Summary: Research of lemurs sheds light on the evolutionary origins of musicality in humans.
Source: Max Planck Institute
Songbirds share the human sense of rhythm, but it is a rare trait in non-human mammals. An international research team led by senior investigators Marco Gamba from the University of Turin and MPI’s Andrea Ravignani set out to look for musical abilities in primates.
“There is a longstanding interest in understanding how human musicality evolved, but musicality is not restricted to humans,” says Ravignani.
“Looking for musical features in other species allows us to build an ‘evolutionary tree’ of musical traits, and understand how rhythm capacities originated and evolved in humans.”
To find out whether non-human mammals have a sense of rhythm, the team decided to study one of the few ‘singing’ primates, the critically endangered lemur Indri indri. The researchers wanted to know whether indri songs have categorical rhythm, a ‘rhythmic universal’ found across human musical cultures.
Rhythm is categorical when intervals between sounds have exactly the same duration (1:1 rhythm) or doubled duration (1:2 rhythm). This type of rhythm makes a song easily recognisable, even if it is sung at different speeds. Would indri songs show this “uniquely human” rhythm?
Ritardando in the rainforest
Over a period of twelve years, the researchers from Turin visited the rainforest of Madagascar to collaborate with a local primate study group. The investigators recorded songs from twenty indri groups (39 animals), living in their natural habitat. Members of an indri family group tend to sing together, in harmonised duets and choruses. The team found that indri songs had the classic rhythmic categories (both 1:1 and 1:2), as well as the typical ‘ritardando’ or slowing down found in several musical traditions. Male and female songs had a different tempo but showed the same rhythm.
According to first author Chiara de Gregorio and her colleagues, this is the first evidence of a ‘rhythmic universal’ in a non-human mammal. But why should another primate produce categorical ‘music-like’ rhythms? The ability may have evolved independently among ‘singing’ species, as the last common ancestor between humans and indri lived 77.5 million years ago. Rhythm may make it easier to produce and process songs, or even to learn them.
“Categorical rhythms are just one of the six universals that have been identified so far”, explains Ravignani. “We would like to look for evidence of others, including an underlying ‘repetitive’ beat and a hierarchical organisation of beats—in indri and other species.”
The authors encourage other researchers to gather data on indri and other endangered species, “before it is too late to witness their breath-taking singing displays.”
About this evolutionary neuroscience research news
What are the origins of musical rhythm? One approach to the biology and evolution of music consists in finding common musical traits across species. These similarities allow biomusicologists to infer when and how musical traits appeared in our species. A parallel approach to the biology and evolution of music focuses on finding statistical universals in human music.
These include rhythmic features that appear above chance across musical cultures. One such universal is the production of categorical rhythms, defined as those where temporal intervals between note onsets are distributed categorically rather than uniformly.
Prominent rhythm categories include those with intervals related by small integer ratios, such as 1:1 (isochrony) and 1:2, which translates as some notes being twice as long as their adjacent ones. In humans, universals are often defined in relation to the beat, a top-down cognitive process of inferring a temporal regularity from a complex musical scene.
Without assuming the presence of the beat in other animals, one can still investigate its downstream products, namely rhythmic categories with small integer ratios detected in recorded signals. Here we combine the comparative and statistical universals approaches, testing the hypothesis that rhythmic categories and small integer ratios should appear in species showing coordinated group singing.
We find that a lemur species displays, in its coordinated songs, the isochronous and 1:2 rhythm categories seen in human music, showing that such categories are not, among mammals, unique to humans.