Looking in the mirror and recognizing oneself was long thought to be an ability reserved for humans. Recently, however, researchers have found that other apes, such as chimpanzees and gorillas, seem to show signs of self-awareness, including recognizing and inspecting themselves in a mirror. Now one group of investigators claims that rhesus macaques have joined this elite group of self-aware animals.

Luis Populin, a neuroscientist at the University of Wisconsin–Madison, noticed that the macaques in his lab were doing something strange. The monkeys, in whom Populin had implanted electrodes for an unrelated study, seemed to be using mirrors to help groom the areas around the implant. They also appeared to be looking in mirrors to view their genitals.

Populin and his colleagues reported in PLoS ONE in September that the monkeys spent significantly less time looking in a mirror covered with black canvas than a regular mirror. He also reported that the macaques were using a large mirror to view areas of their body they could not otherwise see. Both these results, the researchers claimed, indicate that macaques are self-aware.

The problem is that rhesus ma­caques have not yet passed the stan­dard measure of self-awareness, known as the mark test. In this ex­per­iment, researchers anesthetize an animal and attach a small red dot to the middle of its forehead, where the dot will go unnoticed unless the animal can recognize itself in a mirror. Psy­ch­ol­ogist Gordon Gallup, currently at the University of Albany, S.U.N.Y, developed the test with chimpanzees. When Gallup’s chimps woke up and were given a mirror, they peered into the mirror while touching the red dot, indicating that they noticed the change in their appearance. Populin tried the test on his macaques, and they failed to notice the dot—and therefore, Gallup says, they cannot be self-aware.

Populin, however, believes that the problem lies in Gallup’s test. “The standard mark does not seem to be relevant enough for the macaques to care,” Populin says.

Gallup disagrees. “Many people have tested rhesus monkeys for self-recognition, and nobody has ever found compelling evidence,” he notes. “There are too many alternative explanations for why the monkeys touched their acrylic implants.” For instance, they could have been responding to the physical sensation of the implants—and they just happen to like sitting in front of a mirror.