Food-associated calls in chimpanzees: responses to food types or food preferences?

Chimpanzees produce specific vocalizations, ‘rough grunts’, when encountering food, and it has been suggested that these calls vary acoustically depending on the food type discovered by the caller. Nearby listeners often behave as if the calls are meaningful to them, indicating that the calls may function as referential labels for particular foods. We investigated whether rough grunt variants are the result of callers responding to specific food types or relative food preferences. We recorded calls from captive individuals in response to nine different food items, which could be ranked as high, medium or low preference. Individuals consistently produced three acoustically distinct grunt variants to the three food preference classes. There was no evidence that chimpanzees produced individual labels for food types of low and medium preference. However, calls to high-preference food types differed significantly in their acoustic structure. These acoustic patterns remained stable over trials, suggesting that rough grunts have the potential to serve as semantic labels for individual high-preference food types. We were unable to replicate these findings with a set of recordings from the wild, although most other aspects of calling behaviour remained identical. We discuss these discrepancies between the wild and captivity and suggest that the emergence of referential labels for food items may be a by-product of the special circumstances found in captive settings.