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Monkeys See More Than a Stranger in the Mirror

July 18, 2005

Yerkes-based study challenges previous assumptions monkeys only see their mirrored reflections as other

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Lisa Newbern, 404-727-7709,

ATLANTA — It is widely assumed recognizing one’s own reflection is a trait exhibited only by humans and great apes, and that other animals merely see a stranger in the mirror. But new research suggests capuchin monkeys react differently to mirrors and strangers. This study, conducted at the Yerkes National Primate Research Center of Emory University, is the first of its kind to ask whether monkeys can differentiate between their mirror image and a stranger based on a detailed comparison of how they respond to mirrors versus live individuals. This research, reported in the current issue of the Proceedings of the National Academy of Sciences, may lead to further examination of self-awareness and an appraisal of two schools of thought, one that deems only mirror self-recognition (MSR) species possess a concept of self and another that looks at the self concept as an endpoint of gradual change.

Researchers Frans B. M. de Waal, PhD, Marietta Dindo, Cassiopeia A. Freeman and Marisa J. Hall exposed eight adult female and six adult male brown capuchin monkeys twice to three conditions to determine their responses to 1) a familiar same-sex animal, 2) an unfamiliar same-sex animal and 3) a mirror. Females showed more eye contact and friendly behavior, and less signs of anxiety in front of the mirror than toward the unfamiliar animal. Males showed greater ambiguity, but still reacted differently to mirrors and strangers. These findings suggest the monkeys seem to recognize their mirror reflections as special and may not confuse them with an actual stranger.

“It is possible the monkeys reach a level of self-other distinction intermediate between seeing their mirror image as other and recognizing it as self,” said Dr. de Waal. “The capuchins seem to possess a greater understanding of the mirror’s illusory qualities than previously assumed.”

These results challenge the sharp boundary proposed by students of animal behavior that assumes all animals who do not exhibit MSR represent a single cognitive state and possess no concept of self, effectively lumping together in one group species ranging from fish to birds to monkeys. The current research is more closely aligned with the beliefs of some developmental psychologists who argue a level of self-awareness is an end result in a gradual process toward a concept of self.

“The reactions to a mirror and to the stranger are dramatically different, which suggests the monkeys realize the image in the mirror is not a stranger,” continued Dr. de Waal.

The Robert W. Woodruff Health Sciences Center of Emory University is an academic health science and service center focused on missions of teaching, research, health care and public service. Its components include the Emory University School of Medicine, Nell Hodgson Woodruff School of Nursing, and Rollins School of Public Health; Yerkes National Primate Research Center; Winship Cancer Institute of Emory University; and Emory Healthcare, the largest, most comprehensive health system in Georgia. Emory Healthcare includes: The Emory Clinic, Emory-Children's Center, Emory University Hospital, Emory University Hospital Midtown, Wesley Woods Center, and Emory University Orthopaedics & Spine Hospital. The Woodruff Health Sciences Center has a $2.5 billion budget, 17,600 employees, 2,500 full-time and 1,500 affiliated faculty, 4,700 students and trainees, and a $5.7 billion economic impact on metro Atlanta.

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