rhythm makes a song easily recognizable, even if it is sung at different speeds.”
Over a period of twelve years, the researchers visited the rainforest of Madagascar to collaborate with a local primate study group.
They recorded songs from twenty indri groups (39 animals), living in their natural habitat.
They found that indri songs had the classic rhythmic categories (both 1:1 and 1:2), as well as the typical ‘ritardando’ or slowing down found in several musical traditions.
Male and female songs had a different tempo but showed the same rhythm.
“This is the first evidence of a ‘rhythmic universal’ in a non-human mammal,” said Dr. Chiara de Gregorio, a researcher in the Department of Life Sciences and Systems Biology at the University of Turin.
“But why should another primate produce categorical ‘music-like’ rhythms? The ability may have evolved independently among ‘singing’ species, as the last common ancestor between humans and indri lived 77.5 million years ago.”
“Rhythm may make it easier to produce and process songs, or even to learn them.”
“Categorical rhythms are just one of the six universals that have been identified so far,” Dr. Ravignani said.
“We would like to look for evidence of others, including an underlying ‘repetitive’ beat and a hierarchical organization of beats — in indri and other species.”
The results appear in the journal Current Biology.
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Chiara De Gregorio et al. 2021. Categorical rhythms in a singing primate. Current Biology 31 (20): R1379-R1380; doi: 10.1016/j.cub.2021.09.032
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