July 10, 2025, 4:09 am | Read time: 6 minutes

How Does Culture Develop–and Must It Always Be Useful? A new study involving chimpanzees offers surprising insights. Some animals suddenly began to carry blades of grass in their ears or even in their anus–without any apparent purpose. What initially seems like a quirky habit turns out to be a prime example of cultural learning among animals–and sheds new light on the origins of human rituals.

The researchers analyzed how these novel habits emerged and spread. They demonstrated that social learning processes–not individual benefit–were central. The study was published in 2025 in the journal “Behaviour” and offers fascinating insights into the cultural diversity of non-human primates.

Do Chimpanzees Stick Blades of Grass in Their Ears as a Sign of Social Belonging?

Traditionally, research on animal culture focuses on so-called material culture–the use of tools. This provides a clear survival advantage. This includes nut-cracking by chimpanzees or tool use by crows. Increasingly, however, another aspect is coming into focus: social traditions without an apparent purpose, known as non-instrumental behaviors. Examples range from hand-holding in apes to play behavior in capuchin monkeys.

These behaviors might primarily serve social bonding, as Edwin van Leeuwen, a researcher at Utrecht University, explains in the science magazine “Phys.org.” “By copying another person’s behavior, you show that you notice and perhaps even like that person. It could therefore help strengthen social bonds and create a sense of belonging within the group, just as it does with humans.”

As early as 2014, researchers in Zambia documented such a phenomenon for the first time. Back then, a chimpanzee named Julie began sticking blades of grass in her ear. Her group members adopted the behavior–apparently without functional benefit. The current study builds directly on this. It not only documents a recurrence of this ear-grass tradition but also the emergence of a new variant: the so-called “Grass-in-Rectum” behavior. Both show that culture in animals doesn’t always have to be practical–it can also be an expression of social belonging.

Also interesting: How Human Influence Prevents Chimpanzee Sex

Animals Were Observed Passively

The researchers observed 147 chimpanzees in eight groups over about a year (May 2023 to October 2024) at the Chimfunshi Wildlife Orphanage Trust in Zambia. The focus was on a group of eleven animals where the two non-instrumental behaviors developed: inserting grass into the ear (“GIEB”) and into the rectum (“GIRB”).

The animals live in large, naturalistic enclosures where they feed, sleep, and socialize independently. Observations were purely passive–there were no interventions in housing or feeding. To analyze social learning behavior, the scientists used a statistical method called network-based diffusion analysis (NBDA). This method examines whether a behavior spreads according to social relationships within a group. This would be a clear indication of social imitation.

Grass in the Rectum Became the “Newest Trend” with Many Imitators

In the studied chimpanzee group, five of eight animals began sticking grass in their ears within a few days in August 2023–even though this behavior had only been documented in another group ten years earlier. Shortly thereafter, a never-before-seen behavior emerged: six animals inserted grass into their rectums.

In the seven other groups with a total of 136 animals, neither of these behaviors was observed–except for two male animals that belonged to the original GIEB group.

The network analyses clearly showed that both behaviors spread through social relationships. For the ear-grass behavior, a social learning share of about 83.6 percent was estimated, and for the rectum-grass behavior, even 100 percent. The age and gender of the animals had no significant impact on the spread. The study authors conclude that both behaviors did not arise by chance or individual inclination but through imitation within the group–despite having no apparent practical use.

Peer Pressure Could Play a Role in Behavior Adoption

This study provides strong evidence that chimpanzees are capable of adopting seemingly socially “pointless” behaviors through imitation–a phenomenon that has been little documented so far. It challenges existing assumptions that non-human animals only learn functional actions from each other.

Instead, the results suggest that social belonging or peer pressure can also play a role in behavior adoption–similar to human fashion trends or rituals. The research also suggests that social learning ability is not strictly tied to functional benefits but is a flexible tool of social dynamics.

Phenomenon Observed Only in Captivity So Far

So far, the phenomenon of chimpanzees sticking a blade of grass in their ear or rectum has only been observed in captivity. The researchers also have a hypothesis as to why this is: “In captivity, they have more free time than in the wild,” says Edwin van Leeuwen. The animals don’t have to remain as vigilant or spend as much time foraging. “Why they do this particular thing doesn’t really concern me. But that they copy the behavior from each other, that’s the important insight.”

This opens new perspectives on the origins of tradition and culture–not only in chimpanzees but also in the context of human evolution. The study thus expands the understanding of how and why certain behaviors are passed on in animal groups–even if they don’t offer an immediate survival value at first glance.

Do Blades of Grass in Chimpanzee Ears Really Serve No Purpose?

The study is based on intensive observations under natural conditions in a recognized animal welfare project and meets high ethical standards. Its strength lies in the combination of qualitative documentation and quantitative analysis using the NBDA method, which can reliably uncover social learning effects. Nevertheless, the study remains limited in some respects: The exact nature of social learning (e.g., imitation vs. attention sharing) cannot be clearly determined.

Moreover, it remains unclear whether the behaviors actually serve no hidden purpose–such as irritation relief in the ear or anal area. However, no medical symptoms were observed that would support such functions. Possible social advantages–such as increased attention–were also not measured. The study is limited to one chimpanzee group; general statements about other populations are therefore not readily possible.

Another aspect concerns alternative theories like the “Zone of Latent Solutions.” They assume that animals do not imitate but merely develop their own behaviors through stimulation. The authors critically engage with this theory but also emphasize that further long-term monitoring is needed to capture any traditions and their long-term dynamics.

More on the topic

Study Shows Chimpanzees use leaves to wipe themselves and clean up after sex

Loss of Pick-up Gestures How Human Influence Prevents Chimpanzees from Having Sex

According to the study Orangutans Treat Their Wounds With Self-Made Remedy

Conclusion: Animal Culture Is More Diverse Than Previously Thought

This study shows that chimpanzees can not only develop new, non-functional behaviors like wearing a blade of grass in the ear but also pass them on through social imitation. This challenges previous assumptions about the purposefulness of animal culture. The results provide valuable insights into how social practices–similar to fashions or rituals in humans–can also develop in animals.

Long-term observations could reveal whether stable traditions develop from this and what social advantages they might bring. The research impressively demonstrates: Animal culture is more diverse than previously thought–and it doesn’t always have to be useful to be significant.