Wild chimpanzees modify food call structure with respect to tree size for a particular fruit species
The extent to which animal vocalizations are referential has long been debated since it reflects on the evolution of language. Our closest living relative, the chimpanzee, has been shown to have functionally referential food calls in captivity but evidence for such capabilities in the wild is lacking. We investigated the context specificity and function of West African chimpanzee, Pan troglodytes verus, food calls in the wild using all day focal follows of adult males and females of one habituated group in the Taï forest, Côte d’Ivoire. We collected over 750 h of observation and analysed 379 food calls produced for five different food species and found that higher pitched calls were produced for a single fruit species. Additionally, within this species, chimpanzees modified calls according to tree size, whereby smaller trees elicited higher pitched calls. Our results suggest that chimpanzees subtly vary the acoustic structure of food calls with respect to food patch size for a putatively highly valued fruit species, and we propose that arousal alone cannot sufficiently explain the patterns observed. Further work is needed to determine whether variation in food call pitch can influence receiver foraging behaviour. However, in light of our results, we propose that understanding the information content encoded by acoustic variation in chimpanzee food calls requires receiver knowledge about the natural ecological context, specifically spatial memory of tree locations. Therefore, this study highlights the potential significance of feeding ecology in the evolution of flexibly modulated vocal communication.