Dialects in leaf-clipping and other leaf-modifying gestures between neighbouring communities of East African chimpanzees

Abstract

Dialects are a cultural property of animal communication previously described in the signals of several animal species. While dialects have predominantly been described in vocal signals, chimpanzee leaf-clipping and other 'leaf-modifying' gestures, used across chimpanzee and bonobo communities, have been suggested as a candidate for cultural variation in gestural communication. Here we combine direct observation with archaeological techniques to compare the form and use of leaf-modifying gestures in two neighbouring communities of East African chimpanzees. We found that while both communities used multiple forms, primarily within sexual solicitation, they showed a strong preference for a single, different gesture form. The observed variation in form preference between these neighbouring communities within the same context suggests that these differences are, at least in part, socially derived. Our results highlight an unexplored source of variation and flexibility in gestural communication, opening the door for future research to explore socially derived dialects in non-vocal communication.

Figure 1

Number of observations of each leaf-modifying gesture form in each community by age/sex category. Intermediate forms (i.e., cases in which multiple forms are used within the same event) are presented in-between columns of two forms. (e.g., leaf-pull/leaf-tear). Illustrations show what the leaves look like after they have been modified. Moving from the bottom to the top these different forms become more audible. Pie chart colours represent the proportion of cases observed in each behavioural context (Sex, Pant-hoot/drumming, Play, or Unknown).