An eight-year-old juvenile chimpanzee named Kakama trudged along a path among the forest trees, following his pregnant mother. A scientist sat silently at a distance, watching Kakama pick up a log and carry it with him for hours. At one point, Kakama made a nest and placed the log in it, as if it were a small chimpanzee. Months later, two field assistants observed the same thing: Kakama was playing with a similar log, which they labelled "Kakama's toy baby."

Was Kakama simply confused? Did he really think that the log was a smaller version of himself? Or did Kakama know that the log was really a log, and was only pretending that the log was a baby?

Kanzi, the famous bonobo, liked to pretend as well. Primatologist Sue Savage-Rumbaugh described watching Kanzi hide invisible objects under blankets or leaves, later removing them from their hiding spots, and pretending to eat them. "Kanzi also engages the participation of others" in these games, Savage-Rumbaugh notes, "by giving them the pretend object and watching to see what they do with it."

From an early age, human children act out imagined scenes that conflict with reality. Psychologist Robert W Mitchell calls children "proto-typical pretenders", and he writes that pretend play, or make-believe, is "a mental activity involving imagination". Which is, admittedly, useless as a definition.

Dreams could be thought of as being one form of imagination. When researchers measured the brain activity of rats as they were learning to navigate a maze, they saw the same firing pattern while they were asleep as when they were awake. The rats were running through the mazes in their sleep – it was as if someone had pressed the rewind button on a brain activity recorder, and pressed play.

But pretending or "make believe" requires a bit more mental complexity than that. One kind of pretence involves imagining that one object, such as a banana, is actually a second type of object, such as a telephone, or imagining that a lifeless object such as a doll is actually animate – both of which were observed with Kakama.

Flights of fantasy

Another type of pretence involves imagining an object that isn't even there in the first place, such as when children (or adults) play air guitar. An illuminating example of this sort of imagination comes from a chimpanzee named Viki who was raised in a human home. Viki had lots of toys, including some attached to strings that could be pulled along. Primatologists Mary Lee Jensvold and Roger Fouts recount the original description of Viki's behaviour: "Very slowly and deliberately she was marching around the toilet, trailing the fingertips of one hand on the floor. Now and then she paused, glanced back at her hand, and then resumed her progress… She trudged along just this busily on two feet and one hand, while the other arm extended backward this way to pull the toy. Viki had an imaginary pulltoy!" And not only that. Viki sometimes acted as if her pulltoy had got stuck on something. She tugged on the invisible string until she imagined that the toy had gotten free. Once, when her invisible toy was "stuck", she waited until her human caregiver pretended to free the toy, before continuing to play with it.

Some of the more charming examples of animal imagination come from the female gorilla Koko, who was trained to use American Sign Language. Koko routinely pretended that her dolls were her companions, frequently tried to nurse them, and often signed to them, sometimes giving them instructions. In one instance, a five-year-old Koko orchestrated an exchange between two toy gorillas, one blue and one pink. First, looking at the pink gorilla, she signed BAD BAD and then KISS towards the blue one. She then instructed the pair of toys to CHASE and TICKLE before smacking the two dolls together. After wrestling with each doll, Koko stopped and signed, GOOD GORILLA. GOOD GOOD.

Another time, a caregiver showed the ten-year-old Koko a photo of a bird in a magazine. THAT ME, Koko signed. "Is that really you?" KOKO GOOD BIRD, she responded. "I thought you were a gorilla." KOKO BIRD. The caregiver asked, "you sure?" Koko responded, pointing to the bird, KOKO GOOD THAT. "Okay, I must be a gorilla," the caregiver said. BIRD YOU, the gorilla signed. "We're both birds?" Koko responded by signing GOOD. "Show me," the caregiver prodded. FAKE BIRD CLOWN. "You're teasing me. What are you really?" Finally, Koko gave in, with a laugh: GORILLA KOKO.

By pretending to be a bird, Koko is doing something that the youngest human infants can't do. Young infants attempt to grasp objects in pictures as if they're really there. But by nineteen months, on average, grasping is replaced by pointing. By that age, human infants begin to understand that a picture is a representation of another object, not the object itself, much the same as a ten-year-old Koko understood that she wasn't truly a bird.

On the other hand, children under four years seem able to pretend in some instances but not in others. For instance, three-year-old children routinely agree that a balloon on a television screen would float to the ceiling if the top of the set was removed.

Koko, too, in some instances couldn’t distinguish between pretence and reality. In one interaction, a caregiver brought a toy dinosaur and hid it between her legs as she sat with the eleven-year-old ape. The caregiver poked the toy dinosaur out from behind her legs, causing Koko to jump backwards. "I scared you!" said the caregiver, "what's this?" Koko answered FAKETOOTH FAKE. "Yes, it's a fake alligator." Dinosaurs, lizards and alligators are all labelled "alligator" in Koko's vocabulary. After playing with the toy for a while, the caregiver asked, "You like it? You want it?" Koko responded GOOD, but still she didn't take it. The caregiver pretended as if the toy bit her own finger, shouting "Ow!" as if in pain. TOILET STINK, Koko replied. "Give me your finger, Koko," the caregiver instructs. But Koko instead offered a toy doll, letting it get bitten instead. "You funny Koko, let monster bite doll instead of you. Let's try being nice to it. It's a nice animal." The caregiver kissed the doll, to which Koko responded with the sign FAKETOOTH. "You want to kiss it, be nice?" While Koko eventually, though cautiously, kissed the dinosaur, she quickly withdrew.

Koko seemed aware that the dinosaur was only a toy, using the signs FAKE and FAKETOOTH, both of which she regularly used to indicate that objects aren't real. However, she was also scared of it, acting quite wary about touching or kissing it. On some level, perhaps, she thought that the toy dinosaur could actually harm her.

Mind games

Like a human child under four years of age, it might be that Koko didn't understand the concept of pretending, or didn't apply her knowledge of pretence uniformly. Despite the fact that she knew and understood the sign for "pretend", Koko more often used the signs for "know" and "think" when asked about her toys.

That said, one does not have to be aware that they are pretending in order to successfully pretend. In one clever study, researchers asked human children if a troll that hopped like a rabbit was only pretending to be a rabbit, even if he had never heard of rabbits in the first place. It wasn't until children were at least four years old that they answered correctly, suggesting that the younger children did not yet understand that pretending was something associated with the mind rather than the body.

To the extent that animals like Kakama, Kanzi, Viki, and Koko can pretend, their imaginations are probably limited in the same way as young human children. They can imagine, though they might not have a complete awareness of the distinctions between reality and fantasy. They might pretend, but not recognize it as such. Decades of intensive observation have revealed that under some circumstances, animals can imagine the future or the past, can pay attention to imaginary objects, and can pretend that one object is another. In some extraordinary circumstances, non-human animals have been known to feign interest or emotion, a type of pretence, in order to distract a rival from food or a mate.

One type of make-believe that has never been observed in an animal, though, is the sustained relationship with an imaginary other. To the best of our knowledge, no animal has an invisible friend. Still, the likes of Kakama, Kanzi, Viki, and Koko show that even imagination, something that Carl Sagan wrote could "carry us to worlds that never were," is far from uniquely human.

If you would like to comment on this article or anything else you have seen on Future, head over to our Facebook page or message us on Twitter.

Latest Stories

How would nuclear war change humanity?

Deep Civilisation

Deep Civilisation

Deep Civilisation

Mountain

Medical Myths

Are sweeteners bad for us?

Medical Myths

Life:Connected

Medical Myths

Under the Radar

In Depth

Video

Future Thinking

Medical Myths

Around the bbc