The chimps, Brooker says, "have potentially copied it from a human who was walking by the enclosure, or one of the caregivers who was just going about their daily lives."

"Like with all cultures, things change over time and they get refined and new quirks and new traditions pop up," he said.
Chimpanzee influencers

In this case, the team traced the "new quirk" to a male chimp named Juma, who seems to have originated the grass-in-butt variation.

From there, the study shows, it spread rapidly to most of his groupmates within a week.

The same thing happened to Julie's group. She started putting grass in her ear in 2010, and pretty soon, seven other chimps were doing the same.

The phenomenon even continued after Julie died in 2013. The researchers observed Julie's group again for this new study, and found that two chimps, including Julie's son, were still wearing grass in their ears.
A chimpanzee sits on the ground and glances sideways toward the camera, a long reed of yellow grass hanging from its ear.

Much like humans, Brooker says the chimps appear to be willing to suffer for the sake of fashion.

"You see when they're learning this behaviour that it's quite uncomfortable," he said of the ear grass. "They shake their head and they rub the ear a little bit as if they're trying to get used to it."