They have fluffy ears, a penetrating stare and a penchant for monogamy. But it turns out that indris – a large, critically endangered species of lemur – have an even more fascinating trait: an unexpected sense of rhythm.
Indri indri are known for their distinctive singing, a sound not unlike a set of bagpipes being stepped on. The creatures often strike up a song with members of their family either in duets or choruses, featuring sounds from roars to wails.
Now scientists say they have analysed the songs of 39 indris living in the rainforest of Madagascar, revealing that – like humans – the creatures employ what are known as categorical rhythms.
These rhythms are essentially distinctive and predictable patterns of intervals between the onset of notes. For example in a 1:1 rhythm, all the intervals are of equal length, while a 1:2 rhythm has some twice as long as those before or after – like the opening bars of We Will Rock You by Queen.
“They are quite predictable [patterns], because the next note is going to come either one unit or two whole units after the previous note,” said Dr Andrea Ravignani, co-author of the research from the Max Planck Institute for Psycholinguistics.
While the 1:1 rhythms have previously been identified in certain songbirds, the team say their results are the first time categorical rhythms have been identified in a non-human mammal. “The evidence is even stronger than in birds,” said Ravignani.
Writing in the journal Current Biology, the team – which also included researchers from the university of Turin – describe how they made their discovery by studying the songs of 20 groups of indris – typically singing in male and female duets – recorded over a period of 12 years.
The results reveal that the lemurs’ songs matched two rhythmic categories, a 1:1 rhythm, akin to the pace of a metronome, and a 1:2 rhythm. In addition, they found that when indris sing they gradually decrease their tempo – described in musical terms as ritardando.
Both male and female indris showed categorical rhythms but their tempo differed. Rivignani said that might suggest rhythm did not evolve as a way for one sex to show off to the other. “Maybe also in our species, rhythm did not evolve under sexual selection pressure, but rather for group bonding purposes,” he said.
Humans’ and indris’ last common ancestor is thought to have lived 77.5m years ago. “However, the singing capacity and these rhythmic categories seem to be quite rare in mammals,” said Ravignani.
That means it is unlikely that the trait has been passed down from the common ancestor. Instead, the trait has cropped up independently among singing species, possibly to aid “song coordination, processing and potentially learning”, he said.