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By Nicholas Weiler for Science.org
Sometimes a male chimp just needs to drum. Hooting and hollering, he gallops up to the giant buttress root of a tree, grips its crest with his hands, and beats on its wall-like surface with both feet, making a racket that can be heard more than a kilometer away. Now, new research from Uganda suggests these drum solos contain signature rhythmic patterns that may telegraph an individual’s whereabouts to distant troopmates. The findings could provide insight into how rhythm first evolved in humans.
Drumming has puzzled naturalists for years. “Chimps produce this really wonderful resonant sound that goes booming through the forest,” explains chimpanzee researcher Michael Wilson of the University of Minnesota, Twin Cities, who was not involved in the new work. “We knew they were drumming, but we didn’t really know why. This seems to be a pretty good explanation.”
Male chimpanzees use buttress drumming as occasional punctuation to their more common pant hoot calls. Chimps pant hoot for many reasons, including to threaten rivals or intimidate potential mates. Buttress drumming is physically impressive, requiring muscle, skill, and timing to pull off, which has led some naturalists to suppose that drumming, too, is intended to shock and awe. Because the sound of drumming carries further than pant hoots, researchers speculated it could also let chimps communicate over long distances.
Psychologist Katie Slocombe of the University of York in the United Kingdom wondered what information drumming might carry. During fieldwork in Uganda, she noticed that she could identify different chimps in the troop she studied based on their distinctive drumming styles. “I thought, ‘If I can, I’m sure the other chimps can,’ ” she says, and anecdotally noticed that chimps seemed to pay more attention to the drumming of more important troopmates. She hypothesized that large chimpanzee troops use drumming to keep track of one another as they roam their wide territories, which can range from the size of New York City’s Central Park to the size of Manhattan island. Whether chimps want to find or avoid one another, Slocombe says, “having a good idea of where everyone is spatially in the forest could really help.”
To test her idea, Slocombe and her colleagues analyzed the social context of 293 pant hoots recorded over 3 years to see what influenced the troop’s 13 males to add drumming to their calls. If chimps drum merely to flaunt their physical prowess, dominant males should drum most often, the researchers reasoned, particularly when potential rivals or fertile females are nearby. They found that older, more experienced males did drum more than other troop members, but the drummer’s rank and the makeup of the local audience played little role. Instead, the decision to drum seemed most related to whether males were on the move. Chimps paired pant hoots with buttress drumming 75% of the time when traveling, compared with only 40% of the time while resting and 10% while eating.
Traveling males also drummed with distinctive rhythmic flair, the researchers found. Recordings of drumming by eight adult males made between 2003 and 2011 revealed significant differences in how long the males liked to drum, their fondness for doublets and pauses, and how many beats they could squeeze into a drumming bout. The chimps’ drumming styles were distinct enough for a statistical algorithm to correctly identify the drummer 47.5% of the time based on these simple rhythmic features, a good score considering chance identification of one chimp out of eight would be only 12.5%. Subjectively, the team also identified more complex acoustic differences between the chimp’s drum solos that could convey even more information, Slocombe says, though there were too few samples to analyze fully. This led Slocombe and her colleagues to conclude that traveling chimpanzees could use signature drumming styles to convey their location to distant troopmates, they report in this month’s issue of the American Journal of Physical Anthropology.
“This shows that rhythmic abilities are not uniquely human,” Slocombe says. If humans’ common ancestors with chimpanzees had this ability, she speculates, it could illuminate how musical rhythm could have emerged in the human lineage. “Coordinating movement over very long distances might have been why the earliest humans started to drum,” she says.
More research is needed to prove that chimps really use drumming to help find one another in the forest, cautions Adam Clark Arcadi, an anthropologist at Cornell University who was not involved in the current work but also studies buttress drumming. “The possibility that there are drumming signatures is intriguing,” but it’s still not proven that other chimps are paying attention to this information, he says. Observing how chimps respond to recorded drumming bouts could further demonstrate whether drumming contains a threat or a more benign message.
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