Overview

• A new grass-insertion trend emerged independently in mid-2025 among eight chimpanzees at Zambia’s Chimfunshi Wildlife Orphanage Trust, mirroring a behavior first documented in 2010.
• Within weeks, five chimps inserted blades of grass into their ears and six extended the trend by placing grass or sticks in their rears without any evident practical benefit.
• Network-based diffusion analysis led by Utrecht University researchers confirmed that these non-instrumental behaviors spread through social observation rather than individual innovation.
• Interviews with caretakers revealed they sometimes placed grass or matchsticks in their own ears, indicating human modeling influenced the chimps’ adoption of the fashions.
• The persistence of these arbitrary ornamentation habits underscores that non-functional cultural traditions can propagate among chimpanzees and sheds light on the evolutionary origins of fashion-like behaviors.