A bluestreak cleaner wrasse checking itself out in the mirror
Osaka Metropolitan University
Before deciding whether or not to fight another fish, cleaner wrasse check their own reflection in a mirror and size themselves up.
Bluestreak cleaner wrasse (Labroides dimidiatus) are remarkably bright. These finger-sized coral reef fish were the first fish to pass the mirror test, a common assessment to see whether an animal recognises that the reflection is of its own body and not another animal. Researchers have now found that these wrasses use their likeness to construct a mental image of their body size, which they can compare to others.
First, Taiga Kobayashi at Osaka Metropolitan University in Japan and his colleagues tested the fish’s willingness to go on the attack. Against the glass wall of the fish tank, they held up an image of another wrasse that was either 10 per cent larger or smaller than the real fish. Regardless of the size of the model fish in the photograph, the territorial wrasses picked a fight.
Then, they repeated the test but added a mirror, and the fish checked out their reflection. But when the researchers held up a photo of a larger or smaller cleaner wrasse to the glass pane, the fish chose to fight only smaller rivals.
“This was unexpected because we had an image that this fish always shows aggression against rivals, regardless of size,” says Kobayashi.
The tank was partitioned so that the wrasse couldn’t see its reflection and the rival fish’s photo at the same time – so scientists think the fish must be comparing the image to a mental approximation of their own body size.
Cleaner wrasse evolved in an environment without mirrors, so how did they end up with this skill? Whether in the lab or in the wild, it benefits a fish to know what size they are before picking a fight, says Kobayashi. In other words, the fish were clever enough to employ the mirror as a decision-making tool.