Music is often characterized by a rhythm. This rhythm can be found in human music, and it could have its roots back to our ancestors, who were a type lemur called indris.
A male indri (Indri indri) reaching for leaves Nick Garbutt/Nature Picture Library/Alamy
The morning tranquility of eastern Madagascar's lowland rainforest is disturbed by the sounds of eerie wails. Soon, more join them. The song of the endangered, one-metre tall indri lemur is the haunting sound they make. Research suggests that primates' calls may have much in common with humans.
Chiara De Gregorio, a University of Turin, Italy, says that Indris (Indri-indri), sing to communicate with other families or to locate and reunite family members. It is not clear how rhythms in these primates' calls and soulful keenings are synchronized. De Gregorio began to dissect the indris song with her colleagues.
Researchers recorded 20 songs from different indri groups in Madagascar's rainforests over 12 years and analysed their timing.
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The indri had two distinct rhythms. One was 1:1 where notes are spaced evenly like a metronome and the other is 1:2 where the gap between each note is twice as large as the one before it. These rhythm categories, or categorical rhythms, are common in music.
De Gregorio says that this is the first evidence of a characteristic trait of human music being present in another mammal. De Gregorio adds that only two bird species, the thrush nightingales Luscinia luscinia and the zebra finches Taeniopygia Guttata, have this characteristic when they sing. However, each exhibits only one categorical rhythm.
Indris, however, shares with humans two rhythms which make their songs complex and well-articulated, she says.
De Gregorio suggests that these universal musical qualities in indris could indicate that the primate lineage is more deeply rooted than previously believed.
Or, if we consider that humans and lemurs last shared a common ancestor around 77 million years back, then categorical rhythms may have evolved independently twice within primates.
The study only examined the timing of calls. It is not clear what rhythms mean for communication, as the focus was on these properties. The authors suggest that these rhythms could play a role both in song coordination and social bonding.
Simon Townsend, a University of Zurich researcher in Switzerland, said that the study clearly illustrates the importance of comparing humans with other species to discover what aspects of rhythm and music are unique to humans.
Alexandre Celma Miralles, Aarhus University in Denmark, would like to see similar research on gibbons. Gibbons sing and are in closer relationship to humans.
De Gregorio plans to work with her team to determine if indris were born using these rhythm categories, or if they have learned them. Although the primates have much to learn from us, their future looks grim.
She says that every attempt to create captive populations has failed, and their habitat is disappearing at an alarming rate.
Journal reference: Current Biology, DOI: 10.1016/j.cub.2021.09.032
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