A chimpanzee named Juma turned out to be quite the trendsetter. He picked up a blade of grass and stuck it in his ear, a trend that was followed by several other chimps. Then, days later, he stuck another blade into his rectum. This too caught on among members in his group.
There’s no benefit or clear purpose for it, it’s just a fashion fad. This is what makes it so cool: it shows that chimpanzees, like humans, share cultural behaviors that serve no obvious purpose other than reinforcing social bonds. In fact, this suggests that the evolutionary roots of human culture might run deeper than we thought.
Culture isn’t always useful
The grass fashion isn’t new. In 2010, a chimpanzee named Julie at Zambia’s Chimfunshi Wildlife Orphanage began sticking blades of grass in her ears. At first, scientists thought it could be some tool usage; it wasn’t. Julie wasn’t using the blades of grass for anything and she wasn’t rewarded with food or praise. But she kept doing it and over time, several others in her group adopted the same habit.
At the time, scientists described it as a cultural tradition — a socially learned behavior that had no clear survival benefit but was passed on anyway. It looked less like tool use and more like fashion.
Fast-forward to 2023, in a different group of chimps at the same sanctuary. A male named Juma spontaneously began inserting grass into his ear the same way Julie had. Within days, four others followed his lead. Then came the twist.
Juma was soon observed placing grass into his rectum and leaving it there, hands-free. The fashion fad was brought back, but with a new spin. Within six weeks, five more chimps copied him.
These behaviors, which researchers called grass-in-ear (GIEB) and grass-in-rectum (GIRB) weren’t observed in any of the other 136 chimpanzees across seven other groups at the sanctuary. Only two chimps, both from Julie’s original group, still did the ear thing.
“This shows that like humans, other animals also copy seemingly pointless behaviors from one another,” says Utrecht University researcher Edwin van Leeuwen. “And that, in turn, may offer insights into the evolutionary roots of human culture.”
Three female chimpanzees of varying ages in one of Chimfunshi’s diverse social communities. Credit: Jake Brooker/Chimfunshi Wildlife Orphanage Trust.
So why do they do it?
The short answer is we’re not really sure, but there’s probably no practical reason; they just find it fun.
To understand how the behaviors spread, researchers used a method called network-based diffusion analysis. It’s basically a way to study animals and map how ideas or actions travel from one to the other. The results were clear: the chimps weren’t independently inventing these quirks, they were learning them from each other. In fact, for GIRB, the analysis estimated 100% of the spread came from social learning.
The original grass in the ear behavior may have originated in human caretakers.
“Both groups, where chimps put blades of grass in their ears, had the same caretakers. These caretakers reported that they sometimes put a blade of grass or a matchstick in their own ears to clean them,” Van Leeuwen says. “Caretakers in the other groups said they did not do this. The chimps in one group then figured out how to stick the blade of grass in another place as well.”
The grass in butt, however, appears to be a purely chimpanzee fashion invention.
This cultural behavior is all the more remarkable as chimps seem to pick it off from one another and from other species (like humans).
This is a remarkably human behavior
In human society, people do all kinds of things that serve no practical purpose. We wear stylish clothes, pierce our ears, follow TikTok trends. These customs, researchers argue, promote cohesion, identity, and group belonging.
That might be what’s happening here. The chimpanzees in question are relatively young and recently integrated into a new social group, which is prime conditions for trying to fit in.
“In captivity, they have more free time than in the wild. They don’t have to stay as alert or spend as much time searching for food,” Van Leeuwen explains. “Why they do exactly this particular thing, I’m not really concerned about. But them copying the behavior from each other, that is the important insight.”
That’s a profound insight, because human culture is built on exactly that foundation. From language to religion to fashion, much of what defines human life is socially learned and not necessarily useful in a material sense, but cultural.
In other words, this isn’t just a cute story about quirky chimps. It’s a window into the evolutionary roots of culture, and chimps seem to share the same inclination to culture as we do. If non-human animals can invent, copy, and sustain traditions with no clear utility, then the building blocks of human social life may go deeper than we thought.
Dr. Andrei Mihai is a geophysicist and founder of ZME Science. He has a Ph.D. in geophysics and archaeology and has completed courses from prestigious universities (with programs ranging from climate and astronomy to chemistry and geology). He is passionate about making research more accessible to everyone and communicating news and features to a broad audience.
