The researchers were able to rely on the fact that non-human apes have a natural preference for bigger pieces of food. "Great apes have a very strong tendency to choose the larger food of two pieces," Karg says. Chimpanzees, gorillas, bonobos and orangutans would readily snag what they perceived to be larger pretzels. (The primatologists taught the little humans to pick fake "food sticks" with the aid of a sock puppet: "Children generally reacted positively to the [puppet] and seemed to enjoy feeding it," the researchers wrote.)
If just the tips of the pretzels were showing, each primate species frequently chose the deceptively large stick. But when apes and kids watched as the board was slid in place (or if it started in place, was removed, then returned), they were more likely to see through the illusion and point to the actually longer object.
Carla Krachun, a primate expert at Memorial University of Newfoundland, Canada, who was not involved in the study, and her colleagues had previously shown that chimpanzees could overcome an optical illusion of small grapes looking bigger due to magnifying lenses. "It's extremely interesting that they found similar performances across all species," she says in an email to The Dodo.
Apes that see through deception have a leg up, evolutionarily speaking - the natural world is full of things that aren't what they appear. "Camouflage and mimicry are obvious examples," Krachun says, or "objects appear bigger when immersed in water, or appear a different color under different lighting conditions." And mistaking a snake for a branch, for example, could have lethal consequences.
Krachun believes studies like these could help shed light on the way reality and appearance clash in the social world, too. "Other individuals may misrepresent their feelings, intentions, abilities, identity," she says. "And knowing this would allow one to avoid being misled by others, and also to mislead others to one's own advantage."
Anecdotally, there's evidence great apes have tried to visually deceive each other. A chimp might try to bluff about his size by puffing up his hair, Karg points out. And in one instance, a chimp covered "his erect penis with his hands so that a dominant competitor could not see it (this saved him from an aggressive interaction)."
Great ape subterfuge goes beyond bristling chimpanzee fur, of course. "Humans are a very deceptive species - we smile even if we do not mean it, we lie, we betray," Karg says. "In the human world it is thus for sure an advantage if you know what is real and what is fake, so that you can base your decisions on reality."