Are monkeys able to discriminate appearance from reality?

The understanding that the perceptual appearance of the environment can differ in several ways from the reality underlies the ability to discriminate appearance from reality. Being able to realize when a misperception can lead us to behave in inappropriate ways confers an evolutionary advantage and may be a prerequisite to develop a Theory of Mind. Understanding that our own perception can differ from reality seems indeed necessary to attribute to others perceptions or beliefs different than ours. This appearance-reality discrimination ability has recently been demonstrated in great apes but no information is currently available regarding this ability in other nonhuman species. In a comparative study, we tested Tonkean macaques (Macaca tonkeana), an Old World primate species, and brown capuchins (Sapajus apella), a New World primate species. We provided monkeys with two experiments using visual illusions of size and quantity to test their ability to discriminate appearance from reality, with an experimental setup similar to the one developed by Krachun et al. (2016) on chimpanzees. A large number of brown capuchins, from different ages and both sexes, as well as two Tonkean macaques succeeded in the two experiments. By ruling out all alternative explanations (i.e. visual tracking or associative learning), our study brings the first evidence that some Old World and New World monkeys are able to discriminate appearance from reality. Our results suggest moving the evolutionary apparition of this cognitive ability earlier in time. Finally, it suggests that humans could share more Theory of Mind components with more nonhuman species than we previously thought.