300 aspects of each call were cataloged, letting researchers estimate meaning.
Two bonobos sit facing each other, against a lush, green backdrop. Two bonobos sit facing each other, against a lush, green backdrop.
This situation might call for a whistle. Credit: USO
Bonobos, great apes related to us and chimpanzees that live in the Republic of Congo, communicate with vocal calls including peeps, hoots, yelps, grunts, and whistles. Now, a team of Swiss scientists led by Melissa Berthet, an evolutionary anthropologist at the University of Zurich, discovered bonobos can combine these basic sounds into larger semantic structures. In these communications, meaning is something more than just a sum of individual calls—a trait known as non-trivial compositionality, which we once thought was uniquely human.
To do this, Berthet and her colleagues built a database of 700 bonobo calls and deciphered them using methods drawn from distributional semantics, the methodology we’ve relied on in reconstructing long-lost languages like Etruscan or Rongorongo. For the first time, we have a glimpse into what bonobos mean when they call to each other in the wild.
Context is everything
The key idea behind distributional semantics is that when words appear in similar contexts, they tend to have similar meanings. To decipher an unknown language, you need to collect a large corpus of words and turn those words into vectors—mathematical representations that let you place them in a multidimensional semantic space. The second thing you need is context data, which tells you the circumstances in which these words were used (that gets vectorized, too). When you map your word vectors onto context vectors in this multidimensional space, what usually happens is that words with similar meaning end up close to each other. Berthet and her colleagues wanted to apply the same trick to bonobos’ calls. That seemed straightforward at first glance, but proved painfully hard to execute.
“We worked at a camp in the forest, got up super early at 3:30 in the morning, walked one or two hours to get to the bonobos’ nest. At [the] time they would wake up, I would switch my microphone on for the whole day to collect as many vocalizations as I could,” Berthet says. Each recorded call then had to be annotated with a horribly long list of contextual parameters. Berthet had a questionnaire filled with queries like: is there a neighboring group around; are there predators around; is the caller feeding, resting, or grooming; is another individual approaching the caller, etc. There were 300 questions that had to be answered for each of the 700 recorded calls.
But when all this data was finally vectorized and the team started working their distributional semantics magic, gathering it proved worth the effort.
Bonobo dictionary
Berthet started with establishing the tentative meaning of the basic calls: singular grunts or yelps. Grunts appeared in many different contexts, including grooming, feeding, or moving, and the team interpreted them as intended to get another’s attention, a bit like saying “look at me.” Yelps meant “let’s do this” as an imperative, while peeps had a very similar meaning, but were more of a suggestion—think “I would like to…” Bonobos also used peeps or yelps when they wanted others to join them. Low hoots were translated as “I am excited,” while high hoots signaled the presence and location of the caller in dangerous situations. Whistles meant “let’s stay together.”
Once the basic calls were sorted, Berthet started looking at their different combinations. Bonobos combined yelps and grunts into a trivial compositional structure meaning “let’s do what I do.” This was mostly used when the group was building night nests—platforms made high in the trees out of broken branches, sometimes lined with leaves.
However, the team also found examples of non-trivial compositionality, the first such discovery outside of humans.
The first non-trivial combination was high hoot-low hoot that was translated as a distress call. But it was also used to stop other individuals’ display behaviors—dramatic, exaggerated actions or gestures bonobos perform to assert dominance or attract attention. The second was either peep or yelp in the “join” meaning paired with high hoot to form a structure used for coordinating with others before traveling. Finally, the “I would like to” peep followed by “let’s stay together” whistle was used for initiating more romantically inclined interactions bonobos are famous for indulging in.
Berthet said her team managed to record a few more calls but could not use them in her study because they were too rare to gather meaningful context data. Still, she expects we have much more to learn about bonobos’ communication.
Gestures and sounds
One thing the team was not certain about was whether there were more nuanced variations of the sounds they roughly categorized as grunts, peeps, yelps, hoots, and whistles. “There may be subtle acoustic differences that could lead to different meanings, and it may be our dictionary is too rough,” Berthet acknowledges. Another thing the team did not include in their analysis is the gestures that bonobos often accompany their calls with. “They use a lot of gestures, and they may use them to either refine or completely change the meaning of their vocalizations,” Berthet added. Applying the same methodology but with gestures included would be great. That’s definitely the next step to take.”
But she also has a few further steps in mind, and they go way beyond just bonobos. The team argues that the most important contribution of their work is establishing a methodology for deciphering animal communication. “Since now we have this nice tool to investigate compositionality and meaning, what I want to do is apply it to several animal species,” Berthet says. Chimpanzees, gibbons, gorillas, and monkeys are next on her list. The goal of these future studies is to trace when abilities like non-trivial compositionality started to appear in primate evolution. “Maybe we’ll find compositionality in old world primates. Maybe it’s just present in great apes. Maybe it’s been there the whole time. It’s really an open question now,” Berthet says.
Science, 2025. DOI: 10.1126/science.adv1170