Forget indie bands, meet the indri bands

Rhythm is a rare trait in mammals, but it turns out that it’s not unique to humans.

An international team of researchers has spent 12 years studying Madagascar’s critically endangered Indri indri lemur – one of the world’s few ‘singing’ primates – trying to determine if these songsters also have a sense of rhythm.

“There is long-standing interest in understanding how human musicality evolved, but musicality is not restricted to humans”, says Andrea Ravignani, one of the study’s senior investigators, from the Max Planck Institute for Psycholinguistics in the Netherlands.

“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.”

To achieve their musical goal, the team studied 20 indri groups, with a total of 39 animals, recording their songs and analysing them for 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). This type of rhythm makes a song easily recognisable, even if it’s sung at different speeds.

Members of indri family groups tend to sing together, harmonising duets and choruses. The research team 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.

“A precise rhythmic pattern may play an important role in signal’s coordination – indris’ songs can be uttered as a duet or choruses, and a predictive rhythmic pattern may benefit the turn-taking process,” says first author Chiara De Gregorio.

“A vocal emission characterised by the presence of rhythmic categories might also be easier to acquire or learn than a less organised one, in a similar way that for us is easier to remember a song that has a simple, defined pattern – such as a pop song – than a more variable one (as a complex jazz tune).”

While studies have shown rhythm in bird species, the researchers claim the findings are the first evidence of a rhythmic universal in a mammal apart from humans, and suggest that the skill may have evolved independently among singing species, with the last common ancestor between humans and indri living around 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”, explains Ravignani. “We would like to look for evidence of others, including an underlying ‘repetitive’ beat and a hierarchical organisation of beats, in indri and other species.”

The researchers predict that, ultimately, the indri might not be alone among primates in possessing rhythmic features that are shared with human music.

“I think there is the possibility that other primates may show this rhythmic universal, especially the singing ones,” says De Gregorio. “For example, gibbons could be good candidates as their songs are quite long and complex, and gibbons’ songs functions are similar to indris’ ones.”