Strange as it might seem, not all animals can immediately recognize themselves in a mirror. Great apes, dolphins, Asian elephants, and Eurasian magpies can do this—as can human kids around age 2. Now, some scientists are welcoming another creature to this exclusive club: carefully trained rhesus monkeys. The findings suggest that with time and teaching, other animals can learn how mirrors work, and thus learn to recognize themselves—a key test of cognition.

"It’s a really interesting paper because it shows not only what the monkeys can’t do, but what it takes for them to succeed," says Diana Reiss, a cognitive psychologist at Hunter College in New York City, who has given the test to dolphins and Asian elephants in other experiments.

The mirror self-recognition test (MSR) is revered as a means of testing self-awareness. A scientist places a colored, odorless mark on an animal where it can’t see it, usually the head or shoulder. If the animal looks in the mirror and spontaneously rubs the mark, it passes the exam. Successful species are said to understand the concept of “self” versus “other.”

But some researchers wonder whether failure is simply a sign that the exam itself is inadequate, perhaps because some animals can’t understand how mirrors work. Some animals—like rhesus monkeys, dogs, and pigs—don’t recognize themselves in mirrors, but can use them to find food. That discrepancy puzzled Mu-ming Poo, a neurobiologist at the Shanghai Institutes for Biological Sciences in China, and one of the study’s authors. “There must be some transition between that simple mirror use and recognizing yourself,” he says.

So Poo and his colleagues put three young male rhesus monkeys through an intensive training program. Each monkey was secured in a chair facing a mirror, and researchers flashed a red laser pointer at random positions nearby. When the monkeys touched the dot, they received a treat. Sometimes, they could see the dot only by using the mirror. "The monkey has to learn that the hand in the mirror is his own hand. And he has to learn how to control it precisely by watching it in the mirror," Poo says. "That’s the key." Three other monkeys, who served as controls, weren’t trained to respond...

For more details, please refer to http://www.sciencemag.org/news/2017/02/monkeys-master-key-sign-self-awareness-recognizing-their-reflections.