Like most people, something called a theory of mind allows me to mentally step into another's shoes and imagine their thoughts. But whether we share this talent with other primates is still a matter of debate.
A new study adds even more evidence to the view that apes are able to anticipate the others' actions based on their own experiences, suggesting they can model another's thoughts much like we can.
Researchers from Kyoto University in Japan and the University of St Andrews in the UK combined classic animal psychology experiments to determine if our closest cousins can predict the actions of a human by guessing their thoughts.
There's no shortage of research devoted to solving the question of whether – or to what extent – non-human animals can sense the potential behaviours of others by analysing the world from perspectives outside of their own head. We've been chewing on this question for decades.
There are plenty of good reasons to suspect other apes and monkeys might possess at least some of the hallmarks of a theory of mind, not to mention some parrots and even ravens.
But while the evidence is compelling, there's always going to be some wiggle room for non-theory-of-mind explanations. Even animals of seemingly average intelligence can have an uncanny ability to look super smart by reading subtle hints in their environment, after all.
To find ways to identify non-mental explanations, researchers have to find clever new ways to conduct experiments that 'blind' animals to variables that might clue them in unwittingly to the actions of those around them.
In this particular study, the researchers meshed aspects of a classic 'goggles' experiment with previous research they conducted that made use of anticipatory glancing behaviours of apes.
A sample of 29 chimpanzees, 4 orangutans, and 14 bonobos were presented with a short video clip of a human watching an actor in an ape costume place an object into a box.
Once the object was hidden from sight, the human hid behind a screen.
Here's where the blinding part came in. Prior to watching the video, half of the apes had inspected a similar looking screen that was transparent. Meanwhile, the other half of the group were shown one that looked identical, but was completely opaque.
In theory, each ape should assume the screen in the video was more or less the same one they'd encountered before.
The 'watcher' then waited as the actor transferred the object into a second box, before removing it altogether.
The thrilling conclusion to this great ape melodrama involved the human watcher returning to the boxes and reaching their hand towards a spot midway between the two potential hiding places.
Like any of us in the grip of a tense plotline, an ape who suspected the story's protagonist hadn't seen the object's removal would look towards the original hiding spot. After all, that's where the watcher last saw it.
By the same token, if the audience assumed the screen was transparent, a theory of mind should tell them the person was aware that the object was no longer present. The ape would have no reason to look at any particular spot.
Eye-tracking technology allowed the researchers to precisely monitor where the apes were looking, revealing a significant bias of nervous glances towards the target hiding box when the watcher's line of sight was assumed to be blocked.
"In conclusion, we provide evidence that great apes can use their own past experience of visual access to attribute perception and, potentially, resultant beliefs to others," the researchers write.
Psychological concepts like a theory of mind are notoriously hard to prove using any single experiment.
Not only are there plenty of variables to iron out, defining what such a theory should look like outside of our own personal experience is a philosophical nightmare.
These kinds of 'false belief' studies also represent just one aspect of a possible theory of mind. Just how complex this mental trait might be in other animals, or how it might translate into observable actions, is also up for debate.
But given the growing sum of research investigating the topic from different angles, it's becoming increasingly difficult to argue that humans are the only species capable of second-guessing another's actions based on presuming their state of knowledge.
Just think about that.
This research was published in PNAS.