“grass-in-ear behavior”

To figure out if this was really a tradition, and not just chimpanzees sticking grass in their ears at random, van Leeuwen and his colleagues spent a year observing four chimp groups in Chimfunshi Wildlife Orphanage Trust, a sanctuary in Zambia. Only one troop performed the grass-in-ear behavior, although all of the chimps lived in the same grassy territory. There’s no genetic or ecological factors, the scientists believe, that would account for this behavior — only culture.

Lydia Luncz, a primatologist at the Max Planck Institute in Leipzig, Germany, who was not involved with the research, agrees. This study shows how the chimpanzees who learned to put grass in their ears did so through the “natural transmission” of new behavior, she says.

The cultural quirk first popped up in 2010 when a chimpanzee, named Julie, was spotted sporting a long-stemmed piece of grass.

Julie acted as a role model for the other 11 chimpanzees in her group. As van Leeuwen points out, “everybody can wear rings in their ears, but you just have to come to the idea to do it.” The seven chimps who adopted the grass-in-ear tradition — and who would continue it after Julie’s death — repeatedly inspected her behavior before trying it themselves.

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