Chimpanzees socially learn non-instrumental behaviour from conspecifics

Abstract Studying animal culture has been insightful for understanding the complexities of knowledge transmission and tracing human culture’s evolutionary origins. Most studies in this field have focused on material culture — behaviours that involve the use of tools and objects in ways that provide clear practical benefits to the individuals. We have previously documented a tool-use tradition without discernible function in which chimpanzees replicated the practice of inserting blades of grass in their ears from one persistent inventor. Now, over a decade later, we have observed an unrelated group of chimpanzees at the same African sanctuary, where five out of eight individuals began wearing grass in their ears and six out of eight from their rectums within a short period of time. Neither of these behaviours were observed in any of the seven other sanctuary groups ( ), except for two males in the original grass-in-ear group. Network-based diffusion analyses revealed a strong signature of social transmission for both variants. We conclude that chimpanzees adopted non-instrumental behaviours from each other and discuss how the study of social traditions without clear adaptive function could advance the field of comparative cultural evolution.