A chimpanzee puts his arm in front of a neighboring monkey in a video released by researchers. Is he showing off his muscles or asking for a friendly scratch? If you said he wants to be scratched, congratulations: you understand “monkey”.
And you are not the only one. According to a new study recently published in the journal PLOS Biology, many humans are good at understanding apes.
Although all apes vocalize to some extent, no other living species of ape has evolved a human-like spoken language. Instead, our primate cousins communicate using a language of physical gestures.
Kirsty Graham and Catherine Hobaiter, both primatologists at the University of St. Andrews in Scotland were inspired to question human understanding of ape language after realizing that 90% of the gestures used by chimpanzees and bonobos were shared by the two species, as well as 90% of those used by gorillas and orangutans.
“We’re wondering,” Graham said, “whether humans retain this ability to use and understand great ape gestures.”
Graham and Hobaiter developed an online questionnaire to test people’s ability to understand various monkey gestures. Test takers watched videos of one of them and chose what they thought the gesture meant from four options.
Thanks to an announcement of their study on BBC Radio 4 in Great Britain, the researchers had more participants than they expected: 5,656 people responded to the questionnaire.
Respondents correctly identified the meaning of the gestures more than half of the time, with a rate of 52.1%. This is much higher than the 25% that would be expected if participants were guessing at random.
“This study represents an interesting method of obtaining a human interpretation of monkey gestures,” said Frans de Waal, a primatologist at Emory University in Atlanta (USA), who was not involved in the research.
Humans also use non-verbal gestures; a librarian can silence a noisy student by putting a finger to his lips, or a rude customer in a restaurant can wave a waiter away as if he were swatting a fly.
Babies, especially, are known to be prolific gesturers: another study found that 89% of the gestures used by 1- to 2-year-old babies were also used by chimpanzees of all ages.
But as we age, Graham said, nuanced spoken language replaces many of these ape-like gestures, so adults can no longer speak “Ape” but can still understand it, as the new study shows.
In spoken language, Graham said, “we can ask for things more specifically, more politely, more precisely.” This is why an adult would not (hopefully) ask for food by sticking his hand in his mouth, but a baby would.
Perhaps ape gestures aren’t completely lost in adult humans, but are harder to recognize because they’re “sometimes embedded in other hand movements,” like the gestures we sometimes make while talking, De Waal said.
In their article, Graham and Hobaiter point out that the intertwining of our gestures with the use of spoken language makes it difficult to find clear uses of ape gestures by humans.
Charlotte Wiltshire, a doctoral student at St. Andrews, who works with the two researchers, is developing a machine learning program that could facilitate that effort.
“The dream is to one day automate video coding,” Graham said, referring to the process of watching videos of animals and noting observed behaviors. “We will have a trained model capable of detecting gestures. It doesn’t matter the degree of overlap between a chimpanzee’s gestures and ours,” said Graham. “Our ability to understand ape language underscores that we are more like them than different.”
“Humans are really cool, but we’re not unique in all of our abilities,” Graham said.
De Waal concurred: “If you’ve ever seen human children playing with young monkeys, you’ll notice a great deal of overlap in hand gestures and a high degree of mutual understanding and enjoyment, as if there were almost no barrier between them.”
Translated by Luiz Roberto M. Gonçalves
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