Are monkeys self-aware?

Rhesus monkeys may recognize their own reflection in a mirror, indicating self-awareness--a trait traditionally reserved for humans, chimpanzees and orangutans and a topic of much debate among researchers, including Marc Hauser, professor of psychology at Harvard University and the recent subject of misconduct investigations.

Rhesus monkey Image: Wikimedia commons, user 13bobby

The results, published in the September 29th issue of PLoS ONE, question the existence of a stark cognitive divide that separates higher primates from the rest of the animal kingdom.

"In most instances, monkeys do not show [self-awareness]," Christopher Coe, director of the Harlow Primate Laboratory at the University of Wisconsin-Madison, who was not involved in the work, said in an email to The Scientist. But the new study "indicates that rhesus monkeys can acquire this ability in the right setting and with the right tools."

For years, the Gallup mark test has been the standard method for assessing self-awareness. Researchers dye a small tuff of hair on an animal's head, and then give it access to a mirror. If the animal touched the mark while looking in the mirror, researchers concluded it understood the reflection to be its own. Humans over the age of two, chimpanzees, orangutans and potentially gorillas can conclusively pass this test. Monkeys, on the other hand, nearly always fail.

In 1995, Hauser published a controversial paper in Proceedings of the National Academy of Sciences (PNAS), which reported that cotton-top tamarins show signs of self-awareness, despite many failed attempts in the past by other researchers. He used a modified form of the Gallup mark test, however, by dying the entire white head of the tamarin a bright color, such a green or pink. Hauser argued that the small mark was simply not relevant to monkeys, causing them to fail the test in the past.

Yet, monkeys react to a Gallup mark that they can see on their arm or hand, noted Gordon Gallup, lead researcher in the field, professor of psychology at the University at Albany, SUNY, and inventor of the Gallup mark test. "If it's salient on their arm, then it ought to be equally salient on their face."

Skeptical after reading Hauser's paper, Gallup requested to see some tapes recorded during the study. "When I looked at the tapes, I was absolutely shocked," he said. "There was not a shred of evidence in any of the video tapes that suggested that cotton-top tamarins could recognize themselves in mirrors."

In 2001, Hauser reported that he was unable to reproduce the results of the 1995 paper. His new findings, published in the American Journal of Primatology, suggested that cotton-top tamarins do not exhibit behaviors suggesting self-awareness, once again limiting this ability in primates to the great apes.

Despite the irreproducible results of the 1995 paper, Randy Schekman, editor-in-chief of PNAS, said the journal does not have plans to retract Hauser's original paper. "The Harvard committee investigating Hauser has not contacted us about it, and we have no reason to pursue the matter unless someone challenges the paper."

Another study, on the rule learning abilities of cotton-top tamarins, unrelated to his tests of self-awareness, was the subject of the recent misconduct investigation and the retraction of a 2002 Cognition paper.

The new study on rhesus macaques now provides more evidence that some monkeys do possess the ability to recognize themselves in mirrors. Luis Populin, a professor of anatomy also at the UW-Madison, who normally studies the effects of drugs such as Ritalin on monkeys, stumbled upon this project when graduate student Abigail Rajala claimed that she observed a monkey using a small mirror provided for enrichment to groom himself. The monkey paid particular attention to the area around an implant in its head, which the researchers used in their studies on attention deficient disorder.

The observation prompted Populin and his colleagues to test for self-awareness in the monkeys, replacing the traditional splotch of color used in the Gallup mark test with the head implant. They also used monkeys that had years of experience with mirrors, which Populin believed was a necessary ingredient for them to pass the mark test.

While looking into the mirror, the monkeys examined and groomed the area around their implant and other unseen areas on their bodies, such as the genitals. In cases where the implant was removed, the monkeys failed to touch their heads at all, but continued to examine their genitals in the mirror.


The implanted monkeys also showed sparing amounts of aggressive or submissive social responses, another indication that they did, in fact, see the reflections as themselves.

The paper contains a couple of flaws, however, that "render the results inconsequential and uninterpretable," said Gallup. For one, the monkeys can feel the implant in addition to seeing it, unlike the traditional color mark, which controls for tactile cues. Thus, they could be drawn to touch it, despite their reflection in the mirror.

However, Populin believes he controlled for this by presenting the monkeys with a mirror blocked by black plastic. When the mirror was concealed, the monkeys failed to examine their implant and their genitals as often. "Subjects may touch the area because it itches or it is irritated," he agreed. "Although if that were the case, one would see no difference between the mirror and no mirror condition."

The videos of Populin's work, which were published along with the paper, are no help in solving the debate. Some researchers argue the behaviors in the videos do not illustrate self-awareness, and some argue they do. The videos don't "strike me as compelling, self-directed behavior," said Gallup, "but [they do] strike me as investigative behavior coupled with instances of intermittent social behavior." Contrastingly, Charles Snowdon, professor of psychology and zoology at the UW-Madison, who was not involved in the research, said, "the videos are impressive in that rhesus macaques show some evidence of precursors of mirror recognition," in an email to The Scientist.

Whether there exists a blatant cognitive divide separating higher primates from the rest of the animal kingdom still remains open. "For other species, vocal or odor recognition may be more salient," said Coe, "but until paradigms are developed in other modalities we will not know what other species may have [self-awareness]."