From Ears to Rears, a Group of Chimpanzees is Putting Grass in Strange Places, and Scientists Are Fascinated

In a wildlife sanctuary in Zambia, a peculiar trend has emerged among a group of chimpanzees: the appearance of blades of grass, stylishly left dangling from their ears and their behinds. Is this a sign of a new evolutionary milestone, or merely an apish fashion trend?

Detailed in a new peer-reviewed study, the researchers offer a rare window into the social lives and cultural quirks of our closest living relatives, raising some interesting questions about the origins of human culture.

The story begins over a decade ago, when researchers at the Chimfunshi Wildlife Orphanage Trust observed a female chimpanzee named Julie repeatedly inserting a blade of grass into her ear and leaving it there. The behavior had no obvious function. There was no food reward, no grooming benefit, no apparent purpose at all. Yet, it caught on. Other members of Julie’s group began to mimic her, and even after her death, the “grass-in-ear” habit persisted among a few individuals. In colloquial terms, it became “a thing.”

Fast-forward to 2023. In a different group at the same sanctuary, a male chimpanzee named Juma independently began the same odd ritual. He placed grass in his ear, much like Julie had done a decade earlier. Within a week, four more group members followed suit. But the trend didn’t stop there. 

In an unexpected twist, Juma began placing a blade of grass in his rectum and letting it hang out. This, too, was quickly copied by others, with six of the eight group members ultimately participating in the “grass-in-rectum” fad.

The study indicates that the researchers tracked the emergence and diffusion of both the grass-in-ear and grass-in-rectum behaviors. Using network-based diffusion analysis, a statistical method for mapping the spread of innovations through social networks, they found compelling evidence that these were not isolated inventions. Instead, the behaviors were socially transmitted, with chimps copying each other in a pattern reminiscent of human fashion trends. As the old adage goes, “monkey see, monkey do.”

This social learning was so robust that, for the grass-in-rectum behavior, the analysis estimated that 100% of the spread was due to social copying, rather than independent invention. The behaviors were not observed in any of the other 136 chimpanzees across seven other groups at the sanctuary, underscoring their group-specific, cultural nature.

So why would any chimpanzee put grass in its ear, let alone in its rear? 

The researchers and sanctuary staff found no evidence that the chimps were using the grass to alleviate discomfort, clean themselves, or achieve any practical goal. There were no signs of irritation, infection, or distress that might explain the behavior as a form of self-medication or relief.

Instead, the most plausible explanation is that these are arbitrary, non-instrumental behaviors, much like human fashion fads, rituals, or social customs that serve no direct survival function, but may help reinforce social bonds and group identity. 

“It could also serve a social purpose,” explained Utrecht University researcher Edwin van Leeuwen in a press statement. “By copying someone else’s behaviour, you show that you notice and maybe even like that individual. So, it might help strengthen social bonds and create a sense of belonging within the group, just like it does in humans.”

In this sense, the grass trend echoes the seemingly pointless but socially meaningful trends that pervade human culture, such as Gen Z kids wearing socks and sandals, to body piercings and tattoos, or wearing makeup.

While chimpanzees are well-known for their tool use and foraging traditions, the behaviors of grass-in-ear and grass-in-rectum have no clear utility. Instead, they suggest that the roots of humanity’s ability to adopt, transmit, and sustain arbitrary social customs may run deeper in the animal kingdom than previously thought.

One mystery still remained for the researchers. Why did the chimps begin this curious ritual in the first place?

Oddly enough, there is evidence that the original grass-in-ear behavior may have been inspired by the chimps’ human caretakers, who sometimes used blades of grass or matchsticks to clean their own ears in view of the chimps. 

Meanwhile, the researchers say that the grass-in-rectum twist appears to be a purely chimpanzee innovation, highlighting the creativity of these animals. 

The emergence and spread of these behaviors provide a rare example of non-instrumental, socially learned traditions in non-human animals. And this opens new avenues for understanding the evolution of culture, social learning, and the subtle ways in which group identity is constructed and maintained in the animal world.

Moreover, it helps shape our own understanding of what it means to be human.

“This shows that, like humans, other animals also copy seemingly pointless behaviours from one another,” van Leeuwen said. “And that, in turn, may offer insights into the evolutionary roots of human culture.”