Wild bonobos – our closest living relatives – communicate using vocal calls organized in compositionally complex semantic structures that mirror key features of human language, according to a new study. The findings challenge long-held assumptions about the uniqueness of human language and open new avenues for understanding the evolution of communication. 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 (the smallest unit of language with meaning) 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.
Here, Mélissa Berthet and colleagues report strong empirical evidence that wild bonobos (Pan paniscus) use nontrivial compositionality in their vocal communication. Berthet et al. 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 – the authors 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. Berthet et al. 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.
