In the popular children’s series, the mischievous monkey Curious George loves to look at himself in the mirror. But do his real-life cousins have the same hobby? Scientific consensus has long been that rhesus monkeys see the monkey in the mirror as a new playmate, not a reflection of their own image. Recent research, however, suggests that monkeys might really be as probing and self-aware as readers of the Curious George books were led to believe.
In 2010, Abigail Rajala, a neuroscience graduate student at the University of Wisconsin-Madison, noticed that rhesus monkeys in the lab seemed to recognize themselves in mirrors, grabbing them and manipulating the mirrors to see their reflected image. When she told lab director Luis Populin of her observations, they worked together to develop an experiment to determine what the monkeys were perceiving, using an adaptation of the “mark test.”
The test, designed to experimentally measure self-awareness in animals, involves a small mark (a dot of dye or a sticker) placed on a sleeping animal’s forehead and ear, both places that an animal can’t normally see. The animal is then woken and placed before a mirror.
Scientists think that what the animal does next reveals whether or not it is self-aware. If the animal touches the marks on its own body and not on the reflection, then it is aware of its embodied self, according to those scientists who endorse the test. Chimpanzees, one of our closest primate relatives, have passed the test. Rhesus monkeys, however, have failed it since the 1960s, when the mark test was developed.
In their experiment, Populin and his team placed full-length mirrors within reach of the monkeys, so they could move them. Surprisingly, the monkeys reacted to the mirrors, using them to touch their genitals and the medical implants on their heads, both of which are not normally seen without the aid of a mirror and aren’t usually examined. From this, Populin concluded that the monkeys recognized themselves in the mirrors. He and Rajala published their results on September 29 in the journal PLoS ONE.
But not everyone agrees that the rhesus monkeys knew they were looking at their own mirror images. “I think there’s better evidence for recognition in magpies than in these monkeys,” says Daniel Povinelli, a primatologist at the University of Louisiana-Lafayette.
Magpies pass the mark test with flying colors; they recognize and pick at stickers placed on their bodies. Povinelli believes that rhesus monkeys, in contrast, are simply engaging in normal behaviors like grooming and attempting to socialize with the mirror monkeys. He also maintains that the monkeys have not passed the mark test because they still have not recognized a traditional mark of dye or a sticker.
But Populin counters that it’s a matter of having a “salient mark”—a mark interesting enough for a monkey to notice in its reflection. Monkeys find the implants to be more interesting than traditional marks, he believes. Populin hopes to conduct further studies with the rhesus monkeys, using them to examine human conditions like gaze aversion, in which one person avoids eye contact with another, even during a conversation.
The argument over whether monkeys really recognize their mirror images is not just a narrow debate among primatologists. It also has broad implications for evolutionary theory. “There is a huge ideological clash in the field of cognitive evolution,” explains psychologist Phillipe Rochat of Emory University in Georgia, who has researched mirrors and their role in the human psyche. The clash is over how self-awareness evolved: either as a distinct evolutionary jump, or as something that evolved gradually and continuously along the evolutionary ladder. The ability of monkeys to recognize themselves in a mirror might indicate a level of self-awareness that supports the idea of a gradual development of that ability along the evolutionary tree.
Rochat added a caveat, saying that while we may think we know what is going on in an animal’s head during a mark test, we do not know for sure, in part because human interpretations are biased by our own perceptions of self in a mirror. “We see not just ourselves, but what we project to the outside world,” Rochat says. “There is no other animal that shows such a compulsive propensity to self-present.”