Human language is considered distinct from other forms of animal communication because it is referential, context-specific, invokes higher order reasoning, and is voluntary, both on the sending and receiving end. Increasing evidence shows the flexibility and context-specificity of some non-human primate vocalizations. The evidence points to the possibility that cognitive ability was present in primate lineage before the beginning of language. Chimpanzee food calls are highly varied in acoustic structure and are context-specific—they are produced solely when approaching, gathering, or eating food. This study considered West African chimpanzees, P. troglodytes verus, and focused on whether food species, tree size, and fruit count influenced food call structure and if any variation changed nearby chimpanzees’ behavior (i.e. attracting them to a food patch). Based on research done on captive chimpanzees, the authors hypothesized that vocal sound structure will change in relation to the amount of food available and the food species. With increased food, calls were expected to have higher frequencies and longer durations. They also set out to determine to what degree food calls could be considered functionally referential to other chimpanzees who hear the call.
A young male chimpanzee, Pan troglodytes. Source: Wiki Commons
Different Pitches, Different Information
It was discovered that all pitch varied with respect to food species but did not vary in relation to tree size or fruit count. One fruit tree, especially, Nauclea diderichi, elicited higher pitch calls on average over any other species. Furthermore, it contained extensive call variation within its specific call. As tree size of Nauclea increased, the call pitch decreased. Finally, the number of individuals arriving at Nauclea tree decreased as the food call pitch produced for Nauclea increased.
West African chimpanzees modified their call structure with respect to external references, as was seen by captive studies on chimpanzees. Higher fundamental and peak frequencies in food calls correlate with higher preferences for the food. Nauclea elicited a high pitched food call and expressed the most variation in food call pitch associated with tree size. Nauclea was also, of all the fruit species considered, the food with the highest energy.
In terms of higher order thinking and the functionality of food calls, research has shown that wild chimpanzees are familiar with locations of numerous feeding trees within their territories and remember botanical knowledge to find ripe fruits. This study suggests that further research be conducted to determine the intentionality of wild chimpanzees’ food call modification, as well as whether the receivers rely on memory of feeding trees in their territories to decode meaning of a food call, rather than relying solely on the call.
Connection to Human Language
This study is significant in connecting the lines in the evolution of human language. Since it is not directly advantageous to alert others of food sources because it reduces individual food intake, the idea of food-specific vocalizations has interested many researchers. It is possible with larger groups, predator detection and food defense increase and social bonds are formed more deeply. However, if intentionality of modifying food calls becomes more certain with greater research, the theory for the origins of language that suggest foraging was the first context in which primitive language began would gain further support. These shifts in food calls may portray early example of communication in which vocal cues are purposefully and competently changed to express a different meaning (a meaning decided on by the individual). Even further, these different meanings are understood and acted upon differently by the receiver.
References
Kalan, A.K., Mundry, R., & Boesch, C. (2015) Wild chimpanzees modify food call structure with respect to tree size for a particular fruit species. Animal Behavior, 101, 1-9.
