Parents and children, or people who know each other well, often share expressions peculiar to them – phrases and gestures that began by chance and gradually acquired a meaning known only to them.
The same is true for Beryl, a chimpanzee, and her young daughter Lindsay, who live in Kibale National Park, Uganda. When Lindsay wants to travel on her mother's back, she puts one hand over Beryl's eyes. This gesture is known not to be performed by other chimpanzees. It is their own private signature.
“There are so many words and gestures and things that are almost like inside jokes, meaning only to one person,” said Bas van Bockholt, a primatologist at the University of Zurich in Switzerland. “It happens very often to us humans, and now we know it happens to chimpanzees in the wild.”
Dr van Bockholt first noticed this behavior in 2022 during his second field season in a chimpanzee community called Ngogo in Kibale. Scientists have been working with them since the early 1990s. Chimpanzees are now so tame that researchers accompany them for hours at a time, often observing them from just a few yards away, and documenting their lives in great detail.
Dr. Van Bockholt is particularly interested in chimpanzee communication, especially gestures. Chimpanzees have a rich repertoire of languages, and while they may not technically qualify as languages, they certainly use them in ways that resemble languages. More than 80 gestures have been translated, including raising your palm and extending your arm to ask for food. Large, long scratches that encourage grooming. And stomping with both feet means “stop!”
Dr. van Bockholt said that when he saw Lindsey putting her hand over Beryl's eyes, “it was obvious she was doing this for the trip.” “That piqued my interest.” Such a gesture had never been documented before.
Dr. Van Bockholt and his colleagues reviewed recordings made before arriving in Kibale. According to recordings, Lindsay began making signs by putting her hands over her eyes when she was about 3 1/2 years old. Initially, the gesture did not serve as a request to leave on the mother's back. It started happening around the age of 4 and a half.
Several other young chimpanzees in the community were also observed performing this behavior, but none with any regularity or with the same intent.
Researchers don't know how Lindsay and Beryl's unusual interaction happened, but they have a theory. Like any dexterous toddler, Lindsay would have been moving her hands while riding on her mother's back, but Beryl is missing one eye. (Scientists don't know the backstory; when Beryl joined the Ngogo community in 2012, her eyes were already missing.) When Lindsay inevitably covered a good song, she was sure to elicit a reaction.
Perhaps this caused Beryl to repeat the same behavior. As the exchanges continued, it gradually took on meaning. What started as a way to tease mom while riding a bike has become a symbol of the vehicle.
In a study published in the journal Animal Cognition, Dr. van Bockholt and his colleagues put the chimpanzee story into context in the ongoing debate about the nature of chimpanzee gestures, and perhaps the roots of human language. It became.
Some researchers have suggested that the gestures of other great apes (chimpanzees, bonobos, orangutans, gorillas, and primates, including humans) are a fixed part of their species' biological inheritance. If so, gestures would be a relatively limited and inflexible means of communication, bearing no resemblance to language or human gestures. And because all chimpanzees utilize the same genetics, there are probably no examples of what primatologists call “idiosyncratic” behaviors that only one or two individuals use.
Other scientists argue that social learning is most important. This may involve observing and imitating the behavior of other chimpanzees. It may also involve the creation of a shared understanding of movements that were not originally communicated through the informal exchanges that occur when two individuals interact.
It would certainly be a more flexible, language-like system within which unique and idiosyncratic gestures are expected to occur. Lindsay and Beryl's hand-to-eye gesture seems to fit the bill. “We now know that not everything is set in stone,” says Simone Pica, a co-author of the new study and an animal behaviorist at the University of Osnabrück in Germany. “They are creating new signals.”
“Our DNA and chimpanzees' DNA is only 1 percent different, right?” Dr. Pica added. “So why are we always filling in the big gaps instead of saying, 'What do we share?'” And we're sharing gestures. ”
Cat Hovator, a primatologist at the University of St Andrews in Scotland who was not involved in the study, cautioned that the hand-on-eye gesture may not be technically unique. . Perhaps it's just a rarity. But it was clearly “formed as a specific expression between mother and daughter,” Dr. Hobeiter said.
Hobeiter says the nature-versus-nurture dichotomy that has characterized the discussion of chimpanzee behavior is evolving into a more nuanced recognition that both influences are important. Dr. Pica agreed.
Of course, Beryl and Lindsey's story is just one data point. As scientists collect more examples, this pair's private hand-eye code is a poignant reminder of how similar chimpanzees are to their closest living relatives.
“I can't help but notice how human this interaction is,” Dr. Van Buchhold said, adding of Lindsay: “I can't help but notice how human this interaction is. Mom is back. ”
