Once they adjust, he says, they appear largely unbothered. He likened it to people getting their ears pierced.

"There's not a clear benefit that wearing earrings really brings, but some kind of social cultural reason," he said. "I feel like it's similar with the grass in the ear."

It's an apt comparison, says Julie Teichroeb, a primatologist at the University of Toronto who wasn't involved in the study.

"It just looks like an earring, you know, like a fashionable way to present yourself," she said.
'They spend a lot of time looking at each other's butts'

And as for Juma's grass-in-butt variation?

Teichroeb says it's possible they're doing it to make themselves more attractive to potential mates. Females, in particular, she noted, display a swelling on their rear ends to indicate when they're receptive to a little hanky panky.

"They spend a lot of time looking at each other's butts," she said. "So it's kind of not surprising maybe that they were innovating this way to sort of decorate their butts."

Cultural differences are common among primates, and other animals too, but they often boil down to different methods of accessing food and other resources.

Because the Chimfunshi chimps have human caretakers who feed them, Teichroeb says they may have more free time to develop purely social trends.

"We think of, like silly, little pointless cultural ideas that spread amongst people," she said.

"Learning that animals have these kinds of same, pointless little behaviours that become fads and become viral, I think it really shows how closely related we are to them, how much kinship we actually share."

Brooker says it reminds him of the orcas who have recently been spotted wearing salmon on their heads like a hat — a behaviour last reported in the '70s.

"It re-emerged 40 years later, like flared jeans," Brooker said.

In that case, scientists also theorize the trend could be related to an abundance of food after many years of scarcity.