Studying how and why rhythm evolved in these primates could help unravel the mysteries of human musicality.

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.”

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.”