A pointless fashion trend or something else? Chimpanzees wear blades of grass in their ears and rears

A team of researchers from Utrecht University, Durham University, and other institutions have observed something remarkable at a chimpanzee sanctuary in Zambia. Several chimpanzees from one particular group were seen dangling blades of grass from their ear holes or their behinds, for no apparent reason. The behavior was not seen in other chimpanzee groups at the same sanctuary, despite similar living conditions.

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

People regularly do arbitrary things that seem to have no immediate use, like shaving a line into their eyebrow or putting a fashionable scarf on themselves or their dog. Most people do not come up with these things themselves, but copy them from others.

Other animals also adopt behaviours from one another. Often, though, this is useful behaviour, like chimpanzees learning from each other how to find food. But sometimes, animals develop habits that seem to serve no clear purpose. For instance, a fashion trend among orcas, who were seen wearing a dead salmon on their heads, drew quite a bit of media attention last year. Still, these kinds of "useless" trends in animals have rarely been studied in a systematic way.

In 2010, researchers discovered that a female chimpanzee at the Chimfunshi Wildlife Orphanage Trust sanctuary repeatedly stuck a blade of grass in her ear and left it there, for no apparent reason. Later, seven of her group members adopted the behaviour. And even after the female trendsetter died, the behaviour continued, and some chimps in the group still do it today. Researchers therefore interpreted this behaviour as a cultural tradition.

Part 1

More than a decade later, chimpanzees in another group at the same sanctuary also began sticking blades of grass in their ears. They did not copy this behavior from the earlier group, since they had no contact with them. But in the new group, the grass-in-orifice trend did not stop there. While five of the eight chimpanzees in the new group stuck grass in their ears, six of the eight also let a blade of grass dangle from their behinds.

The researchers found no evidence that the chimpanzees were bothered by their ears or behinds and used the blades of grass, for example, for relieving an itch.
By carefully tracking which animals displayed the behavior over time, the researchers showed it was likely that the animals did not each invent the behavior on their own, but copied it from one another. When the researchers looked deeper into how the behavior started, they found something striking.

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. 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.

In the wild, similar "useless" trends have not been seen in chimpanzees. So why do they do it in captivity?
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, say the researchers.
The question of why humans are more culturally evolved than other animals is still being debated by scientists. Some scientists think the key lies in humans' unique ability to copy, including small, seemingly useless details. Other animals would not be able to do this and would have to constantly reinvent the wheel, which limits their cultural evolution.
But this study shows that chimpanzees are able to copy small, useless behaviours from each other.
And this behaviour could also serve a social purpose. By copying someone else's behavior, 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.

 Edwin J.C. van Leeuwen et al, Chimpanzees socially learn non-instrumental behaviour from conspecifics, Behaviour (2025). DOI: 10.1163/1568539X-bja10313

Part 2

Vampire bats’ mutual grooming helps spread innovative rabies vaccine

A gel that bats lick off one another’s fur could help prevent rabies outbreaks in cattle, a growing problem in Latin America

Bat vaccine can be spread lickety-split

An oral vaccine could curb rabies infections among vampire bats (Desmodus rotundus) in Central and South America. The vaccine is applied to the bats’ fur in a thick gel. The bats can then spread the vaccine among themselves through mutual grooming — licking one another’s fur to keep clean. In a small test, researchers applied the gel to 24 bats in a colony of 117. After seven days, they found that the vaccine had been spread among 88% of the colony. Vaccinating the bats against rabies could stop them from spreading the virus to farm animals without resorting to harmful measures such as poisons.

https://www.biorxiv.org/content/10.1101/2025.06.03.657068v2

https://www.science.org/content/article/vampire-bats-mutual-groomin...