"Our observation is quite unique in the sense that nothing seems to be communicated by it," says study author Edwin van Leeuwen, a primate expert at the Max Planck Institute in The Netherlands.
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.