In the quest to understand the minds of animals, few questions are as compelling as whether they possess self-awareness. This concept, often considered a uniquely human trait, suggests an inner recognition of one’s distinct existence. Scientists have long focused on primates, our closest relatives, to explore this profound cognitive ability. Determining the presence of a “self” in monkeys requires looking beyond simple intelligence and examining the deeper structures of their cognition, often through behavioral tests and observations of their complex social lives.
Defining the Cognitive Concept of Self-Awareness
Self-awareness is the capacity to recognize oneself as a separate individual from the external environment and other beings. Researchers often describe this as a form of metacognition, or “knowing about knowing,” which goes beyond basic consciousness—the state of being aware of one’s surroundings, like sensing a loud noise.
True self-awareness involves the realization that one possesses a body, intentions, and a distinct and enduring existence. This includes bodily self-awareness, the sense of one’s physical body as separate from the rest of the world and subject to one’s control. For social animals, this may also extend to social self-awareness, which is the understanding of one’s role and position within a group hierarchy.
This cognitive function suggests the ability for introspection, an internal focus on one’s own mental states, such as feelings, beliefs, and thoughts. Since animals cannot verbally communicate these internal states, researchers must rely on observable behaviors to infer self-recognition.
The Benchmark: Mirror Self-Recognition Testing
The primary method developed to test for visual self-recognition in non-human animals is the Mirror Self-Recognition (MSR) test, also known as the mark test. Developed by psychologist Gordon Gallup Jr. in 1970, this test is the standard for assessing this specific form of self-awareness. The procedure determines if an animal recognizes its reflection as an image of itself rather than another animal.
The test begins by exposing the animal to a mirror to observe its initial reaction, which typically involves social behaviors like threat displays. The next phase involves marking the animal with an odorless dye or sticker on a part of the body, such as the forehead, which the animal cannot see directly without the mirror. The animal is often anesthetized during marking to ensure it is unaware of the mark’s placement.
After the animal recovers, it is again presented with the mirror. Scientists look for mark-directed behavior, where the animal touches, rubs, or inspects the mark on its own body while looking at its reflection. Passing the test indicates the animal understands the image is its own. Animals that fail typically react to the reflection as if it were a strange conspecific or ignore the mark.
Specific Findings: Do Monkeys Pass the Test
In the classic MSR test, most monkey species—including Old World monkeys like rhesus macaques and baboons, and New World monkeys like capuchins—generally fail to demonstrate spontaneous self-recognition. When first introduced to a mirror, monkeys usually exhibit social behaviors, such as vocalizing or displaying aggression toward the reflection, treating it as another individual. They do not use the mirror to explore or remove the mark placed on their bodies.
This contrasts with great apes (chimpanzees, orangutans, and bonobos), who typically pass the mark test, suggesting a cognitive difference between these two primate groups. The failure of most monkeys led to the scientific consensus that this specific form of self-awareness is absent in the monkey lineage. However, some studies suggest the MSR test might not be a perfect measure for all species.
Studies involving rhesus macaques have shown that MSR behavior can be induced through extensive, specialized training. Researchers used visual-proprioceptive training, where monkeys learned to associate a laser spot on their face, visible only in the mirror, with a tactile sensation or reward. After weeks of training, the monkeys began to spontaneously touch the dye mark on their face when seeing their reflection, thus passing the standard mark test. These findings suggest that while the capacity for mirror self-recognition may be latent in monkeys, it is not an innate, spontaneously expressed behavior as it is in great apes.
Evidence of Complex Cognition Beyond MSR
The inability of monkeys to spontaneously pass the MSR test does not mean they lack advanced intelligence or other forms of self-awareness. Their complex social structures and problem-solving abilities point to sophisticated cognitive skills. Old World monkeys, such as macaques and baboons, have demonstrated cognitive competencies in certain tasks comparable to those of great apes.
Monkeys navigate intricate social hierarchies, requiring them to constantly track the relationships and dominance ranks of dozens of individuals. They display advanced deliberation and decision-making, weighing multiple factors to solve complex optimization problems. This suggests a level of thoughtful processing and mental flexibility.
Certain monkey species, notably capuchins, are known for tool use in the wild, using stones to crack nuts and manipulate resources. Additionally, some primates exhibit tactical deception, manipulating the behavior of others to their own advantage, which suggests an understanding of another individual’s knowledge state. These behaviors indicate a robust awareness of their own body, agency, and social standing, even without visual self-recognition.