These Singing Lemurs Are the Only Other Mammal Species to Possess Rhythm

The shrieking, cacophonous call of an indri—a large, black-and-white lemur with buggy eyes—may not sound like much of a song, but it is. Indri indri, a critically endangered species native to Madagascar, use these tunes to communicate with their social groups, reports Nicola Davis for the Guardian.

Scientists recently discovered that their songs have a component that no other mammal species—besides humans—possesses: rhythm. The team of researchers published their findings in the journal Current Biology this week.

To better understand the origins of rhythm in humans, scientists are looking into how musically inclined other primate species might be.

"There is longstanding interest in understanding how human musicality evolved, but musicality is not restricted to humans," says Andrea Ravignani, a cognitive biologist at the Max Planck Institute for Psycholinguistics in the Netherlands, in a press release. "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."

That's what led Ravignani and his collaborators to spend 12 years collecting data in Madagascar. With microphones in hand, they ventured into the forests to record the lemurs' bellows. Ultimately, they documented 636 songs from 20 different social groups, reports Jack Tamisiea for Scientific American.

The duet of the indris: the female is singing with the reproductive male (out of view) and her son.

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"We isolated every syllable in each song, where it started and ended, and who it belonged to," Ravignani tells Virginia Morell for Science. And once the songs were analyzed, "we could see it at once: the rhythm."

The team's analysis revealed that indris follow a set of patterns known as categorical rhythms. They possess two types—1:1 and 1:2—that are also common in our songs. In the first, the pacing follows that of a metronome; the pause between each note is the same duration. A 1:2 rhythm has some intervals that are twice as long as others, like the stomp-stomp-clap of the opening of Queen's "We Will Rock You," reports Science.

"When you’re listening to a musical piece and dancing to it, you’re basically processing this very complex stream of sounds, extracting some regularities from it, and then predicting what’s coming next," Ravignani tells Sam Jones for the New York Times. "If an indri had some sort of metronome in its head going 'tac, tac, tac,' then they would likely produce what we see. It’s so close to human music — it’s quite astonishing."

Before this study, scientists only knew that humans and nightingale thrushes follow categorical rhythms, reports Jason Bittel for National Geographic. The thrushes tend to stick to a 1:1 rhythm, though they've been documented following 1:2. But indris follow 1:2 more frequently, and their flexibility is impressive, Ravignani says, since most animals tend to stick to one set pattern.

Humans and indris are the only known mammal species to have a sense of rhythm, but our lineages diverged over 77 million years ago, when dinosaurs still roamed the planet. Ravignani tells Scientific American that a knack for music likely evolved twice independently after our lineages split. Though the evolutionary benefit of having rhythm is still unclear, scientists suspect that they could help young indris learn the songs, which is important when family groups need to communicate.

"After our recent discovery, I think these giant lemurs hide [even] more shared traits with humans than previously thought," Chiara De Gregorio, a biologist at the University of Turin in Italy, tells Scientific American.

Scientists have only recently started to unravel the origins of musicality, and their work is ramping up. Only last year did Ofer Tchernichovski, a biologist at Hunter College in New York, publish the study on nightingale thrushes. And Ravignani tells National Geographic that studying rhythm in whales and dolphins—notoriously chatty and music creatures—hasn't been done yet to his knowledge.

"We only recently discovered that in birds, and now this paper is the first one that discovered that in mammals," Tchernichovski, who was not involved with the new study, tells National Geographic. "My feeling is that the more people look, the more people will find it."

But time to study the critically endangered indris is running out, reports National Geographic. Experts estimate that 1,000 and 10,000 individuals remain, and they are threatened by deforestation and hunting. They're also incredibly important for the Malagasy, the people native to the island of Madagascar.

"We are very worried about indris’ situation, which is very critical," De Gregorio tells National Geographic. "They cannot survive in captivity, so once the forest is gone, they are gone too."

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Rasha Aridi is a science journalist based in Richmond, Virginia. She has written for Science magazine and Science News for Students. You can find her portfolio at rashaaridi.com.