Dab a mark on a sleeping chimp's face, and it checks the spot in a mirror

In 1970, Gordon Gallup marked anesthetized chimpanzees with an odorless red dye above the brow, then watched them wake and see a mirror. Chimps with mirror experience immediately touched and inspected the mark on their own face, something they'd never do without a mirror. The test became the standard way to check whether an animal, or a human toddler, recognizes itself as separate from its reflection.

— Gordon G. Gallup Jr., Chimpanzees: Self-Recognition — Science, vol. 167, 1970