Rhesus monkeys pass the mirror test, but only after training

[Robert]

If you passed out at a party and someone drew a mustache on your face, you’d know it when you next looked in a mirror. There are a few other species—including great apes, dolphins, and corvids—that are also able to recognize themselves in the mirror. Mostly, though, other animals won't try to rub off any hand-drawn mustaches, because they don't seem to understand that they're looking at themselves. Puppies, for instance, have a hard time figuring out this whole reflection business.

Rhesus monkeys are one species that doesn't pass this test—spontaneously. A paper in PNAS this week describes how to teach them to earn a passing grade. The result raises some important questions about the test, and it could help in understanding what mental tools need to be in place before any individual can recognize their own reflection.