Mammals make all manner of sounds, but their calls rarely resemble music. The culprit? A lack of rhythm—the temporal sequence that organizes sounds, and the pauses between them, into a repeatable pattern. Humans were previously the only mammals known to use rhythm to create music. To discover how we could have acquired this ear for timing, scientists are exploring the musical capabilities of other species.

This is why a team of researchers recently trekked into the jungles of Madagascar, armed with microphones to record the remarkable calls of the indri lemur (Indri indri). Indris resemble gangly, black-and-white teddy bears with piercing green eyes. They often perch high in the rain forest canopy, where—despite being the largest species of lemur—they can be tough to spot. The animals are easy to hear, though; their bellowing cries are recognizable from more than a mile away. In addition to their prodigious pipes, indris sport a varied vocal repertoire, including a high-pitched, wail-like “song” that reverberates through the forest.

Indris live in family groups, and their distinctive songs help the groups communicate with one another. Like a family band, the adults perform sorrowful-sounding duets before offspring join in for a cacophonous chorus. These songs are “hauntingly beautiful,” says Andrea Ravignani, a biomusicologist at the Max Planck Institute for Psycholinguistics in Nijmegen, the Netherlands.

Indri songs may seem chaotic at first, but University of Turin biologist Chiara De Gregorio says they begin to sound less so the more one hears them. “When you get used to them, you can really recognize a pattern in those songs,” she says. “When they start a phrase, you know what to expect, note after note.” In a new study published in Current Biology, De Gregorio, Ravignani and their colleagues analyzed recordings of 636 individual indri songs collected from 20 different family groups to determine whether these vociferous lemurs do, in fact, possess rhythm.

Two-month-old lemur.
Two-month-old indri. Credit: Bernard Castelein/Nature Picture Library/Alamy Stock Photo

Because rhythm is all about timing, the team dissected each recording into its base elements to measure the duration of notes and the pauses between them. The researchers compared the lengths of the intervals between each sound and found that the songs often broke down into rhythmic ratios of 1:1 (when the intervals between sounds have the same duration, like singing along with a metronome) or 1:2 (when the first interval is half as long as the second). Both are common in human music.

The researchers say their study shows indris use these patterns to structure their songs—making this the first confirmed case of a nonhuman mammal possessing humanlike categorical rhythm.* Their findings applied to all the indris they recorded; females and males sang at different tempos, but both employed the same rhythmic patterns. Indris also showed the ability to maintain a constant rhythm as they decreased the tempo of their songs—a process known in classical music as ritardando.

Although indris and human composers may utilize some similar structures, each likely developed musical abilities separately. It has been 77.5 million years since the common ancestor of humans and indris existed, a chasm of evolutionary time that makes it unlikely rhythm was an ancestral trait. Ravignani instead suggests similar social pressures molded both indris and humans into singers at different times—an example of convergent evolution. The exact benefits this ability confers to indris are still unknown, but the authors speculate that organizing songs into repeatable patterns may make them easier for young lemurs to learn, or it could help indri families quickly coordinate when they need to defend territory or gather together.

Elizabeth St. Clair, a biological anthropologist at Johns Hopkins University who studies the evolution of the primate vocal tract, says she was surprised by the rhythmic similarities between indri and human songs. “It seems like it’s an individual characteristic of indris that is not seen in many other mammals or even birds,” says St. Clair, who was not involved in the new study. She suspects that gibbons, small Southeast Asian apes known to coordinate their calls, may also use rhythm to structure their songs.

Dissecting indri songs indicates these animals share an underlying sense of rhythm with humans, but it raises more questions about how indris communicate. “After our recent discovery, I think these giant lemurs hide [even] more shared traits with humans than previously thought,” De Gregorio says. She wonders how an indri’s singing develops as the primate matures and what specific purposes can be served by communicating through repetitive sounds.

Unfortunately, time to answer questions such as these may be running out. Deforestation and hunting have devastated indri (as well as gibbon) populations; some experts estimate that as few as 1,000 indris remain in the wild, and all signs point to that number decreasing. Madagascar’s forests are predicted to decline by as much as 93 percent by 2070.

The researchers hope conservation efforts will help indris survive well into the future. As scientists attempt to untangle why primates began using music, hearing the songs of wild indris will be crucial. “Their communication system, it’s an indirect window into their minds,” Ravignani says. “They have a lot of secrets hidden in their heads, and by looking at their sound production, we can uncover them.”

Editor’s Note (12/9/21): A version of this article with the title “Lemur Rhythm” was adapted for inclusion in the January 2022 issue of Scientific American. This text reflects that version, with the addition of some material that was abridged for print.

*Editor’s Note (1/4/22): This sentence was edited after posting to clarify the type of categorical rhythm the indris demonstrated.