The ability to sense rhythm seems almost instinctual in human beings. We have crafted many different types of percussive instruments across cultures. The body can also be used as a drum through hitting, stomping and vocalizing.
As we can predict when the next beat will drop, our perception of rhythm seems to be directly related to our experience with time. Studies have shown that humans are limited in their ability to predict beats. If the interval between beats is longer than 3 seconds, it can cause us to struggle to place them accurately.
What are the roots of our species' proclivity for rhythm? This question can be addressed by looking at rhythm in other species, especially our primate relatives. A recent study in Current Biology does exactly this.
Andrea Ravignani, coauthor of the study, said, "Looking for musical characteristics in other species allows me to build an ‘evolutionary tree’ of musical traits, which will allow us to understand how rhythm capabilities originated and evolved in people."
The researchers sought to find universal rhythmic characteristics in all cultures before they could identify rhythm in primates.
The authors of the paper note that categorical rhythms are a universal production. They are defined as intervals where the temporal intervals between note beginnings are distributed categorically, rather than uniformly.
People are familiar with the popular rhythmic categories that involve intervals that are related by smaller integers. This includes isochrony and 1:1, which refers to notes that are twice as long than their next ones.
These categorical rhythms were previously identified in birdsong. However, in order to better understand rhythmic perception in humans, researchers looked at singing mammals. This is in large part because of their genetic closeness to humans.
Indri Indri, a group of singing mammals, is one such example. This species of lemur is known for its large frame and striking black-and white coats. Researchers recorded songs from 19 indri groups, which included 39 individuals, over the course of twelve years.
These recordings included approximately 1% of the total population of this critically endangered species. They were captured in their natural habitat, Madagascar's rainforests.
I. indri in a tree. (Filippo Carugati)
Researchers extracted the temporal characteristics of indris songs through analysis of intervals of notes, and their ratios.
These songs were found to match two categories of rhythmic ratios: 1:1, which is similar to a metronome's pace, and 2:2, which is a fundamentally smaller integer ratio.
This is direct evidence of one musical universal, categorical rhythms. According to the authors, the small integer ratios of 1:1 and 1:2 are precisely those that are expected to be used for beat production and binary, mathematical subdivisions. Both of these aspects are rare in other species.
Additional findings showed that the interval lengths between notes for male and female indris were different, but that the ratios remained constant at 1:1. These differences could be related to selective hypotheses about music. It may play a part in sexual selection or social bonding.
Why should another primate such as the Indri produce a categorical rhythm like humans?
The authors point out that there is no common ancestor of categorical rhythm with both species, despite the fact that they share a common ancestor from 77.5 million years.
This ability may have evolved in concert among songbirds, humans, and other singing species like indris and songbirds. They write that indris rhythmic categories and isochrony in songbirds may aid in song coordination, processing and learning.
Although the brain mechanisms supporting rhythmic categories in Indris are not yet known, this new discovery provides a powerful tool to analyze rhythmic behavior in other species as well as in ourselves.
Current Biology published the study.
