Chimpanzees modify their food calls with respect to tree size for a high valued fruit species. The vocalization capabilities of our closest living relatives, the great apes, often pale in comparison to their flexible gestural repertoire. However, the vast majority of literature on great ape communication, gestural and vocal, comes from studies conducted in captivity where the surrounding environment is vastly different from the socio-ecological context in which wild apes naturally communicate.
Researchers of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, have now investigated a specific call type, the chimpanzee food call, which has already been shown to be produced solely during a foraging context, and a study in captivity has also provided evidence for the call being functionally referential to conspecifics.
“We recorded wild chimpanzee food calls from nine adult chimpanzees, male and female, along with detailed feeding data including the species, tree size, and quantity of fruits present upon arriving to a food source” says lead author Ammie Kalan. The data were collected using all day focal follows of individual chimpanzees from the habituated South group of the long-term Taï Chimpanzee Project in Côte d’Ivoire. “Using robust statistical analyses, our results show that chimpanzees produced food calls of a higher dominant and peak frequency for a particular food species, Nauclea diderichi, compared to four other species that were also included in the analysis. Additionally, the variation in food call structure for this fruit species was further driven by differences in Nauclea tree size, whereby larger trees produced food calls with lower dominant call frequencies, also known as call pitch” says Kalan.
“This is the first time chimpanzee vocalizations have been shown to significantly vary in acoustic structure with respect to naturally occurring food sources and their associated characteristics” says Kalan. Crucially, Nauclea fruits, like all five food species investigated in this study, were eaten by chimpanzees while sitting on the ground, meaning the size of the tree crown predicts the size of the patch where chimpanzees can find and feed on ripe fruits. Hence, differences in food call pitch for Nauclea trees could provide nearby conspecifics with information about the size of the patch, which may influence their own foraging behaviour. More specifically, whether they should join others to feed at the Nauclea patch since more individuals can feed at larger patches. “An important finding from our study shows that for Nauclea feeding events where the first food call recorded was one with a low call pitch, and thus associated with a larger food patch, those events tended to attract more out-of-sight individuals to join the on-going feeding event”, says Kalan.
In light of additional research from the Taï forest, we know chimpanzees have a sophisticated spatial memory of tree locations in their territory; therefore it is not surprising that food calls in this study were found to not be necessarily species-specific but patch-size specific. However, we caution that more food species should be included in the analysis since it appears that Nauclea was the fruit species with the highest energy content and was the food species that chimpanzees spent the most amount of time feeding on during the study period. This would suggest that Nauclea fruits may have been highly valued or preferred by the chimpanzees.
Therefore, it is worth investigating more food species that may also be of similar value, for other wild chimpanzee populations as well, to see if we observe similar food call modification as was observed in this study. “It also would be worthwhile to conduct similar studies on other populations to determine whether the results are specific to this group of chimpanzees or whether it is a more commonly observed vocal phenomenon” says Kalan. “This study highlights the often neglected contribution of ecological complexity as a driving force for flexibly modulated animal vocal communication, and potentially also for the advent of language within our own hominoid ancestry”, says Christophe Boesch.
This work is published in the journal Animal Behaviour.
Ammie K. Kalan, Roger Mundry, Christophe Boesch. Wild chimpanzees modify food call structure with respect to tree size for a particular fruit species, Animal Behaviour, Volume 101, March 2015, Pages 1-9, ISSN 0003-3472, DOI:10.1016/j.anbehav.2014.12.011.