Fish measure themselves in a mirror to determine if they can win a fight

Before deciding whether to fight another fish, cleaner wrasses first look at their own reflection in the mirror and assess themselves.

Blue cleaner wrasse (Labroides dimidiatus) are remarkably bright. These finger-sized coral reef fish were the first to pass the mirror test, a common test to see if an animal recognizes that its reflection is of its own body and not another animal. Now, researchers have discovered that these wrasses use their likeness to construct a mental image of their body size, which they can compare with others.

First, Taiga Kobayashi of Osaka Metropolitan University in Japan and his colleagues tested the fish’s willingness to attack. They held up a picture of another wrasse that was 10 percent larger or smaller than the real fish to the glass wall of the aquarium. Regardless of the size of the model fish in the picture, the territorial wrasses went to war.

They then repeated the test, but added a mirror, and the fish watched their reflection. But when the researchers held a photo of a larger or smaller cleaner wrasse up to the glass, the fish chose to fight only smaller rivals.

“This was unexpected, because we had a perception that this fish always shows aggression towards rivals, regardless of size,” says Kobayashi.

The tank was divided so that the wrasse could not see its own reflection and the photo of the rival fish at the same time, so scientists believe the fish is comparing the image with a mental approximation of its own body size.

Cleaner wrasses evolved in an environment without mirrors, so how did they get this ability? Whether in the lab or in the wild, it’s useful for a fish to know how big it is before it gets into a fight, Kobayashi says. In other words, the fish were smart enough to use the mirror as a decision-making tool.