Overview

• Network-based diffusion analysis published in Behaviour confirms that grass-in-ear and grass-in-rectum behaviors spread through social learning rather than independent invention.
• The trends emerged independently in two groups at Chimfunshi Wildlife Orphanage Trust, with only those under caretakers who cleaned their own ears with grass or matchsticks adopting the fad.
• Researchers found no evidence that the grass served any practical purpose such as itch relief, classifying the actions as non-instrumental traditions.
• These arbitrary behaviors have not been observed in wild chimpanzees and appear only in captivity, where consistent provisioning frees time for experimentation.
• Study authors suggest the fads may strengthen social bonds within groups and offer comparative insights into the evolutionary origins of human cultural practices.