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The mirror test (also known as the mark test, or the mark mirror test) is a measure of self-recognition in animals. Mirror tests involve exposing animals to mirrors, hence the name. Animals are marked with a dye (often on or near their faces) to distinguish themselves from other members of the same species. Most of self-recognition research involving mirror test is focused on non-human primates, although animals of other species also have been studied. The mirror test for human infants is called the rouge test and is differentiated from the mirror test for non-human animals. Comparative psychologists use the mirror test to study the self-concept in different species of animals. The psychologists attempt to answer when the ability for self-recognition first appeared in the history of evolution, what derived the emergence, and why certain species possess or lack such ability. The study of self-concept in nonhuman animals allows deeper understanding of human cognition and consciousness.

Typical Mirror Test
Typical mirror test methods used today are based on the methods used by Gordon G. Gallup (1970) in his study of self-recognition in chimpanzees. Mirror tests involve exposure of mirrors to animals. All animals tend to exhibit social behaviours toward the mirror reflection when they are first exposed to the mirror, but the social behaviours decrease in frequency over time. After the first mirror exposure, animals are marked with a dye on body parts that are only visible through mirrors. Usually, animals are marked on or near their faces (e.g. above the eyebrow or on the tip of an ear). Then, animals are again exposed to mirrors. Researchers observe the animals’ behaviour toward their mirror reflection. If an animal displays social behaviours, then the animal is said to lack the ability to recognize itself in the mirror. If the animal displays self-directed behaviours, then the animal is said to be capable of recognizing itself in the mirror. Social behaviours and self-directed behaviours are further explained in detail in the following sections.

Social Behaviours

 * Social behaviours or other-directed behaviours refer to the actions that animals often make in the presence of other members of the same species. Gallup’s logic was that an animal would display social behaviours towards its mirror image because it recognizes the reflection as another member of its species. Thus, if an animal behaves socially toward its mirror reflection, it is inferred that it lacks the ability for self-recognition and the cognition of self-concept. For example, when chimpanzees are exposed to mirrors, they may demonstrate a number of social behaviours such as bobbing, vocalizing, and threatening.

Self-Directed Behaviours

 * Self-directed behaviours refer to animals’ actions that are responses to themselves. In mark tests, animals touch their marked parts while looking at the mirror. Animals also use the mirror reflection to explore their body parts that are out of direct visual access. Chimpanzees in Gallup’s studies (1970, 1977) exhibited a number of the self-exploratory behaviours. The chimpanzees used the reflection to groom parts of the body that could not be seen directly, and to pick bits of food from between their teeth. The animals also blew bubbles and made faces while looking at the mirror. Thus, when animals display self-directed behaviours, it is interpreted as the presence of self-recognition ability.

The First Mirror Test

 * Gallup (1970) is the first who tested animals' ability to recognize self in the mirror. Two male and two female preadolescent chimpanzees were the subjects of the study. Each of them was isolated by itself and put into a cage. The chimpanzees were exposed to the mirror for approximately 80 hours in total prior to the test. The animals were anesthetised for the purpose of marking and were marked on two body parts (one eye brow and ear on the opposite side) with a red dye. The dye did not cause irritation to the skin and the marked spots were only visible through mirror reflection. Results suggested that, as time progressed, chimpanzees displayed less social behaviours and more self-directed behaviours. Thus, chimpanzees were found to be capable of recognizing themselves in the mirror and were found to possess the cognition of self-concept.

Modified Mirror Tests
It was brought to many psychologists’ attention that the first mirror test developed by Gallup (1970) had some limitations in regards to studying self-concept in animals. Researchers have developed some modified versions of mirror test to better account for different problems the original mirror test elicited.

Using Different Marking Substance

 * Heschl and Burkart (2006) challenged the typical mirror tests, arguing that the standard versions do not provide conclusive statements when the subjects fail the test. To tackle such problem, the researchers developed a slightly altered mark test using chocolate cream (instead of a typical dye) as the marking substance. The study involved a number of marmosets (certain species of the New World monkeys) as subjects. Marmosets were tested in both the standard and the modified versions of mirror test. The results suggested that some marmosets were able to pass the standard test, whereas all marmosets could not pass the modified version. Thus, the researchers claim that the new mark test with chocolate cream helped distinguishing the true lack of self-recognition ability in monkeys from the false positive results due to the methodological limitations of the standard test.

Preventing Direct Gaze at the Face

 * Machellini and colleagues (2010) considered the fact that direct gaze at the face is often an aggressive gesture among many monkey species. The researchers modified the typical mirror test slightly by marking the monkeys on the chest, rather than on their faces. This minimized direct gaze at the face. The results indicated that pig-tailed macaque monkeys still did not pass the mirror test and do not have the ability recognize self in the mirror. It was also suggested that the same cognitive ability is used for both face and body recognition.

Training the Animals to Touch the Mark

 * Roma et al. (2007) considered the idea that monkeys are less likely than humans or apes to touch the marked spots even when the marks are visible without the aid of mirrors. Capuchin monkeys were trained to touch marked spots on their bodies to motivate them to touch the marks. When the monkeys were tested with a novel mark only visible by mirror reflection, none of the monkeys passed the test. The results supported the previous claim that monkeys do not possess the ability of self-recognition in mirror images.

Considering New Measures
Itakura (2001) suggested a different measure for self-recognition of non-human primates. Itakura found that the heart rate of a Japanese macaque when the monkey saw other Japanese macaques was different than its heart rate when it saw its mirror reflection. The heart rate of the monkey was increased at the first sight of other member of the species but declined soon after. However, in the mirror reflection condition, the heart rate decreased at first, then increased after. Although Itakura was cautious about drawing conclusions from a single subject, he suggested that measurement of heart rates may be used to study self-recognition in mirrors.

Decline of Self-Recognition Ability with Age
A group of chimpanzees that passed a typical mirror test were tested again 8 years later. The animals were not exposed to mirrors after the first mirror test until the same test 8 years later. The results indicated that evidence for self-recognition was still present in the animals. This study strengthened the theory for self-recognition in primates. However, it was also found that the ability may be weakened with age. It was comparable to the self-recognition ability of humans, because humans also have tendency to decrease in the ability with age.

Human vs. Non-Human Comparison
Swartz (1997) argued that self-recognition in the mirror by non-human primates does not necessarily imply self-concept as in humans. The understanding of self-recognition in non-human primates is different from the understanding of self-recognition in humans. The mark-mirror test used in non-human primate studies is different from the one used in human infant studies. This makes direct comparison of the results of mirror self-recognition studies in humans and non-human primates problematic.

To reconcile this problem, Bard et al. (2006) applied nearly identical mark-mirror tests to human and chimpanzee infants. Results indicated that human and chimpanzee infants performed comparably; both groups were found to develop self-recognition using mirrors around at the age of two years. How the infants discover the mark did not seem to play an important role in passing the test. The results also revealed that emotional distress may negatively affect the ability of self-recognition via mirror reflection. In typical mirror tests, animals are often isolated and anesthetised for making. This would cause emotional distress to the animals. In comparison, human children infants are rarely isolated and never anesthetised in the test situations, and are often accompanied by their parent. When the conditions were equivalent, human and chimpanzee infants gave similar results.

Animals that Passed the Mirror Test
Anderson and Gallup (2011) concluded that the great apes, but not monkeys, pass the mirror test.
 * Great apes
 * Chimpanzees
 * Humans

Animals that Failed the Mirror Test

 * Monkeys
 * Rhesus monkeys
 * Capuchin monkeys


 * Cotton-Top Tamarins


 * Jungle Crows

Rhesus Monkeys
A pair of rhesus monkeys who were continuously exposed to a mirror from the age of 3 months showed increased social behaviour after the mirror was withdrawn shortly and placed at a different location. This suggested that the monkeys did not recognize their reflection as themselves.

Capuchin monkeys
Anderson and Roeder (1989) used five different ways of mirror presentation to study self-recognition abilities in capuchin monkeys. The results suggested that the monkeys are not capable of recognizing self in mirror images. Paukner, Anderson, and Fujita (2004) presented two different types of mirrors simultaneously to capuchin monkeys. The results supported the previous claim that monkeys are not capable of recognizing self through their mirror images.

Cotton-Top Tamarins
Hauser and colleagues (1995) found that cotton-top tamarins displayed more self-directed behaviours when the monkeys were exposed to mirrors prior to test and species-distinct physical feature was marked. Every cotton-top tamarins in this study was reported to have self-recognition ability. However, the study was later criticized for its lack of important controls. Hauser and colleagues (2001) failed to replicate their results in a study conducted a few years later. Cotton-top tamarins were unsuccessful at passing the mark-mirror test, which indicates lack of self-recognition ability. The amount of mirror-exposure time prior to the test did not affect the results. Thus, it was concluded that cotton-top tamarins do not have the ability of self-concept cognition.

Jungle Crows
Researchers Kusayama, Bischof, and Watanabe (2000) found that jungle crows recognize their mirror images as other members of the same species. Thus, jungle crows lack the ability of self-recognition through mirror images.

Inconsistencies in Findings
Marino, Reiss, and Gallup (1994) studied self-recognition in bottlenose dolphins and found no evidence for the dolphins' ability to recognize self via mirrors. The researchers suggested that the typical mirror testing methods used in non-human primate studies may not be suitable for studying self-recognition in species whose primary sense of perception is not visual. However, another study with bottlenose dolphins found evidence for the animals’ self-recognition ability through mirror test. Further research is required to draw a firm conclusion about presence of self-concept in bottlenose dolphins.
 * Bottlenose Dolphins

Controversies over the Theory of the Mirror Test
Since Gallup’s (1970) first report of the study of mirror test with chimpanzees, many research studies have been conducted and replicated Gallup’s findings. The great apes were found to be capable of recognizing self in mirror reflections. Researchers also continued to fail to find evidence for mirror self-recognition in monkeys and other nonhuman primates. Gallup drew the following conclusion: animals that pass the mirror test possess the cognitive ability for self-awareness, or self-concept. Gallup explained that the animals must have a concept of self before knowing who they were seeing in a mirror.

Gallup (1977) also attempted to link the mirror test phenomenon with social influence. He found that socially deprived chimpanzees failed to show self-directed behaviours, whereas chimpanzees that grew up in natural, social environment exhibited self-directed behaviours. Gallup suggested that early social experiences influence the development of the self-recognition ability. In other words, the concept of self may emerge through interaction with others.

However, Gallup’s interpretation was challenged by other psychologists of the field. Critiques of Gallup’s theory pointed out that it is difficult to find evolutionary drive in favouring the mirror self-recognition ability in natural environment. The critiques argued that Gallup’s theory does not explain the fact that this ability is limited to humans and the great apes. Moreover, the fact that gorillas, a member of the great ape group, continue to fail the mirror test was a puzzling case that could not be easily explained. The link between mirror test and social influence failed to explain why many highly-social primates do not pass the mirror test, but orangutans, known for their solitary lifestyle, do pass the mirror test.

Barth, Povinelli, and Cant (2004) offered a different approach to the phenomenon of mirror test. They argued that as the great apes evolved to have increased body mass, they were required to possess a more detailed and clear representation of their bodies. The great apes and humans have much heavier bodies compared to other primate species. The common ancestor of the great apes and humans is thought to be closely represented by orangutans. Orangutans are arboreal (tree-dwelling), and spend most of their times on trees. When they move from one tree branch to another, their heavy body causes deformation of tree limbs, creating big gaps between the branches. This problem forces orangutans to come up with effective solutions. The solution, as the researchers suggest, is the capacity for planning and executing movements. The researchers argue that a better representation of their bodies was required, because the animals needed to keep track of how their body affects the surrounding environment. The interaction between body mass and deformability of the arboreal environment may have triggered the evolution of the capacity for more explicit body representation. This helped the emergence of self-concept, and thus the great apes are able to pass the mirror test. The researchers called their model, the Self Evolved for Locomotor Flexibility (SELF) model.

The SELF model also attempts to provide explanation for the odd case of gorillas. The researchers stated that gorillas are terrestrial animals, which means they live on the ground and do not spend most of their time on trees. This terrestrial nature did not require the explicit representation of the body, or the capacity to keep track of their movements. Thus, the SELF system might have been traded off with other traits such as more rapid physical maturation. The researchers noted that it is possible that gorillas still have the SELF system, but the system may have been turned off. Therefore, it enables gorillas to keep more developed representations of the body than other primates (outside the great apes).