The example of 'grass in the ear' trend becoming popular among chimpanzees suggests that the primates often form new traditions by copying the habits of one of their group members, though these traditions are often seen only among one specific group.
According to the researchers, a female chimp named Julie repeatedly put a stiff, strawlike blade of grass for no apparent reason in one or both of her ears and left it there even when she was grooming, playing or resting, after which other chimpanzees also followed suit.
The research team, including Zambians who monitor the chimpanzees daily, collected and analyzed 740 hours of footage that had been shot during the course of a year of 94 chimpanzees living in four different social groups in the sanctuary. Only two of these groups could see one another.
The observations show that there's nothing random about individual chimpanzees sticking grass into their ears. They spontaneously copied the arbitrary behavior from a group member. Chimpanzees have a tendency to learn from one another - clearly a case of "monkey see, monkey do" in fact.
The study was published in Springer's journal Animal Cognition.