The indri is a lemur, a primate with opposable thumbs; a short tail; and round, tufted, teddy-bear-like ears. They share a branch of the evolutionary tree with humans, but our paths diverged some 60 million years ago. Still, one very striking similarity has stuck around: Indris are one of the few mammals that sing. Family groups create choruses in the treetops of their rain forest home in Madagascar; their voices ringing out for miles. Those songs—which biologist Andrea Ravignani describes as sounding like a cross between several jazz trumpeters jamming, a humpback whale, and a scream—are also the only songs other than those made by humans to be structured with regular, predictable rhythms.
In fact, indri rhythm can be the same as human rhythm, says Ravignani, who studies bioacoustics at the Max Planck Institute for Psycholinguistics. He is part of an international team of researchers whose recent paper in Current Biology is the first to document rhythm in lemurs.
Analyzing how, and when, the lemurs’ songs use a rhythmic structure could help researchers understand musicality in humans, the evolutionary purpose of which remains mysterious. Traits like color vision, bipedal ambulation, and prolonged infanthood have all been attributed to evolutionary pressures that favored the people who carried certain genes. But music, which is so pervasive across human cultures, is unexplained. “As a music lover I am fascinated by the beauty of music,” says Ravignani. “As a biologist, I’m puzzled about why we still haven’t found an answer when many other things are so obvious in human evolution.”
The origin of rhythm—and even the term itself—has been challenging to nail down. “There’s no universally accepted definition,” says Anirrudh Patel, a cognitive psychologist at Tufts who was not involved in the lemur study. He points out that rhythm is often confused with beat. Both are the underlying, propulsive forces that make you move your hips or snap your fingers in time to the music. But the two are not always synonymous. Think of a Gregorian chant, which has no beat and yet is still rhythmic. While a beat is generally an isochronous, steady pulse, the rhythm is the relationship between events like notes, clicks, or drum beats.
Patel defines rhythm as a systematic arrangement of events in time. That encompasses everything from the bouncing oompah-pah notes of a polka to the John Cage composition Organ2/ASLSP (As Slow as Possible), an ongoing performance expected to take 639 years, in which the notes are divided by years of silence.
For decades, scientists had thought perceiving rhythm was a distinctly human capacity until Snowball, a cockatoo and YouTube star, bobbed his way onto the scene in 2007. In viral videos, Snowball taps his talons and nods his head in time to hits by the Backstreet Boys, Queen, and Michael Jackson. When Patel saw the clips, he immediately brought Snowball into his lab and started experimenting to see if these dances were a coincidence or if the bird really could discern the rhythm in the songs. Patel’s research showed that this was no accident. When his team sped up or slowed down the music, Snowball changed his movements to match.
Patel hypothesizes that animals like humans and cockatoos have a complex set of connections between the region of their brains that perceives auditory stimuli and the region that controls movement. He points to research that shows that when people listen to songs with strong rhythmic beats, they get very strong activations in brain regions that control movement, even if they’re not moving or intending to move. He thinks those connections require special brain circuitry that only some animals have. “Those are animals that have evolved to mimic complex sounds,” he says, like cockatoos, which can learn a song just from listening to it.
It’s also possible that these abilities are more widespread than we realize. Subsequent studies have concluded that seals and sea lions can also perceive rhythm, even though they’re not particularly gifted vocal mimics.
But in all of these cases so far, the animals have learned to follow along to the rhythm, which is not the same as producing their own. So far, indris and humans seem to be the only ones that can do that, or at least are the only two to share the same rhythmic structures.
Chiara De Gregorio, a bioacoustics researcher at the University of Turin who worked with Ravignani and others to analyze lemurs' songs, wasn’t surprised. De Gregorio spends months deep in the mountain rain forest in Madagascar, tracking lemurs and recording them singing. She’d follow the indris, which live in family groups, for eight or nine hours a day as they moved through the trees above her. After spending so much time with them, De Gregorio got used to their songs and could predict when they might be getting ready to sing and which notes might be next in the song. Still, she is amazed by how regular the indris’ rhythms are. “I didn’t think that they were so perfect,” she says. “It’s very precise.”
The team analyzed 12 years’ worth of recordings of one type of song that the lemurs sing in the morning to announce their presence. The scientists then isolated each singer’s contribution to each song and isolated each note that the singer produced, measuring how long each note lasts. They found that lemur sounds fell into three rhythmic categories. In one type, the notes were all the same length, meaning they had a 1:1 rhythm, like a metronome. Others had a 1:2 rhythm, meaning the second note lasted twice as long as the first. Still others had a 2:1 rhythm, like the stomp-stomp-clap in “We Will Rock You.”
Darwin theorized that musical ability is a sexually selected trait, but scientists say that hypothesis doesn’t seem to fit human behavior. Usually in sexually selected traits, one sex has the trait and the other chooses their mate based on it, like the way female birds seek out males with fancy feathers or sophisticated singing styles. “That’s clearly not true for music” when it comes to people, says Greg Bryant, a cognitive scientist at the University of California, Los Angeles who studies communication. Men and women are equally able—and interested in—playing and listening to music. The same is true for indris; both sexes display the same rhythms and both sing.
Ravignani says the team’s data supports a different theory: that rhythm helps members of a group organize and coordinate, making them able to predict which note is coming next and how long it will last. For animals like the indris, which live and sing in groups, the rhythm could help everyone understand when to sing. “In a way, it’s kind of maybe the first step toward having a beat-based communication, like music,” says Patel.
Bryant agrees this could be a possibility. “When you have these particular acoustic features, then it allows you to coordinate your action with others and potentially communicate something to outsiders through that coordination,” he says. Rhythmic music not only helps everyone participate at the right times, it signals which group you belong to and how well organized that group is.
Ravignani’s team’s work on the indris’ rhythm is just beginning. In addition to their morning announcement song, the animals also sing when they’re lost, as a warning, or as a threat, so De Gregorio is curious about whether those songs also have these rhythms.
Next, Ravignani wants to apply these research techniques to other singing primates, like gibbons, and then to marine animals like seals. “And then who knows?” he asks. “Every year or so, we discover that at least one animal species has something that we previously thought was uniquely human. So I think we’re up for a lot of surprises.”