Researchers studying wild chimpanzees have discovered that our closet living relatives convey information about different foods and the size of fruit trees to each other. Higher pitched calls were reserved for their favorite fruits, according to a new study published in Animal Behaviour. 

Previous studies on great ape communication—both gestural and vocal—were mostly conducted in captivity, a socio-ecological context that’s very different than in the wild. To study how chimps naturally communicate, a Max Planck Institute for Evolutionary Anthropology team led by Ammie Kalan recorded calls made by nine adults living in the Taï forest of Côte d’Ivoire in West Africa. They focused on food calls, which are produced only during foraging contexts, as they gathered feeding data such as the species, tree size, and the number of fruits. 

The team analyzed 379 calls produced for five different food species and found that food calls are context specific, acoustically graded vocalizations. Chimps modified the pitch of their call according to the fruit. In particular, they produced calls of a higher dominant and peak frequency for the highly valued fruit called Nauclea diderichi. "I never tried these fruits myself, but they do smell very good in the forest," Kalan tells Discovery News. "They are also quite big and easy to ingest, and we also know that they have a high energy content, which is important for wild animals." 

Additionally, the Nauclea fruit calls were further driven by tree size: Smaller trees elicited higher pitched calls. And nearby chimps paid more attention to lower pitched calls, which were associated with larger patches of food. “Those events tended to attract more out-of-sight individuals to join the on-going feeding event,” Kalan says in a news release.

Here’s an adult male named Woodstock eating Nauclea fruit. “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,” Kalan explains. 

Study co-author Christophe Boesch of Max Planck adds: “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.”

Images: shutterstock.com (top), Ammie Kalan (middle, bottom)