What does passing the mirror test suggest about an animal?

Some sense of self - the ability to recognise that the reflection is you rather than another animal. The standard version marks the animal somewhere it cannot see without a mirror and watches whether it investigates the mark on its own body. Great apes, dolphins, elephants, magpies and cleaner wrasse have passed.

The test is much weaker than its reputation. It assumes vision is the animal's primary sense and that it cares about its own appearance - dogs fail comprehensively and yet plainly recognise themselves by smell, and a version using scent suggests they pass. Gorillas mostly fail, which is odd given chimpanzees pass, and one explanation is that gorillas find prolonged eye contact threatening and look away. Passing is evidence of something; failing is evidence of very little.

The cleaner wrasse result in 2019 made it worse. A fish passing a test designed to identify self-awareness in primates either means fish have more going on than assumed, or means the test does not measure what it claims. The researchers themselves argued for the second reading, which is an unusually honest thing for people who had just produced a headline result.