The mirror test was developed by psychologist Gordon Gallup Jr.1 in 1970 as a method for determining whether a non-human animal has the ability of self-recognition. It’s also known as the “mark test” or “mirror self-recognition test” (MSR).
When conducting the mirror test, scientists place a visual marking on an animal’s body, usually with scentless paints, dyes, or stickers. They then observe what happens when the marked animal is placed in front of a mirror. The researchers compare the animal’s reaction to other times when the animal saw itself in the mirror without any markings on its body.
Animals that pass the mirror test will typically adjust their positions so that they can get a better look at the new mark on their body, and may even touch it or try to remove it. They usually pay much more attention to the part of their body that bears a new marking.
Even if an animal doesn’t pass the test, they may still have interesting reactions to their reflections.
Many species respond aggressively, or even show affectionate behavior. In such cases, it might be that the animal mistakes its reflection for another of its kind. This can lead to some amusing sights for human observers.
Humans are able to pass the mirror test when they are around 18 months old. But how do other animals fare?
Currently, there are several non-human animal species pass the mirror test. Not all individuals of each species pass, but many do.
If an animal can pass the mirror test, it’s certainly strong evidence of self-recognition, and indicates the possibility of self-awareness (i.e. a “sense of self”). However, it’s not definitive proof. And if an animal isn’t able to pass, that doesn’t necessarily mean that they do not possess these abilities.
This list of animals that have passed the mirror test examines how each species responded during testing.
1. Rhesus Macaques
It was only recently, in 2010, that scientists accidentally discovered that monkeys can pass the “cognitive divide” between the highest primates and the rest of the animal kingdom.
Researchers had placed head implants in two rhesus macaque monkeys, in advance of doing a study on attention deficit disorder. To their surprise, the macaques – who would normally fail the mirror test by reacting to their reflection as if it were an invading monkey – showed clear signs of self-awareness.
The monkeys observed themselves in the mirror while grooming and examining their foreheads near the implant. They also examined parts of their body that they hadn’t seen before and turned themselves upside down to do so. Finally, they grasped and adjusted the mirror to get an enhanced view. However, the self-realization had no permanence: when the mirror was covered up these behaviors disappeared.
In 2015, scientists published research that suggests some ants can recognize themselves when looking in a mirror. When viewing other ants through glass, ants didn’t divert from their normal behaviors.
However, their behavior did change when they were put in front of a mirror. The ants would move slowly, turn their heads back and forth, shake their antennae, and touch the mirror. They’d retreat and re-approach the mirror. Sometimes they would groom themselves.
The ants were next given a classic mirror test. The team of researchers would use blue dots to mark the clypeus of some of the ants, which is a part of their face near their mouths.
When in an environment without mirrors, these ants would behave normally, and wouldn’t touch the markings. But this changed when they could see their reflections in a mirror. The ants with blue dots on their face would groom and appear to try to remove the markings.
Very young ants, and other ants with brown dots that blended in with the color of their face didn’t clean themselves. Interestingly, neither did ants with blue dots put on the back of their heads.
When put in the company of those with blue-dotted faces, other ants would respond aggressively, presumably because the difference caused them to think the blue-dotted ant was an outsider (not a member of their colony). All of this lead the researchers to conclude that the clypeus is a species-specific physical characteristic that is important for group acceptance.
Given that these ants tried to clean the mark rather than respond aggressively, the ants likely didn’t think their reflection was just another ant. The team thinks their study shows that self-recognition is not an “unrealistic” ability in ants.
Gallup’s initial subjects – two male and two female chimpanzees – were each put in a room alone for two days. Then a full length mirror was added to the room and their reactions scrutinized.
Initially, the chimps (who had never seen a mirror before) made ᴛʜʀᴇᴀᴛening gestures at their reflections. But eventually they began to use the mirror for self-directed responding behaviors, like grooming hard-to-see places, picking their noses, making faces and blowing bubbles at themselves.
4. Orca Whales
In a study, orca whales were allowed to view themselves in a mirror. Afterward, they were marked, then allowed to view themselves in the mirror once more. Their behavior showed that they expected the image would be altered as a result of their being marked – a strong indication that they understand the image in the mirror is indeed of themselves.
5. Bottlenose Dolphins
Bottlenose dolphins usually show extreme interest when they catch sight of their reflections. They will open their mouths, stick out their tongues, and make a series of novel movements while observing themselves in the mirror. When marked, dolphins regularly inspect the marking.
6. Asian Elephants
Asian elephants display a wide range of reactionary behaviors when they see their reflections in mirrors, and will respond to colored markings placed in-view on their bodies. However, not all of the elephants in a study by Joshua M. Plotnik passed. This might be because the normal behavior of elephants conflicts with what passing the mirror test requires.”
The mark test can be difficult to apply across species because it assumes that a particular animal will be interested in something weird on their body. Primates are interested in such things—we’re groomers. But elephants are different. They’re huge and they’re used to putting things on, not taking things off of their bodies, like mud and dirt.” said Plotnik.
Bonobos are an enᴅᴀɴɢᴇʀed ape species and more peaceful and social than the common chimpanzee.
In fact, genetically, they are most similar to us – and share many behavioral traits such as walking upright and having very similar facial expressions. Like humans, it is also easier to distinguish facial features between different individuals, such that humans can easily differentiate one bonobo from another.
They, too, have passed the mirror test, back in 1994. Two well-known bonobos have been taught language: Kanzi and Panbanisha have a vocabulary of about 400 words which they type using a keyboard of lexigrams (geometric symbols). They can also respond to spoken sentences.
Quite possibly our most intelligent cousins, the leading primatologist, Professor Franz de Waal, claims that bonobos are also capable of altruism, compassion, empathy, kindness, patience and sensitivity. For a fictional take on the study of language in bonobos, check out Sara Gruen’s Ape House.
Although most gorillas fail the mirror test, one specific gorilla named Koko has passed it. Koko is famously known as the “talking” gorilla, recognizing more than 1,000 words in American Sign Language and 2,000 words in spoken English.
Born in 1971 in San Francisco Zoo, Koko is the subject of a groundbreaking science experiment to determine the true intelligence of gorillas. Although she can’t vocalize like a human, she can understand spoken words and communicate her thoughts and feelings with hand signals.
Her trainer, Dr Francine Patterson, has also seen her invent new signs of her own – like the combination of signs for “finger-bracelet” to describe a ring. Koko even demonstrated that she feels emotions much like a human being. She once asked if she could have a cat and chose out a gray male Manx as her pet, which she cared for as a baby. Later that year, the cat escaped and was hit by a car. When Patterson explained the cat had gone, Koko signed “bad-sad-bad” and “frown-cry-frown-sad”. After that, Koko the gorilla was able to pick out two new kittens which became her surrogate babies.
Today, Koko is 42 years old and describes herself as a “fine-gorilla-person”, amazing her friends and caregivers with her intelligence and emotional depth. Her astonishing story is revealed in the documentary Koko: A Talking Gorilla.
9. European Magpies
The final (and quite unexpected) entry into the list of animals with self-awareness is the European magpie. Not only is it the only bird to have passed the mirror test, it’s the only non-mammal species, too. Go magpies.
Closely related to crows (who are also known for their intelligence, if not self-awareness) magpies officially gained self-awareness in the scientific archives in 2008.
A German study involved placing various colored dots on the necks of the birds where they couldn’t see them. While the magpies didn’t react to the feel of the dots, they began scratching their necks when placed in front of a mirror. Those with black dots, which were camouflaged against their feathers, didn’t react at all.
It was once thought that self-awareness arises from the neo-cortex of the brain – but magpies don’t have one. Franz de Waal points out that magpies do, nonetheless, have large brains with lots of connectivity.
“Magpies are known for their ability to steal shiny objects and to hide away their loot,” he said. “It’s not too far-fetched that a master thief like a magpie has that perspective-taking ability.”
10. Promising Candidates: Manta Rays
When it comes to fish, manta rays have the largest brains. This fact lead Dr. Csilla Ari to suspect that they might be the fish species most likely to pass the mirror test. When she exposed captive manta rays to a large mirror, they showed great interest in their reflections.
The rays would repeatedly swim in front of the mirror, turning over to show their undersides and moving their fins. When in front of the mirror, they even blew bubbles, an unusual behavior. What the rays didn’t do is try to socially interact with the mirror image.
All of this suggests that the rays might recognize it’s themselves they’re seeing in the mirror, not another ray. However, a classic mirror test using marks on the rays’ bodies has yet to be done. If rays can pass a mark test, it’s a more solid indication of self-recognition abilities. The presence of exploratory behavior and a lack of social behavior doesn’t automatically indicate self-recognition.