Bluestreak Cleaner checking himself out in the mirror
Osaka Metropolitan University
Before deciding whether to fight another fish, wrasse look at their own reflection in the mirror to gauge their size.
Blue Streak Cleaner Lass (Loveroid) are astonishingly bright. This finger-sized coral reef fish is the first to pass the mirror test, a common assessment of whether an animal can recognize its own body and not another animal in a mirror. Researchers discovered that these wrasses use their own reflection to build an image of their own body size and compare it to others.
First, Taiga Kobayashi and his colleagues at Osaka Metropolitan University in Japan tested whether the fish were willing to attack. They held up images of different wrasses, each 10 percent larger or smaller than the real fish, against the glass wall of an aquarium. Regardless of the size of the model fish in the photo, the territorial wrasses put up a fight.
The researchers then repeated the test with additional mirrors, and the fish saw their own reflection in the mirror, but when the researchers held up pictures of larger or smaller wrasses on the glass plate, the fish chose to fight only the smaller rivals.
“This was unexpected, as this fish has always been known to be aggressive towards rivals, regardless of its size,” Kobayashi says.
Because the tanks are partitioned, the wrasses can’t see both themselves and pictures of rival fish at the same time, so the scientists think the fish must be comparing the pictures to a mental approximation of their own size.
How did wrasses develop this ability, given that they evolved in an environment without mirrors? In both the lab and in the wild, it’s advantageous for fish to know the size of their opponent before fighting, Kobayashi says. In other words, the fish were smart enough to use the mirror as a decision-making tool.
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