Clear plastic cups and pitchers adorned a wooden table in Des Moines, Iowa. Invisible juice was poured and presented to Kanzi, who enthusiastically chose the fake-filled cup, playing along with his visitor. This was the quintessential scene of a children's imaginary tea party, except Kanzi was a 44-year-old bonobo.
The experiment, conducted at the Ape Initiative facility in 2024, was the first to empirically test and document pretend play in a great ape species. Results published in Science in February 2025 add to a growing body of research over the past decade that has uncovered robust similarities between ape and human behaviours, upending long-held beliefs about what distinguishes humans from their 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," said study lead author Amalia Bastos, a comparative psychologist at the University of St Andrews. "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. In one scenario, two cups were "filled" with juice, then one was "emptied" into a jug. Kanzi correctly indicated which cup contained juice in 34 out of 50 trials, suggesting he grasped the concept of a pretend beverage. In another test, he chose real juice over pretend juice in 14 out of 18 trials.
Transforming Our Understanding of Ape Minds
Kanzi's ability to pretend would have been unthinkable just a few decades ago. In the 1990s, scientists acknowledged that apes were intelligent, could solve puzzles, use tools, build strong social relationships, learn symbols and sign language, and pass the mirror test. But researchers were only beginning to explore abstract concepts like culture, representation, and theory of mind in great apes.
In recent years, scientists have unearthed dozens of stunning findings. Chimpanzees and bonobos can remember past groupmates for decades. When presented with stronger evidence, chimpanzees rationally revise their beliefs. Western lowland gorillas engage in kissing behaviours. Orangutans, bonobos, chimpanzees, and gorillas playfully tease one another. Bonobos cooperate across social borders. And chimpanzees show a quirky fascination with crystals, akin to human "New Age" folk.
Advancements in Theory of Mind
Scientists have made significant strides in understanding apes' theory of mind, once believed to be a distinctly human trait. Theory of mind is the ability to understand that others have their own thoughts, beliefs, desires, intentions, and knowledge that may differ from one's own.
"In the last several decades, we've seen transformative insights from different research groups that all point to the idea that chimpanzees and other apes are extremely sensitive to their social partners," said Christopher Krupenye, a cognitive scientist at Johns Hopkins University. "Thirty years ago, the dominant view was there is no evidence whatsoever. Today, the consensus is that there is an exciting capacity here."
The Rational Mind
Greek philosopher Aristotle defined humans as "the rational animal," possessing a unique capacity for reason. But comparative psychologist Hanna Schleihauf of Utrecht University questions what rationality truly means. A 2025 study of semi-captive chimpanzees at Ngamba Island sanctuary in Uganda tested whether chimpanzees revise beliefs as evidence changes.
Researchers presented chimpanzees with two boxes, rattling one to indicate food might be inside. After the chimp chose, they turned the second box to reveal something resembling an apple. The chimp could then choose again. If rational, it would change its mind because the first choice was based on weak evidence. The chimps did just that, showing humans are not the only rational beings.
Long-Term Memory and Recognition
Krupenye, co-author on the Kanzi imagination study, tested long-term memory in zoo-housed chimpanzees and bonobos. Using non-invasive eye tracking, they showed images of former groupmates alongside strangers. Apes spent more time looking at former social partners, with one bonobo recognizing individuals not seen for 26 years.
Cultural Diversity in the Wild
To understand the full range of great ape intelligence, researchers look to the wild. In June 2022, primatologists observed an adult male Sumatran orangutan named Rakus chewing leaves of a liana vine known for antibacterial properties and applying the paste to a facial wound. The wound healed completely, marking the first report of active wound healing with a plant in a wild animal.
Isabelle Laumer, a cognitive biologist at the Max Planck Institute, has studied playful teasing across four great ape species and compared exploratory behaviour in zoo-housed and wild orangutans. She found that zoo-housed orangutans were more likely to explore objects, likely because they have all limbs free on the ground, unlike wild orangutans that must hold onto trees.
Kristin Andrews, a philosophy professor at the City University of New York, focuses on social cognition and animal minds. She notes that chimpanzees have unique cultures differing between groups. For example, in one community, biting a leaf means play, while in another, it signals sex. Similarly, some groups use wooden hammers, others stone hammers. These cultural variations have serious implications for conservation strategies.
In a February 2025 paper in Learning and Behavior, Andrews questions whether protecting animal cultural diversity should become a new conservation goal. "Should culture be an important consideration in prioritising populations for conservation?" she asks. Conservation often focuses on preventing extinction, but unique cultures can be lost if a particular group is wiped out. "If we preserve chimpanzee DNA somewhere, but that organism doesn't know anything about being a chimpanzee, that's not a chimpanzee," she says.
The seven great ape species—Bornean orangutan, Sumatran orangutan, Tapanuli orangutan, eastern gorilla, western gorilla, chimpanzee, and bonobo—are all listed as endangered or critically endangered. Time may be running out to fully understand their inner worlds. As Krupenye puts it, "Part of the reason to study great apes is that they are our closest relatives, so we're learning something about ourselves. But we're working with these remarkable creatures with rich mental lives that have so much more going on under the surface than people give them credit for."
