In a series of tea party-like experiments, Johns Hopkins University researchers demonstrate for the first time that apes can use their imagination and play pretend, an ability thought to be uniquely human.
Consistently and robustly across three experiments, one bonobo engaged with cups of imaginary juice and bowls of pretend grapes, challenging long-held assumptions about the abilities of animals.
The findings suggest that the capacity to understand pretend objects is within the cognitive potential of, at least, an enculturated ape, and likely dates back 6 to 9 million years, to our common evolutionary ancestors.
"It really is game-changing that their mental lives go beyond the here and now," said co-author Christopher Krupenye , a Johns Hopkins assistant professor in the Department of Psychological and Brain Sciences who studies how animals think. "Imagination has long been seen as a critical element of what it is to be human but the idea that it may not be exclusive to our species is really transformative.
"Jane Goodall discovered that chimps make tools and that led to a change in the definition of what it means to be human and this, too, really invites us to reconsider what makes us special and what mental life is out there among other creatures."
The findings are published today in Science.
By age two, human children can engage in pretend scenarios, like tea parties. Even at 15-months-old, infants show measures of surprise when they see a person "drinking" from a cup after pretending to empty it.
There had been no controlled studies of pretense in nonhuman animals, despite several anecdotal reports of animals seemingly engaging in pretending behavior from both the wild and captivity.
For example, in the wild, young female chimpanzees have been observed carrying and playing with sticks, holding them like mothers would hold their infants. And a chimpanzee in captivity seemed to drag imaginary blocks along the floor after playing with real wooden blocks.
Krupenye and co-author Amalia Bastos , a former Johns Hopkins postdoctoral fellow who is now a lecturer at Scotland's University of St. Andrews, wondered if they could test this capacity to pretend in a controlled environment.
They created experiments very similar to a child's tea party to test Kanzi, a 43-year-old bonobo living at Ape Initiative, who had been anecdotally reported to engage in pretense and could respond to verbal prompts by pointing.
In each test, an experimenter and Kanzi faced one another, tea party-style, across a table set with either empty pitchers and cups or bowls and jars.
In the first task there were two transparent cups on the table, both empty, alongside an empty transparent pitcher. The experimenter tipped the pitcher to "pour" a little pretend juice into each cup, then pretended to dump the juice out of one cup, shaking it a bit to really get it out. They then asked Kanzi, "Where's the juice?"
Kanzi pointed to the correct cup that still contained pretend juice most of the time, even when the experimenter changed the location of the cup filled with pretend juice.
In case Kanzi thought there was real juice in the cup, even if he couldn't see it, the team ran a second experiment. This time there was a cup of real juice alongside the cup of pretend juice. When Kanzi was asked what he wanted, he pointed toward the real juice almost every time.
A third experiment repeated the same concept, except with grapes. An experimenter pretended to sample a grape from an empty container, then placed it inside one of the two jars. They pretend emptied one of the containers and asked Kanzi, "Where's the grape?" Kanzi again indicated the location of the pretend object.
Kanzi was never perfect, but he was consistently correct.
"It's extremely striking and very exciting that the data seem to suggest that apes, in their minds, can conceive of things that are not there," Bastos said. "Kanzi is able to generate an idea of this pretend object and at the same time know it's not real."
The findings inspire continued study, especially trying to test whether other apes and other animals can engage in pretend play or track pretend objects. The team also hopes to explore other facets of imagination in apes, perhaps their ability to think about the future or to think about what's going on in the minds of others.
"Imagination is one of those things that in humans gives us a rich mental life. And if some roots of imagination are shared with apes, that should make people question their assumption that other animals are just living robotic lifestyles constrained to the present," Krupenye said. "We should be compelled by these findings to care for these creatures with rich and beautiful minds and ensure they continue to exist."
