The test used to see if animals are self-aware might not actually work

The famed mirror test might not be as useful as we thought

Humans use mirrors so reflexively that we’ll often use shop windows or phone screens to preen ourselves without a second thought. But it didn’t always come so easy. Before the age of about two, kids don’t see themselves when they look in the mirror—they have to develop that ability over time. Until they do, they just think they’re looking at another baby. And new evidence suggests the same might be true for some monkeys.

Great apes and humans have long been amongst the few species to pass the mirror test, also known as the mark test. When researchers put an ink mark on a great ape’s forehead without the ape realizing, then put it in front of a mirror, the ape can recognize that it’s looking at its own reflection and will reach up to touch the unfamiliar mark. There’s some evidence that dolphins, killer whales, Asian elephants, and magpies can do similar things, though some scientists contest the issue. Most primates can’t even pass the test. And what that’s told primatologists for years is that many species aren’t self-aware. If you can’t recognize yourself in a mirror, that means you don’t have a sense of self. But what if we just weren’t testing those primates properly?

A new study in the Proceedings of the National Academy of Sciences suggests just that: we weren’t doing a thorough enough test. If you teach monkeys how to use a mirror well enough, they can demonstrate an understanding of self. “I’ve never seen a monkey do that before,” says Annika Paukner, a staff scientist at the National Institutes of Health who wasn’t involved in the new study. “We’ve never known why they don’t do it when great apes and humans do. No one had any idea. This is the most convincing paper I’ve seen so far that they do.”

The really impressive part wasn’t the main experiment, though. The main experiment involved training the monkeys to touch a point of light projected onto their faces (another form of the mark test), which they were able to do after a few weeks of training. But rhesus monkeys can do a lot of things if you train them. Even pigeons can peck at spots on their own bodies via a mirror image if you train them with food rewards. Rhesus monkeys will even inspect their genital areas in a mirror, though there’s some debate about whether that’s truly a sign of self-awareness. Paukner argues it probably isn’t (or that it’s at least up for debate), while Abigail Rajala, a neuroscientist at University of Wisconsin-Madison (who also wasn’t involved in the new study) argues that it is a sign of self-awareness. Both agreed that what the rhesus monkeys demonstrated went above and beyond. They inspected their faces in their mirrors on their own time—as if the light experiment had taught them to appreciate the fundamental nature of a mirror and its potential uses.

“That’s what I would expect to see from a great ape,” says Paukner. When these monkeys went back to their cages, they weren’t preening with their mirrors because someone trained them to do so. They’d generalized the knowledge they’d accumulated over time, explains Rajala. The rhesus monkeys she’s worked with have exhibited similar learning. Her lab puts mirrors into the cages when the monkeys are just infants, giving them ample time to learn and understand how the surfaces work. And in 2010, she published a paper showing that rhesus monkeys use the mirrors to examine their own bodies, arguing that it was a clear sign of self-awareness—regardless of the mark test results. “Monkey subjects are used to being trained to do stuff,” she says, “that’s how they make their living. They do whatever they realize is going to get their reward, but eventually you make these connections. It’s the same thing that happens with infants.”

These rhesus monkeys aren’t just doing what they’re told. They’re using the mirror to their own, self-motivated ends. It’s possible that the extra training these monkeys got helped them to understand something more fundamental about mirrors. Most studies train monkeys to get to 100 percent accuracy on whatever test they’re doing, explains Paukner, but these neuroscientists went beyond that. After the monkeys could perform with 100 percent accuracy on multiple tests, the researchers trained them for another two weeks or so. That over-training may have taught the monkeys not just to locate a spot on their face, but how to actually use a mirror. Paukner says she’s not sure that the monkeys have a different sense of themselves, but rather that the monkey’s understanding of how a mirror works has changed.

It’s not unreasonable to think that monkeys don’t inherently understand what a mirror is, even if they are self-aware. Children as old as six in non-Western countries struggle to pass the mark test, but probably not because they have no sense of self. It’s just not something they’ve encountered before, so they don’t behave in the way we expect them to. Western infants are around mirrors all the time—we show babies their reflection and say “who’s that? It’s you!” over and over again until everyone but the parent is bored. If you grew up without all that, it would make sense that you wouldn’t immediately know how to react to your own reflection.

“We have a tendency to put things in a way that would make sense for us,” says Rajala. “That makes studying other species difficult, especially higher-order species.” Gorillas, for example, seem to recognize their own reflection, but because eye contact is a potentially aggressive behavior, they tend to hide from mirrors—which makes it seem like they think they’re hiding from another gorilla. Just like your dog, they misunderstand what they’re seeing. Or maybe they’re just not used to looking at their own reflection and will choose instead to rub off the mark in a secluded area—we’re not in their heads, so we can’t be sure.

And any animal that doesn’t rely on vision as its primary sense is unlikely to do well on the test. Rajala offers up rodents as an example. Rodents use smell to guide them through the world, so even though they may have some elementary understanding of self they probably won’t be able to demonstrate that to us on a mark test.

“You have to meet the animal where it is,” says Rajala. It’s not that the mark test doesn’t tell us anything useful, it’s that it’s just one piece of the puzzle. It’s a starting point. Self-awareness doesn’t boil down to a yes or no question, it’s more of a continuum. We can’t know unequivocally whether another being is self-aware or not. All we have are test results and interpretations. Welcome to science.

Video footage used in GIFs courtesy of authors

Sara Chodosh

Sara Chodoshis an associate editor at PopSci where she writes about everything from vaccine hesitancy to extreme animal sex. She got her master's degree in science journalism at NYU's Science Health and Environmental Reporting Program, and is getting a second master's in data visualization from the University of Girona. Contact the author here.