Mirror test

The mirror test—sometimes called the mark test, mirror self-recognition (MSR) test, red spot technique, or rouge test—is a behavioral technique developed in 1970 by American psychologist Gordon Gallup Jr. as an attempt to determine whether an animal possesses the ability of visual self-recognition.^[1] The MSR test is the traditional method for attempting to measure physiological and cognitive self-awareness. However, agreement has been reached that animals can be self-aware in ways not measured by the mirror test, such as distinguishing between their own and others' songs and scents,^[2] and being aware of their own bodies, while humans have abnormally good vision, and thus intelligence that is highly visual.

In the classic MSR test, an animal is anesthetized and then marked (e.g. paint or sticker) on an area of the body the animal normally cannot see (e.g. forehead). When the animal recovers from the anesthetic, it is given access to a mirror. If the animal then touches or investigates the mark on itself, it is taken as an indication that the animal perceives the reflected image as an image of itself, rather than of another animal.

Very few species have passed the MSR test. Species that have include the great apes, a single Asian elephant, manta rays, dolphins, orcas, the Eurasian magpie, and the cleaner wrasse. A wide range of species has been reported to fail the test, including several species of monkeys, giant pandas, and sea lions.^[3]^[4]

Method and history

The inspiration for the mirror test comes from an anecdote about Charles Darwin and a captive orangutan. While visiting the London Zoo in 1838, Darwin observed an orangutan named Jenny throwing a tantrum after being teased with an apple by her keeper. This started him thinking about the subjective experience of an orangutan.^[5] He also watched Jenny gaze into a mirror and noted the possibility that she recognized herself in the reflection.^[6]

In 1970, Gordon Gallup Jr. experimentally investigated the possibility of self-recognition with two male and two female wild preadolescent chimpanzees (Pan troglodytes), none of which had presumably seen a mirror previously. Each chimpanzee was put into a room by itself for two days. Next, a full-length mirror was placed in the room for a total of 80 hours at periodically decreasing distances. A multitude of behaviors was recorded upon introducing the mirrors to the chimpanzees. Initially, the chimpanzees made threatening gestures at their images, ostensibly seeing their reflections as threatening. Eventually, the chimps used their reflections for self-directed responding behaviors, such as grooming parts of their body previously not observed without a mirror, picking their noses, making faces, and blowing bubbles at their reflections.

Gallup expanded the study by manipulating the chimpanzees' appearance and observing their reaction to their reflection in the mirror. Gallup anesthetized the chimps and then painted a red alcohol-soluble dye on the eyebrow ridge and the top half of the opposite ear. When the dye dried, it had virtually no olfactory or tactile cues. Gallup then removed the mirror before returning the chimpanzees to the cage. After regaining full consciousness, he recorded the frequency with which the chimps spontaneously touched the marked areas of skin. After 30 minutes, the mirror was reintroduced into the cage, and the frequency of touching the marked areas was again determined. With the mirror present, the frequency increased from four to ten, compared to only one when the mirror had been removed. The chimpanzees sometimes visually or olfactorily inspected their fingers after touching the marks. Other mark-directed behavior included turning and adjusting the body to better view the mark in the mirror or tactile examination of the mark with an appendage while viewing the mirror.^[1]

An essential aspect of the classical mark test (or rouge test) is that the mark/dye is nontactile, preventing attention from being drawn to the marking through additional perceptual cues (somesthesis). For this reason, animals in the majority of classical tests are anesthetized. Some tests use a tactile marker.^[7] If the creature stares unusually long at the part of its body with the mark or tries to rub it off, then it is said to pass the test.

Animals that are considered to be able to recognize themselves in a mirror typically progress through four stages of behavior when facing a mirror:^[8]

social responses
physical inspection (e.g., looking behind the mirror)
repetitive mirror-testing behavior
realization of seeing themselves

Gallup conducted a follow-up study in which two chimps with no prior experience with a mirror were put under anesthesia, marked, and observed. After recovery, they made no mark-directed behaviors before or after being provided with a mirror.^{[citation needed]}

The rouge test was also done by Michael Lewis and Jeanne Brooks-Gunn in 1979 for the purpose of self-recognition with human mothers and their children.^[9]

Implication and alternate explanations

The default implication drawn from Gallup's test is that those animals who pass the test possess some form of self-recognition. However, a number of authors have suggested alternative explanations of a pass. For example, Povinelli^[10] suggests that the animal may see the reflection as some odd entity that it is able to control through its own movements. When the reflected entity has a mark on it, then the animal can remove the mark or alert the reflected entity to it using its own movements to do so. Critically, this explanation does not assume that the animals necessarily see the reflected entity as "self".

Criticism

The MSR test has been criticized for several reasons, in particular because it may result in false negative findings.^[11]

Perception

It may be of limited value when applied to species that primarily use senses other than vision.^[12]^[2]^[13] Humans have been determined by biologists to have some of the best eyesight amongst animals, exceeding the overwhelming majority in daylight settings, though a few species have better.^[14] By contrast, dogs for example mainly use smell and hearing; vision is used third. This may be why dogs fail the MSR test. With this in mind, biologist Marc Bekoff developed a scent-based paradigm using dog urine to test self-recognition in canines.^[15]^[12] He tested his own dog, but his results were inconclusive.^[16] Dog cognition researcher Alexandra Horowitz formalized Bekoff's idea in a controlled experiment, first reported in 2016^[17] and published in 2017.^[18] She compared the dogs' behavior when examining their own and others' odors, and also when examining their own odor with an added smell "mark" analogous to the visual mark in MSR tests. These subjects not only discriminated their own odor from that of other dogs, as Bekoff had found, but also spent more time investigating their own odor "image" when it was modified, as subjects who pass the MSR test do.^[19] A 2016 study suggested an ethological approach, the "Sniff test of self-recognition (STSR)" which did not shed light on different ways of checking for self-recognition.^[20] Dogs also show self-awareness in the size and movement of their bodies.^[21] Garter snakes, a relatively social snake species, have also passed an odor based "mirror" test.^[22]

Social motivation

Another concern with the MSR test is that some species quickly respond aggressively to their mirror reflection as if it were a threatening conspecific, thereby preventing the animal from calmly considering what the reflection actually represents. This may be why monkeys fail the MSR test.^[23]^[24]

Disinterest

In an MSR test, animals may not recognise the mark as abnormal, or may not be sufficiently motivated to react to it. However, this does not mean they are unable to recognize themselves. For example, in an MSR test conducted on three elephants, only one elephant passed the test, but the two elephants that failed still demonstrated behaviors that can be interpreted as self-recognition. The researchers commented that the elephants might not have touched the mark because it was not important enough to them.^[25] Similarly, lesser apes infrequently engage in self-grooming, which may explain their failure to touch a mark on their heads in the mirror test.^[11] In response to the question of the subject's motivation to clean, another study modified the test by introducing child subjects to a doll with a rouge spot under its eye and asking the child to help clean the doll. After establishing that the mark was abnormal and to be cleaned, the doll was put away and the test continued. This modification increased the number of self-recognisers.^[26]

Ambiguity

Frans de Waal, a biologist and primatologist at Emory University, has stated that self-awareness is not binary, and the mirror test should not be relied upon as a sole indicator of self-awareness, though it is a good test to have. Different animals adapt to the mirror in different ways.^[27]

Non-human animals

European magpies have demonstrated mirror self-recognition.

Several studies using a wide range of species have investigated the occurrence of spontaneous, mark-directed behavior when given a mirror, as originally proposed by Gallup. Most marked animals given a mirror initially respond with social behavior, such as aggressive displays, and continue to do so during repeated testing. Only a few species have touched or directed behavior toward the mark, thereby passing the classic MSR test.

Findings in MSR studies are not always conclusive. Even in chimpanzees, the species most studied and with the most convincing findings, clear-cut evidence of self-recognition is not obtained in all individuals tested.^[28] Prevalence is about 75% in young adults and considerably less in young and aging individuals.^[29]

Until the 2008 study on magpies, self-recognition was thought to reside in the neocortex area of the brain. However, this brain region is absent in nonmammals. Self-recognition may be a case of convergent evolution, where similar evolutionary pressures result in similar behaviors or traits, although species arrive at them by different routes, and the underlying mechanism may be different.^[11]

Animals that have passed

Mammals

Cetaceans

Bottlenose dolphin (Tursiops truncatus): Researchers in a study on two male bottlenose dolphins observed their reactions to mirrors after having a mark placed on them. Reactions such as decreased delay in approaching the mirror, repetitious head circling and close viewing of the eye or genital region that had been marked, were reported as evidence of MSR in these species.^[30]^[31]
Killer whale (Orcinus orca): Killer whales and false killer whales (Pseudorca crassidens) may be able to recognise themselves in mirrors.^[32]

Primates

Bonobo (Pan paniscus)^[33]^[34]
Bornean orangutan (Pongo pygmaeus):^[35] However, mirror tests with an infant (2-year-old), male orangutan failed to reveal self-recognition.^[36]
Chimpanzee (Pan troglodytes):^[1]^[37]^[38] However, mirror tests with an infant (11 months old) male chimpanzee failed to reveal self-recognition.^[36] Two young chimpanzees showed retention of MSR after one year without access to mirrors.^[39]
Western gorilla (Gorilla gorilla): Findings for western gorillas have been mixed; more so than for the other great apes. At least four studies have reported that gorillas failed to show self-recognition.^[35]^[40]^[41]^[42] However, other studies have shown self-recognition in captive gorillas with extensive human contact. Such gorillas show less aversion to direct eye contact than wild gorillas. In wild gorillas, as in many other animals, prolonged direct eye contact is an aggressive gesture, and gorillas may fail the mirror test because they deliberately avoid closely examining or making eye contact with their reflections.^[43]^[44] Gorillas who have passed the MSR were habituated to the mirror before testing and were not subject to anesthesia during the marking process.^[45]^[46] Koko was among the gorillas who passed the MSR test under these circumstances.^[43]^[44]

Proboscidea

Asian elephant (Elephas maximus): In a study performed in 2006, three female Asian elephants were exposed to a large mirror to investigate their responses. Visible marks and invisible sham-marks were applied to the elephants' heads to test whether they would pass the MSR test.^[8] One of the elephants showed mark-directed behavior, though the other two did not. An earlier study failed to find MSR in two Asian elephants;^[47] it was claimed this was because the mirror was too small.^[8]^[48]

Birds

Video of the responses of a European magpie in an MSR test: The magpie repeatedly attempts to remove the marks.

Eurasian magpie (Pica pica): The Eurasian magpie is the first non-mammal to have been found to pass the mirror test. In 2008, researchers applied a small red, yellow, or black sticker to the throat of five Eurasian magpies, where they could be seen by the bird only by using a mirror. The birds were then given a mirror. The feel of the sticker on their throats did not seem to alarm the magpies. However, when the birds with colored stickers glimpsed themselves in the mirror, they scratched at their throats—a clear indication that they recognised the image in the mirror as their own. Those that received a black sticker, invisible against the black neck feathers, did not react.^[28] In 2020, researchers attempted to closely replicate the 2008 study with a larger number of magpies, and failed to confirm the results of the 2008 study. The researchers stated that while these results did not disprove the 2008 study, the failure to replicate indicated the results of the original study should be treated with caution.^[49]

Some pigeons can pass the mirror test after training in the prerequisite behaviors.^[50] In 1981, American psychologist B. F. Skinner found that pigeons are capable of passing a highly modified mirror test after extensive training.^[51]^[52] In the experiment, a pigeon was trained to look in a mirror to find a response key behind it, which the pigeon then turned to peck to obtain food. Thus, the pigeon learned to use a mirror to find critical elements of its environment. Next, the pigeon was trained to peck at dots placed on its feathers; food was, again, the consequence of touching the dot. The latter training was accomplished in the absence of the mirror. The final test was placing a small bib on the pigeon—enough to cover a dot placed on its lower belly. A control period without the mirror present yielded no pecking at the dot. When the mirror was revealed, the pigeon became active, looked in the mirror and then tried to peck on the dot under the bib. However, untrained pigeons have never passed the mirror test.^[53]

Fish

Bluestreak cleaner wrasse (Labroides dimidiatus): According to a study done in 2019, cleaner wrasses were the first fish observed to pass the mirror test.^[54]^[55] The bluestreak cleaner wrasse is a tiny tropical reef cleaner fish. Cleaner fish have an adapted evolutionary behavior in which they remove parasites and dead tissue from larger fish. When put through the mirror test, using a benign brown gel injected into the skin of the fish, and resembling a parasite, the cleaner wrasse showed all the behaviors of passing through the phases of the test. When provided with a colored tag in a modified mark test, the fish attempted to scrape off this tag by scraping their bodies on the side of the mirror. Gordon Gallup believes the cleaner wrasses' behavior can be attributed to something other than recognizing itself in a mirror. Gallup has argued that a cleaner wrasse's job in life is to be aware of ectoparasites on the bodies of other fish, so it would be hyper aware of the fake parasite that it noticed in the mirror, perhaps seeing it as a parasite that it needed to clean off of a different fish. The authors of the study retort that because the fish checked itself in the mirror before and after the scraping, this means that the fish has self-awareness and recognizes that its reflection belongs to its own body.^[56]^[57]^[58] The cleaner wrasses, when tested, spent a large amount of time with the mirror when they were first getting acquainted with it, without any training. Importantly, the cleaner wrasses performed scraping behavior with the colored mark, and they did not perform the same scraping behavior without the colored mark in the presence of the mirror, nor when they were with the mirror and had a transparent mark.^[59] Following various objections, the researchers published a follow-up study in 2022, where they did the mirror test on a larger sample of wrasses and experimented with several marking techniques. The new results "increase[d] [the researchers'] confidence that cleaner fish indeed pass the mark test", although wrasses attempted to scrape off the mark only when it resembled a parasite.^[60]^[61] Another study in 2024 found that cleaner wrasse that initially showed aggression to photographs 10% larger and 10% smaller than themselves ceased confrontation with 10% larger photographs upon encountering their reflection.^[62]

Giant oceanic manta ray (Mobula birostris): In 2016 a modified mirror test done on two captive manta rays showed that they exhibited behavior associated with self-awareness (i.e. contingency checking and unusual self-directed behavior).^[63]

Crustaceans

Atlantic ghost crab (Ocypode quadrata): A 2023 study found that these crabs seem to be capable of recognizing themselves in a mirror. The study's author concluded that the data indicate that the crabs have "a rudimentary form of self-awareness".^[64]