Clear plastic cups and pitchers adorned the wooden table in Des Moines, Iowa. Invisible juice was poured and presented to Kanzi, who enthusiastically chose the fake filled cup, playing along with the man who had come to visit. In many ways, it was the quintessential scene of a children’s imaginary tea party. Only Kanzi, at 44 years old, was a bonobo.
The experiment, carried out at the Ape Initiative facility in 2024, was the first to empirically test and document pretend play in a great ape species, with the results published in the journal Science in February. The study adds to an expansive repertoire of research over the past decade that has uncovered robust similarities between ape and human behaviours, upending long-held beliefs about how we distinguish ourselves from our closest kin.
“It seems to be a recurring thing in our field where people come up with reasons why humans are special and unique, and then scientists like me test it out, and we find that, actually, maybe we’re not that special after all,” says the study’s lead author, Amalia Bastos, a comparative psychologist at the University of St Andrews in Scotland. “That animals, too, are capable of secondary representations or imagination.”
Kanzi, who died last year, participated in several experiments with Bastos and her colleagues to test his ability to make believe. “We start all of these trials by saying, ‘Kanzi, let’s play a game. Let’s find the juice.’ That way we can at least verbally scaffold as much as possible,” Bastos says.
In one scenario, two cups were “filled” with juice, and then one was “emptied” into the jug. Kanzi was then asked to indicate which cup contained juice. He chose correctly in 34 out of 50 trials, suggesting he could grasp the concept of a pretend beverage. In another test, he was asked to choose between real orange juice and pretend juice. In 14 out of the 18 trials, he chose the cup containing real juice.
Kanzi’s ability to pretend – to envision a reality beyond the one in front of him – would have been unthinkable just a few decades ago. In the 1990s, scientists acknowledged that apes were intelligent, that they could solve puzzles and use tools, that they built strong social relationships and could learn symbols and sign language, and that they could pass the mirror test, recognising their reflections to suggest some level of self-awareness.
But primatologists and cognitive researchers were only just beginning to think about some of the more abstract and complex questions of culture, representation and theory of mind in great apes.
