Study shows: Bonobos use compositionality in their communication

Bonobos, our closest living relatives, form complex vocal sequences that resemble human word combinations.

A new study by researchers from the University of Zurich and Harvard University challenges the long-held assumption about what makes human communication unique. It suggests that key aspects of language have deep evolutionary roots.

In a new study, researchers from the University of Zurich and Harvard University observed the vocal behaviour of wild bonobos in the Kokolopori Community Reserve in the Democratic Republic of Congo. Using novel linguistic methods, they demonstrate for the first time that bonobos combine their vocalisations according to the principle of compositionality, much as humans do.

According to this principle, words are combined into phrases whose meaning depends on the individual words and their order. In simple compositionality, meaning arises from the addition of components — a “blond dancer” is both blond and a dancer. In more complex compositionality, one element influences the other: a “bad dancer” is not simply a dancer with an additional attribute, but the adjective alters the overall meaning of the phrase.

Creating a bonobo dictionary

As a first step, the researchers applied a method developed by linguists to quantify the meaning of human words. “This allowed us to create a kind of bonobo dictionary — a complete list of bonobo calls and their meanings,” says Mélissa Berthet, lead author and postdoctoral researcher at the UZH Institute of Evolutionary Anthropology. “This is an important step towards understanding the communication of other species, as we are for the first time determining the meaning of sounds across the entire vocal repertoire of an animal.”

Compositionality: not exclusively human

After the researchers had determined the meaning of individual bonobo calls, they examined their combinations using a further linguistic approach. “This allowed us to measure how the meaning of individual calls relates to that of call combinations,” explains last author Simon Townsend, Professor of Primate Communication at UZH. The researchers identified numerous combinations whose meaning derived from their constituent parts — a central hallmark of compositionality. Some even showed surprising similarities to the more complex structures of human language. “This suggests that the ability to combine sounds in complex ways is not as unique to humans as we thought,” says Berthet.

Compositionality is older than previously thought

One important finding of the study is that it provides evidence for the evolutionary roots of linguistic compositionality. “Humans and bonobos shared a common ancestor 7 to 13 million years ago and share many traits from their lineage — likely including the compositionality of their communication,” says co-author and Harvard Professor Martin Surbeck. “Our results suggest that our ancestors already possessed this ability at least 7 million years ago — if not earlier,” adds Simon Townsend. The study also shows that the ability to form complex meanings from smaller sounds existed long before human language. And it underscores that the vocal communication of bonobos is more similar to human language than previously assumed.