Humans are adept combiners. As it turns out, so, too, are bonobos. According to a new study in Science, bonobos can combine their calls a lot like humans can, indicating that a hallmark of human communication — an ability called compositionality — is just as much a hallmark of the communication of our closest relatives.
“Since humans and bonobos had a common ancestor approximately 7 to 13 million years ago, they share many traits by descent,” said Martin Surbeck, a study author and an evolutionary biologist at Harvard University, according to a press release. “It appears that compositionality is likely one of them.”
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Bonobos and Compositionality
When we humans communicate, we combine our words into sequences, some more complicated than others. In some cases, these sequences are as meaningful as their individual words, a type of structure called trivial compositionality. In other cases, these sequences are more meaningful, a type of structure called nontrivial, or complex, compositionality.
For a complex compositionalist, the meaning of one component in a combination of words alters the meaning of another component. Take the case of the combination “skilled speaker,” for instance. Its trivial interpretation represents a person who is skilled and who is also a speaker, while its complex interpretation represents a person who is skilled at speaking.
All sorts of species combine calls in the trivial sense, though it isn’t clear how many communicate nontrivially. Indeed, the animal studies that have found unambiguous support for trivially composed compositions in primates and birds have always stopped short of finding the same for complex ones.
Hoping to find this support for the first time, a team of researchers turned to the bonobos at the Kokolopori Community Reserve in the Democratic Republic of Congo. Studying three sets of bonobos from the reserve, the researchers found that these primates composed compositional calls — and a lot of them — some of which were trivial and some of which weren’t.
“This suggests that the capacity to combine call types in complex ways is not as unique to humans as we once thought,” said Mélissa Berthet, a study author and an evolutionary anthropologist at the University of Zurich, according to a press release.
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Creating a Bonobo Dictionary
Capturing the audio and recording the context of 700 calls and call combinations, the team connected the sound of individual calls to the occurrence of specific situations and outcomes.
“This allowed us to create a bonobo dictionary of sorts — a complete list of bonobo calls and their meaning,” Berthet said in the release. “This represents an important step towards understanding the communication of other species, as it is the first time that we have determined the meaning of calls across the whole vocal repertoire of an animal.”
The team then compared the contexts in which calls were used individually and in combination, allowing them to see whether the bonobos were trivial or nontrivial combiners.
“With our approach, we were able to quantify how the meaning of bonobo single calls and call combinations relate to each other,” said Simon Townsend, another study author and evolutionary anthropologist at the University of Zurich, according to the release.
The bonobos’ use of both trivial and complex call compositions suggests that bonobos and humans are closer to each other than traditionally thought, at least in terms of the structure of our communication. The primates’ use of complex compositionality also suggests that certain components of human communication are old, tracing back to the last common ancestor between humans and bonobos.
“Our study, therefore, suggests that our ancestors already extensively used compositionality at least 7 million years ago, if not more,” Townsend said in the release.
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Article Sources
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Sam Walters is a journalist covering archaeology, paleontology, ecology, and evolution for Discover, along with an assortment of other topics. Before joining the Discover team as an assistant editor in 2022, Sam studied journalism at Northwestern University in Evanston, Illinois.
