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.