Versatile use of chimpanzee call combinations promotes meaning expansion

Language is a combinatorial communication system able to generate an infinite number of meanings. Nonhuman animals use several combinatorial mechanisms to expand meanings, but maximum one mechanism is reported per species, suggesting an evolutionary leap to human language. We tested whether chimpanzees use several meaning-expanding mechanisms. We recorded 4323 utterances in 53 wild chimpanzees and compared the events in which chimpanzees emitted two-call vocal combinations (bigrams) with those eliciting the component calls. Examining 16 bigrams, we found four combinatorial mechanisms whereby bigram meanings were or were not derived from the meaning of their parts-compositional or noncompositional combinations, respectively. Chimpanzees used each mechanism in several bigrams across a wide range of daily events. This combinatorial system allows encoding many more meanings than there are call types. Such a system in nonhuman animals has never been documented and may be transitional between rudimentary systems and open-ended systems like human language.