Bluestreak cleaner wrasse defend territories from unfamiliar members of their species, and body size helps decide those confrontations. Researchers showed individual fish photographs of unfamiliar wrasse scaled to 10 percent larger or 10 percent smaller than the viewer. Mirror-naive fish attacked both images, behaving aggressively even toward the slightly larger opponent.

After mirror experience and the study’s mirror-recognition procedure, the pattern changed. The fish continued challenging the smaller photographs but refrained from attacking the larger ones. They did not need to inspect their reflection again before every choice, suggesting that they could retain information about their own size. When shown an intimidating larger image, however, mirror-experienced fish frequently revisited the mirror, as if updating the comparison before acting.

The experiment treats a mirror as a source of practical body information rather than a stage for human-like vanity. Its authors interpret the behavior as evidence of a mental body-size representation and an element of private self-awareness. Mirror tests remain debated across animal cognition, and aggression toward photographs is a controlled proxy rather than a natural fight. The measured result is narrower: experience with a reflection changed size-based decisions about unfamiliar rivals in a direction that matched the fish’s real physical odds.