A group-specific arbitrary tradition in chimpanzees (Pan troglodytes).

Social learning in chimpanzees has been studied extensively and it is now widely accepted that chimpanzees have the capacity to learn from conspecifics through a multitude of mechanisms. Very few studies, however, have documented the existence of spontaneously emerged traditions in chimpanzee communities. While the rigour of experimental studies is helpful to investigate social learning mechanisms, documentation of naturally occurring traditions is necessary to understand the relevance of social learning in the real lives of animals. In this study, we report on chimpanzees spontaneously copying a seemingly non-adaptive behaviour ("grass-in-ear behaviour"). The behaviour entailed chimpanzees selecting a stiff, straw-like blade of grass, inserting the grass into one of their own ears, adjusting the position, and then leaving it in their ear during subsequent activities. Using a daily focal follow procedure, over the course of 1 year, we observed 8 (out of 12) group members engaging in this peculiar behaviour. Importantly, in the three neighbouring groups of chimpanzees (n = 82), this behaviour was only observed once, indicating that ecological factors were not determiners of the prevalence of this behaviour. These observations show that chimpanzees have a tendency to copy each other's behaviour, even when the adaptive value of the behaviour is presumably absent.