We’ve been underestimating how appreciative apes can be of others’ ignorance
If you hide something from someone while they step out of the room, you should have a guess about where they’d expect that object to be when they return. For just about any adult, it’s not hard to see why the subject of your deception would first look where they had last seen something, even if you know it’s no longer there. This may feel incredibly basic, but it actually requires putting yourself in another person’s shoes and imagining how the information they know might be different than your own. This concept, usually referred to as theory of mind, isn’t actually very common, making it onto the special list of Things only Humans Can Do™. Or at least, that’s what we thought based on the information we had at the time.
What do they know, and when did they know it?
Part of why theory of mind has been considered such an exclusive ability is that even young humans don’t seem to grasp it. Studies of kids under four years old often found that they didn’t make the separation between what they knew and what someone else might know, which supported the idea that it was a complicated task requiring a more developed brain to handle. Other species usually fail tests as well, although ravens did manage to strategize along lines that would suggest they some grasp of considering what other ravens might know.
A study in 2007 broke this barrier though, showing that human infants just over two-years-old would guess where person would look for an object that had been hidden from them. Based on that experiment, researchers designed a similar experiment for various primates to see if that presentation would click for other species. Chimpanzees, who have engaged with watching movies before, seemed to respond to this format, showing interest in where an on-screen character would likely expect something to be found.
Watching humans with sticks and stones
Keeping the presentation lively and somewhat relatable, chimps watched short movies involving a human coming into conflict with an ape, as played by another human in a gorilla costume. The drama included stones being hidden under boxes, or even an angry human with a stick trying to find the hiding ape in fake haystacks. The chimpanzee audience had their eyes being tracked, and their focus was always on parts of the images that were important to the story at that moment, such as a door the human went through, or the stick the human was about to swing at a haystack. The key moments were when the chimps paid attention to where the human last saw the stone or ape, even though the chimps knew those targets were no longer in those locations. They were apparently watching according to the on-screen human’s expectations, not their own experience as witnesses.
This seems like chimps may be part of the theory of mind club, but there’s still a bit of hesitation to grant them a full membership. While the eye-tracking is a decent indicator of the apes’ expectations, it’s not as definitive as it could be. There are also concerns that the audience may have been reacting to learned rules about what to expect on screen, especially since there were segments where the human did not get tricked, and found the stone or ape where they left it. Further studies will hopefully clarify the limits of apes’ understanding, although even if this hasn’t demonstrated a true theory of mind, it still pushes the boundaries on what we thought they were capable of.
Source: Apes can tell when you've been duped by Helen Shen, Nature