Compositionality, the capacity to combine meaningful elements into larger meaningful structures, is a hallmark of human language. Compositionality can be trivial (combination’s meaning is the sum of the meaning of its parts) or nontrivial (one element modifies the meaning of the other element). In new research, scientists studied the vocal behavior of wild bonobos (Pan paniscus) — our closest living relatives — in the Kokolopori Community Reserve in the Democratic Republic of Congo and discovered robust empirical evidence for the presence of nontrivial compositionality in these primates.
Tupac, a young male bonobo scratching its head. Image credit: Lukas Bierhoff, Kokolopori Bonobo Research Project.
A hallmark characteristic of human language is its ability to combine discrete elements to form more complex, meaningful structures.
This principle, known as compositionality, allows for the assembly of morphemes into words and words into sentences.
The meaning of the whole is determined by its constituent parts and their arrangement.
Compositionality can take two forms: trivial and nontrivial. In trivial compositionality, each word maintains its independent meaning.
Nontrivial compositionality involves a more complex, nuanced relationship where meaning is not simply a direct sum of the words involved.
Compositionality may not be unique to human language; studies in birds and primates have demonstrated that some animals are capable of combining meaningful vocalizations into trivially compositional strucutres.
However, to date, there is no direct evidence that animals use nontrivial compositionality in their communication.
In their new study, University of Zürich biologist Mélissa Berthet and her colleagues discovered strong empirical evidence that wild bonobos use nontrivial compositionality in their vocal communication.
The authors analyzed 700 recordings of bonobo vocal calls and call combinations and documented over 300 contextual features associated with each utterance
Employing a method derived from distributional semantics, a linguistic framework that measures meaning similarities between words, they analyzed these contextual features to infer the meanings of individual bonobo vocalizations and quantify their relationships.
Then, to assess whether bonobo call combinations follow compositional principles, they applied a multi-step approach previously used to identify compositionality in human communication.
They discovered that bonobo call types integrate into four compositional structures, three of which exhibit non-trivial compositionality, suggesting that bonobo communication shares more structural similarities with human language than previously recognized.
“With our approach, we were able to quantify how the meaning of bonobo single calls and call combinations relate to each other,” said University of Zürich’s Professor Simon Townsend.
“Since humans and bonobos had a common ancestor approximately 7 to 13 million years ago, they share many traits by descent, and it appears that compositionality is likely one of them,” added Harvard University’s Professor Martin Surbeck.
“Our study therefore suggests that our ancestors already extensively used compositionality at least 7 million years ago, if not more,” Professor Townsend said.
The study was published in the journal Science.
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M. Berthet et al. 2025. Extensive compositionality in the vocal system of bonobos. Science 388 (6742): 104-108; doi: 10.1126/science.adv1170
