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.
ESM_2 Val (adult male, focal individual) sits and watches Julie, Jack, and Kathy from a distance. Upon seeing Julie with grass in her ear, Val picks a straw of grass (1:01) and puts it in his ear (1:03) after which he shivers. He puts the grass back in his ear (1:11) and remains seated while occasionally looking at Julie, Jack, and Kathy. (MP4 27112 kb)
ESM_3 Miracle sits close to Julie (who is holding a piece of grass in her ear), looks at her over her shoulder (0:03), picks a straw of grass (0:30), and attempts to put it in her ear (0:35). Around 0:59, Miracle again attempts to put grass in her ear, which becomes explicitly visible as of 1:14 (after which she is seen leaving the grass in her ear). (MP4 30533 kb)