Chimpanzees in the wild use gestures that are not common to the group, showing they can develop their own sign language, Dutch researcher Bas van Boekholt has found.

Van Boekholt, who is a PhD researcher at the University of Osnabrück, studied footage he took of the communication between a group of chimpanzees in Uganda and found that a mother and her daughter used a sign with a particular meaning. When the daughter wanted to make clear she wanted to travel on her mother’s back she would cover one of her mother’s eyes with her hand.

“This gesture has meaning but only for the mother and daughter,” Van Boekhold told Nu.nl. “The same can be seen among humans, a way of communicating which outsiders don’t understand,” he said. An example would be a baby’s language which no one except the parents understand.

Van Boekholt’s finding means chimpanzees don’t just display instinctive behavior with gestures that are used by the whole group but are able to invent their own individual signs.

Chimpanzees in captivity also learn new gestures but always in connection with human beings. “It’s the first time we see it happening in the wild,” Van Boekholt said,

The research will shed light on the evolution of communication, both in humans and their nearest relations, primatologist Iris de Winter said. “By studying the chimpanzees’ behaviour we can test hypotheses about the origin of human communication and the role of social interaction in this process,” she said.

Van Boekholt will be returning to Uganda in January to study how the mother and daughter pair combine gestures, facial expressions and sound.