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 administering the mirror test, scientists will place a visual marking on a part of the 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 sessions when the animal viewed 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 than they did in past un-marked sessions.
Even if an animal doesn’t pass the test, they may still have interesting reactions to their reflections. Many species have been observed putting on aggressive displays, or even showing affectionate behavior. In such cases, it might be that the animal mistakes its reflection for that of another animal. 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, 9 non-human animal species pass the mirror test. Not all individuals of each species pass, but the many do. This list of animals that have passed the mirror test examines how each species responded during testing.
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 study2 by Joshua M. Plotnik passed. This might be because the normal behavior of elephants is at odds 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.3
The Great Apes
Bonobos, chimpanzees, orangutans, and gorillas have all passed the mirror test.4,1,5 However, it is not uncommon for many individuals to fail, especially if they are particularly young or old. In chimpanzees, there is evidence for self-recognition in about 75% of young adults.6 This percentage lowers with individuals closer to either end of the age spectrum.
Many gorillas have failed the mirror test. When shown a mirror, a number of silverbacks have exhibited aggressive behavior. It has been speculated that most of the gorillas and other primates who do not pass the test may have inhibited behavior due to the presence of observing humans.
“Ironically, it may have been the gorillas’ very capacity for self-consciousness that prevented them from exhibiting behaviours indicative of self-recognition in the test situation.”7
Additionally, gorillas will often avoid eye contact with their reflection. By avoiding eye contact, gorillas are likely not able to look at their reflection long enough to realize that it is themselves who they are seeing. Koko the gorilla, well known for having learned sign language and performing well in other cognitive experiments, was the first of her species to pass the test.
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.8
In a study9, 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.
The Eurasian magpie is the first non-mammalian species to pass the mirror test.10 When contrasting colored stickers were placed on their feathers, the magpies reacted to their reflections by trying to remove the mark. Birds who were marked with invisible stickers showed no altered behavior.
Prior to this experiment, scientists believed that self-recognition abilities came from the neocortex, a part of the brain found only in mammals. Since magpies are birds, they do not have this part of the brain. Their passing of the mirror test shows that self-recognition ability can arise from other brain parts in other types of brains. Such a thing is a case of “convergent evolution”. This happens when similar abilities, behaviors, and traits independently evolve in species that are not closely related.
In 2015, research was published11 that suggests some ants can recognize themselves when looking in a mirror. When viewing other ants through a 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 finally given a classic mirror test. The team of researchers would use blue dots to mark the clypeus of some of the ants, 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 clypeus 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 clypeus 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 clypeus’, 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 characteristic that is integral to group acceptance.
The fact that the ants with blue-dotted clypeus’ would try to remove the marking that made them appear alien is thought by the team to be a survival behavior. Given that these ants would try to clean the mark rather than respond aggressively, the ants likely didn’t think their reflection was the image of another ant. The team thinks their study shows that self-recognition is not an “unrealistic” ability in ants.
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.12, 13 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.
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, if an animal isn’t able to pass, that doesn’t necessarily mean that they do not possess these abilities. For example, animals that rely on other senses more heavily than their vision may not take much interest in the sight of their reflections. Dogs, who recognize others mainly by their sense of smell, might quickly conclude that their mirror image is not of themselves or any other animal, because it lacks a corresponding scent. Furthermore, some animals may be able to recognize themselves in the mirror and see that they have been marked, but do not find the mark important enough to warrant touching or inspection.
To learn more about the mirror test and some of the animals that have passed it, view the videos below.
This article was written by Amanda Pachniewska, founder & editor of Animal Cognition.
1 – Gordon G. Gallup Jr.
2 – Joshua M. Plotnik, Frans De Waal, Diana Reiss
Self-recognition in an Asian elephant
Proceedings of the National Academy of Sciences of the United States of America
3 – Kids (and Animals) Who Fail Classic Mirror Tests May Still Have Sense of Self
4 – G. C. Westergaard, C. W. Hyatt
The responses of bonobos (Pan paniscus) to their mirror images: Evidence of selfrecognition
5 – Gordon G. Gallup, Jr., James R. Anderson, and Daniel J. Shillito
The Mirror Test
6 – S. Robert
Ontogeny of mirror behavior in two species of great apes
American Journal of Primatology
7 – Francine Patterson, Wendy Gordon
The Case for the Personhood of Gorillas
The Great Ape Project
8 – Kenneth Martin, et al.
Evidence of self-awareness in the bottlenose dolphin (Tursiops truncatus).
Self-awareness in animals and humans: Developmental perspectives
9 – F Delfour, K Marten
Mirror image processing in three marine mammal species: killer whales (Orcinus orca), false killer whales (Pseudorca crassidens) and California sea lions (Zalophus californianus)
10 – Helmut Prior, et al.
Mirror-Induced Behavior in the Magpie (Pica pica): Evidence of Self-Recognition
11 – Marie-Claire Cammaerts , Roger Cammaerts
Are Ants (Hymenoptera, Formicidae) Capable of Self Recognition?
Journal of Science
12 – Manta Cognition
Manta Pacific Research Foundation
13 – Csilla Ari, Dominic P. D’Agostino
Contingency checking and self-directed behaviors in giant manta rays: Do elasmobranchs have self-awareness?
Journal of Ethology