Alex Jordan had just surfaced from a dive off the coast of Corsica when he called me back last summer. “We’ve put mirrors in the wild,” he said. “It’s always a bit of a nightmare.” With the help of ...

Bluestreak cleaner wrasse are small, territorial fish that aggressively fend off intruders. But when they have access to a mirror, the fish size themselves up before deciding whether or not to fight.

A finger-length fish has reignited a long-running debate about self-awareness, cognition and how much we’ve underestimated ...