Apes are on a par with human infants in being able to tell when people have an accurate belief about a situation or are actually mistaken, researchers say.
While previous work has shown that great apes understand the goals, desires and perceptions of others, scientists say the latest finding reveals an important cognitive ability.
"For the last 30 or more years people thought that belief understanding is the key marker of humans and really differentiates us from other species -- and this does not seem to be the case," said David Buttelmann, co-author of the research from the Max Planck Institute for Evolutionary Anthropology in Germany.
The results follow on the heels of a study published last year which also suggests that apes understand the concept of false beliefs -- after research that used eye-tracking technology to monitor the gaze of apes exposed to various pranks carried out by an actor dressed in a King Kong suit.
But the new study, says Buttelmann, is an important step forward, showing that apes not only understand false belief in others, but apply that understanding to their own actions.
Writing in the journal Plos One, Buttelmann and colleagues described exploring the understanding of false belief in 34 great apes, including bonobos, chimpanzees and orangutans, using a test that can be passed by human infants at one to two years of age.
After training the apes to open bolted boxes regardless of whether or not they contained an object, the team created a scenario in which an ape watched as an experimenter put an object into a box.
Two scenarios then played out. In the first, a second individual approached the boxes and, in full view of the experimenter, took the object out of the first box and put it in a second box. Both boxes were then locked.
In the second scenario, the individual switched the location of the object to the second box and locked it only once the experimenter had momentarily left the room -- creating a situation in which, upon return, the experimenter would have a mistaken belief in which box contained the object.
In both scenarios the apes subsequently saw the experimenter struggling to open the box in which the object was originally placed, but was in fact empty.
The team watched to see whether, when given the chance, the ape would help the experimenter to unlock the empty box, or instead unlock the box in which the object was now located.
The results showed that, in the case when the experimenter had not seen the switch, the apes were more likely to open the box containing the object, choosing it 76.5% of the time.
By contrast in the case where the experimenter had observed the switch take place, the apes unlocked each box with almost equal likelihood.
Together with a further experiment, the results, says Buttelmann, show the apes were not merely trying to show the experimenter which box the object was in, but that the apes understood when the experimenter had a belief about where the object should be, and what that belief was, as well as an understanding about whether that belief was true or false.
"That is very nice because in evolution there is nothing that comes out of the blue from nowhere, it has to build on earlier existing capacities," said Uta Frith, a developmental psychologist at University College London who was not involved in the research. "I would say now the search should go on for other species as well because somewhere we might trace the origin of this amazing ability -- tracking invisible mental states to some extent."
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