Kanzi the bonobo demonstrates symbolic play, challenging what we thought made humans unique

Kanzi is forty-three years old, a bonobo living in captivity, and he has just done something that scientists thought only human children could do. He has played an imaginary game—not with toys or props, but with the invisible contents of empty cups and bowls. The researchers Christopher Krupenye and Amalia Bastos, working from Johns Hopkins University, set up a simple test. They showed him two transparent cups and a pitcher, all of them empty. One of the researchers pretended to pour juice into both cups, then deliberately spilled one. When asked where the juice was, Kanzi pointed to the cup that had not been spilled—the one that existed only in the shared fiction between ape and human. Even when the researchers moved the imaginary-full cup around the table, Kanzi tracked it, distinguishing it from the other. The game broke only when they presented him with a cup containing actual juice, which he recognized without hesitation.

Symbolic play—the kind where a child sits with stuffed animals at a pretend tea party, pouring invisible liquid and having imaginary conversations—has long been held as a marker of human uniqueness. It requires the ability to hold two realities in mind at once: the physical world as it is, and the world as it is being imagined. Animals play, certainly. Puppies wrestle and chase. Young primates mimic their elders. But this kind of play, where nothing is real except the agreement that something is real, has never been documented in any species but our own. Until now.

The researchers chose Kanzi deliberately. He had already shown signs of what they call figurative capacity—the ability to point at things when given verbal signals, to understand that a symbol can stand for something else. Because bonobos have no interest in tea, Krupenye and Bastos adapted the classic children's game. They used juice pitchers and fruit bowls, objects Kanzi knew and had interacted with many times. In a second test, a researcher pretended to pluck imaginary grapes from a bunch and place them into cups. Then, with exaggerated hand movements, they pretended to empty the cups. When asked where the grapes had gone, Kanzi pointed to the spot where they had been pretend-emptied. There was nothing there. He was pointing at absence.

Miquel Llorente, a primatologist at the University of Girona, calls the study methodologically excellent and notes that it addresses a fundamental challenge in primate research: how do you prove that an animal is imagining something that does not exist? The answer, it turns out, is to watch where they point. But Llorente also sounds a note of caution. Kanzi is not a typical bonobo. He is, in his words, an extraordinarily enculturated genius—trained in artificial language, raised in a human environment, his cognitive development amplified by decades of interaction with researchers and caretakers. The study shows the ceiling of what a bonobo's mind can reach, not the floor of what bonobos in the wild actually do.

Juan Carlos Gómez, a psychologist at the University of St. Andrews, raises another possibility: perhaps Kanzi has simply learned the game. Perhaps he has figured out that when his caretakers present him with two empty cups and ask him to choose, the correct answer is to point at the one that has not been tipped over—because that is the one where something might invisibly remain. He may not be imagining at all. He may be solving a puzzle, reading the behavior of the humans around him, and responding with the answer they seem to want. The ambiguity matters. It is the difference between discovering that apes possess an innate capacity we thought was ours alone, and discovering that they are simply very good at learning what humans teach them.

Yet even with these caveats, the research points toward something significant. Great apes, bonobos especially, are closer to humans in their cognitive architecture than we have previously documented. They can plan for the future. They can attribute mental states to others—understand that someone else knows something, wants something, believes something. Kanzi's performance with the invisible juice is not necessarily proof that all bonobos imagine. But it is proof that at least one bonobo can. And that changes what we thought we knew about the boundary between human and animal minds.