Are Rhesus Monkeys Self-Aware?

Conventional wisdom from cognitive science posits that a variety of animals can recognize themselves in the mirror and, therefore, possess self-awareness. Traditionally, macaque monkeys have not been included among them, but a new study utilizing refined behavioral methods reveals that rhesus monkeys can indeed recognize themselves in the mirror. The results appear to reconcile a decades-old conundrum about presumably variable self-recognition abilities among evolutionarily distinct primates.

Traditionally, scientists have assessed mirror self-recognition abilities in animals based on their performance on the “mark test.” In this test, marks are placed on an animal’s face and, subsequently, its behavior in front of a mirror is observed. If the animal spends increased time touching the marks or looking at them in the mirror, then it passes the mark test and is assumed to possess at least a rudimentary form of self-awareness. Select chimpanzees, orangutans, elephants, dolphins, and even magpies pass the mark test while gorillas and macaque monkeys do not.

A research team headed by Dr. Luis C. Populin at the University of Wisconsin-Madison suspected that macaque monkeys might be self-aware even though they fail the mark test consistently, even in their own laboratory. This suspicion arose when the researchers noticed the macaques grooming themselves while looking in the mirror after having a surgical implant affixed to their skulls.

Rhesus monkeys that had been exposed to mirrors throughout most of their lives were videotaped during exposure to one of the following hung outside their cage: a small mirror, a large mirror, or a mirror covered with black, non-reflective plastic. Multiple independent observers viewed and scored the videotapes for the occurrence of social and self-directed behaviors. Occurrence of the former (for example, signs of aggression such as open-mouth threats) suggests that a monkey may interpret his mirror image as another monkey, while the latter (self-examination, for example) indicates mirror self-recognition. All of the monkeys studied had previously been prepared with skull implants to facilitate physiological recordings from their brains. However, for purposes of comparison, some monkeys were also observed prior to receiving their skull implants.

In a study recently published in PLoS One, the research team reports that macaques looked at the small mirror at an increased rate, and at an even greater rate in the large mirror. These effects were not observed when the mirrors were covered with black, non-reflective plastic. In addition, the rate at which the monkeys touched unseen body parts was increased almost tenfold with either the small or large mirror compared to when no mirror was present. Moreover, when the large mirror was present, the rate at which social behaviors occurred declined significantly while the rate of self-directed behaviors remained elevated and stable. Interestingly, monkeys without skull implants did not observe themselves in the mirror, but proceeded to do so after the implants were affixed to their skulls.

Mirror-self recognition is a learned ability. Even in children at a specific developmental stage, its expression varies depending on intelligence level, cultural background, and testing conditions. The findings of this study indicate that, although macaque monkeys fail the traditional mark test, they are nonetheless able to recognize themselves in the mirror when the saliency of the mirror image is increased (in this case, with the skull implant). The researchers suggest that the skull implant serves as a “super mark” which facilitates learning of mirror self-recognition in rhesus monkeys. Thus, the traditional mark test may not be an adequate way to assess mirror self-recognition in all species and this ability indeed appears to exist on an evolutionary continuum.


Plotnik JM, de Waal FB, & Reiss D (2006). Self-recognition in an Asian elephant. Proceedings of the National Academy of Sciences of the United States of America, 103 (45), 17053-7 PMID: 17075063

Prior H, Schwarz A, & Güntürkün O (2008). Mirror-induced behavior in the magpie (Pica pica): evidence of self-recognition. PLoS biology, 6 (8) PMID: 18715117

Rajala AZ, Reininger KR, Lancaster KM, & Populin LC (2010). Rhesus monkeys (Macaca mulatta) do recognize themselves in the mirror: implications for the evolution of self-recognition. PloS one, 5 (9) PMID: 20927365

Reiss D, & Marino L (2001). Mirror self-recognition in the bottlenose dolphin: a case of cognitive convergence. Proceedings of the National Academy of Sciences of the United States of America, 98 (10), 5937-42 PMID: 11331768

  • Obp

    Isn’t it odd how the things that we thought defined us as human are proving to be shared by many animals?

  • What form did the implants take? Was there any particular reason they thought the implant contributed? Could the subjects feel the implant?

  • Pingback: What Makes Us Human? | Brain Blogger()

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Dario Dieguez, Jr, PhD

Dario Dieguez, Jr., Ph.D., spent over a decade conducting neuroscience research relevant to cognitive brain aging. He worked as a Science Writer in the Office of the National Institutes of Health (NIH) Director Elias Zerhouni, M.D. and at NIH’s Center for Scientific Review. He taught Cellular Biology and Neurochemistry at the University of Texas at San Antonio and Cognitive Psychology at Boston University. For several years, he worked as a consultant for Pearson, Inc. and as a freelance science writer, with several clients in the U.S., Canada, Australia, and Germany. As a Research Program Manager at the Lupus Foundation of America, he oversaw the awarding of millions of dollars for research and was integral to the launching of Lupus Science and Medicine, an open access journal. Currently, he works as a Health Scientist Administrator at the Society for Women’s Health Research and is an Adjunct Professor of Psychology and Bioethics at The Washington Center.

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