Functionally referential communication in a chimpanzee.

The evolutionary origins of the use of speech signals to refer to events or objects in the world have remained obscure. Although functionally referential calls have been described in some monkey species, studies with our closest living relatives, the great apes, have not generated comparable findings. These negative results have been taken to suggest that ape vocalizations are not the product of their otherwise sophisticated mentality and that ape gestural communication is more informative for theories of language evolution. We tested whether chimpanzee rough grunts, which are produced during feeding contexts, functioned as referential signals. Individuals produced acoustically distinct types of "rough grunts" when encountering different foods. In a naturalistic playback experiment, a focal subject was able to use the information conveyed by these calls produced by several group mates to guide his search for food, demonstrating that the different grunt types were meaningful to him. This study provides experimental evidence that our closest living relatives can produce and understand functionally referential calls as part of their natural communication. We suggest that these findings give support to the vocal rather than gestural theories of language evolution.