A new study suggests wild chimpanzee vocalizations are used to coordinate their hunts, much like humans use communication as part of cooperative efforts
Chimpanzees in Kibale National Park in Uganda. “The fact that chimpanzees and humans both show evidence of a vocal communication signal being used to facilitate cooperation suggests it is not something unique to the human lineage,” says Zarin Machanda. Photo: Ronan Donovan
From an evolutionary point of view, cooperating with a group to achieve an outcome—putting food on the table, say—is a pretty rare thing. Usually it’s every animal for himself or herself; that’s how you survive and pass on your genes.
But humans are a very cooperative species—we work together to do everything from feeding ourselves to fighting wars. Communication plays a big role in cooperation; we have to be able to share our goals to achieve them. It’s long been thought that communication and cooperation evolved together only in humans—but is that right?
New research by Tufts primate researcher Zarin Machanda and her colleagues at the University of Zurich suggests that it’s been used longer than we modern homo sapiens have been around. In a paper published in Science Advances, the researchers describe how wild chimpanzees use vocal signals in hunting, hallmarks of cooperative behavior.
“This is really the first example of this in the apes that we are aware of,” says Machanda, an assistant professor of anthropology.
Chimpanzees and bonobos are our closest living relatives, and can help us understand our own evolutionary history, she notes. “The fact that chimpanzees and humans both show evidence of a vocal communication signal being used to facilitate cooperation suggests it is not something unique to the human lineage—that it is something that most likely existed in the last common ancestor between chimps and humans, about 6 million years ago,” says Machanda.
For the study, Machanda, who is also director of long-term research for the Kibale Chimpanzee Project at the Kibale National Park in Uganda, examined the data collected by workers at the site for more than 20 years, covering 227 hunts by chimpanzees. She had help from Miranda Yu, A21, who was a student in Machanda’s lab as a Laidlaw Foundation scholar and one of the authors on the paper.
Researchers at the site meticulously document every action, vocalization, and gesture by the chimpanzees on the site, from sunup to sundown, pretty much 365 days a year. It takes about five years for the wild chimps to completely acclimate to the presence of human researchers, when the real research can begin.
The Chaos of a Hunt
Chimpanzees mostly eat fruit and other plant matter, but sometimes band together to hunt and eat other monkeys, particularly red colobus monkeys. While they cross paths with red colobus monkeys almost every day in the forest, sometimes when chimpanzees are satiated with food and have extra energy to spare, they start a hunt—often with a hunting “bark,” a vocalization that is distinct from the 60-plus other vocalizations they make, says Machanda.
The hunting bark seems to be a signal to other chimps to take part in the hunt. Usually an especially good hunter—an “impact hunter”—heads up a tree, initiating the hunt, trying to reach the lowest monkey in the forest canopy. What follows is pretty much immediate chaos, says Machanda, who also holds the Usen Family Career Development Professorship at Tufts.
Chimpanzees grabbing a young red colobus monkey during a hunt in the forest canopy. Photo: Ronan Donovan
“The red colobus monkeys just freak out—the female monkeys and the young ones scatter,” says Machanda. Other chimpanzees start to climb up different trees, while some might stay on the ground and not participate.
“The chimps in the trees are running after the monkeys, and at this point, as researchers you’re trying to avoid getting hit with things—branches are dropping, monkeys are falling out of trees,” says Machanda. “There’s a lot of alarm calling, a lot of excitement and screaming.”
Analyzing the data from all the hunts, the research team found that “individuals who bark are more likely to hunt,” Machanda says. “So it’s some kind of honest signal of whether they’re participating.”
Hunts with barks recruit more hunters, which “suggests that individuals who are hearing someone else bark use that information to join the hunt, since it lowers that individual’s cost of participation,” she says. That’s because hunts are always more successful with more individuals.
Hunts that begin with bark vocalizations tend to begin faster, too. Instead of waiting around to see who else is hunting, the bark seems to signal other chimps should join quickly. Though hunts with barks were not necessarily more successful, “the hunting bark certainly meets some interesting criteria about participation in this cooperative event,” Machanda says.
Chimpanzees grooming each other in Kibale National Park in Uganda. Photo: Ronan Donovan
While the chimpanzee barks are not as rich as human language, and don’t qualify as language, chimpanzees “are a communicative animal,” she says. “They use vocal communication in all sorts of contexts, and gesture at each other and have lots of facial expressions.”
The findings, the researchers write, suggest “that the behavioral dynamics we observed in hunting chimpanzees could represent precursors to a more typically hominin interplay of communication and cooperation. ... In wild chimpanzees, not only do barks possibly facilitate decisions regarding whether and how quickly to join a hunt, they also influence the outcome of the group behavior: hunting is more effective following a bark.”
To more fully verify the research findings, a follow-up study might use recorded playback of hunting barks, and note the reactions, Machanda notes. But those types of studies are difficult, and fraught with complexities, she says, and the researchers will be cautious about using them.
