Giant lemurs are the first mammals (besides us) found to use rhythm

Mammals make all kinds of sounds, but their calls rarely sound like music. The guilty? A lack of rhythm – the temporal sequence that organizes sounds, and the pauses between them, into a repeatable pattern. Humans have long been the only mammals known to use rhythm to create music. To find out how we were able to acquire this rhythmic ear, scientists are exploring the musical abilities of other mammals.

This is why a team of researchers recently went to the Madagascan jungle, armed with microphones to record the remarkable cries of the Indri lemur (Indri indri). Indris look like gangly black and white teddy bears with piercing green eyes. They often perch high in the rainforest canopy, where (although they are the largest species of lemur) they can be difficult to spot. The animals are easy to hear, however; their bellowing cries are recognizable from more than a kilometer and a half away. In addition to their prodigious bagpipes, the Indris sport a varied vocal repertoire, including a high-pitched moan-like “song” that resonates through the rainforest.

Indris live in family groups, and their distinctive songs help different groups communicate with each other: Like a family group, adults perform what might seem to humans as sad duets, before their offspring join in a cacophonous chorus. “When you hear an indri song, it sounds hauntingly beautiful,” says Andrea Ravignani, bio-musicologist at the Max Planck Institute for Psycholinguistics.

Indri’s songs may sound chaotic at first, but University of Turin biologist Chiara De Gregorio says they start to sound less, so the more you hear them. “When you get used to it, you can really recognize a pattern in these songs,” she says. “When they start a sentence, you know what to expect, note after note. In a study published Monday in Current biology, De Gregorio, Ravignani and their colleagues analyzed recordings of 636 individual Indri songs collected from 20 different family groups to determine if these thriving lemurs in fact possess a rhythm.

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

Researchers say their study shows indris use these patterns to structure their songs, making it the first confirmed case of a non-human mammal possessing a categorical rhythm. Their conclusions applied to all the indris they recorded; women and men sing at different tempos, but both use the same rhythmic patterns. Indris also showed his ability to maintain a constant rhythm by decreasing the tempo of his songs, a process known as ritardando in classical music.

Although indris and human composers can use similar structures, each has likely developed their own musical abilities. It has been 77.5 million years since the common ancestor of humans and indris existed, an evolutionary chasm of time that makes it unlikely that rhythm was an ancestral trait. Rather, Ravignani believes that similar social pressures have shaped both indris and humans into singers at different times – an example of convergent evolution. The exact benefits this ability confers on the indris are still unknown, but the authors believe that organizing songs into repeatable patterns may make it easier for young lemurs to learn or help indri families get on the same length quickly. wave when they have to defend a territory or assemble. .

A two-month-old indri. Credit: Bernard Castelein Alamy

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

Dissection of indris songs indicates that 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 are hiding [even] more traits shared with humans than previously thought, ”says De Gregorio. She wonders how an indri’s song develops as the primate grows and for what specific purposes communication through repetitive sounds can be used.

Unfortunately, the time to answer such questions may be running out, as deforestation and hunting have pushed indris (as well as gibbons) to the brink of extinction. The Indris are particularly vulnerable; some experts estimate that only 1,000 are left in the wild, and all signs are that this number is declining, as Madagascar’s forests are expected to decline to 93 percent by 2070.

Researchers hope conservation efforts will help indris survive for a long time. As scientists try to understand why primates have started using music, it will be crucial to hear the songs of wild indris. “Looking at their communication system, it’s an indirect window on their mind,” says Ravignani. “They have a lot of secrets hidden in their heads, and by looking at their sound production, we can uncover them.”

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