Chimpanzees have been observed copying quirky social behaviours from one another – wearing grass in their ears and bottoms – despite these actions offering no apparent practical benefit.
It reveals that chimpanzees, like humans, are capable of socially transmitting arbitrary customs, suggesting that our closest relatives may share deeper roots with human culture than previously thought.
Researchers observed 147 chimpanzees across eight social groups living in the Chimfunshi Wildlife Orphanage in Zambia, Africa.
They documented two unusual traditions emerging within one of the closely bonded groups.
Within weeks, five of the eight chimpanzees in the group had started inserting grass or small sticks into their ears, while six began doing the same with their rectums.
These behaviours appeared spontaneously, spread rapidly, and were not seen in any of the 136 chimpanzees across seven other groups – with the exception of two individuals in a group known for a similar ‘grass-in-ear' trend observed over a decade ago.
The research was led by Dr Edwin van Leeuwen of Utrecht University, Netherlands, and involved Dr Jake Brooker of our Department of Psychology.
Dr Brooker said: “What’s remarkable is that these customs have no obvious utility.
“This isn’t about cracking nuts or fishing for termites – it’s more like chimpanzee fashion.
“It mirrors how human cultural fads spread: someone starts doing something, others copy it, and it becomes part of the group identity even if it serves no clear purpose.”
While social learning is well-documented in the animal kingdom, it typically focuses on behaviours that help with survival, such as hunting, tool use or foraging.
In the wild, similar “useless” trends such as the grass fad have not been seen in chimpanzees.
This may be because chimpanzees living in captivity have more free time than in the wild without the need to stay as alert or spend as much time searching for food.
The findings suggest that the capacity for shared, non-functional traditions may not be exclusive to humans.
The research has been published in the journal Behaviour and was supported by the Templeton World Charity Foundation.
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