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Female cleaner fish are sensitive to what their partners can and cannot see while working on client fish. This means they may have theory of mind, a concept built on awareness of other’s perspectives, often associated with humans and other primates.

Cleaner wrasse (Labroides dimidiatus) typically work in male-female pairs to “clean” client fish by eating their dead skin cells and skin parasites. The wrasse actually prefer to eat the mucus produced by these client fish, but the clients can react to this by terminating the relationship – leaving the cleaners without food.

This means a lot is at stake when a male-female cleaner wrasse pair work as a team. If one fish cheats by attempting to eat mucus while their partner is cooperating with the client, this may leave both fish without food. If a male cleaner fish knows his female partner has cheated, he will sometimes punish her by chasing and even attempting to bite her, says at Boston College in Massachusetts.

But this made McAuliffe and her colleagues wonder whether females had developed ways to cheat without the knowledge of the males. “Because punishment is on the line and females would benefit from getting away with cheating, we had reason to suspect that they might show this sensitivity to what their male partner can and cannot see,” she says.

In an experimental set-up, females had the choice of feeding in a tank with transparent or opaque barriers while their male partner was in a separate part of the tank with either a transparent or opaque partition. The researchers demonstrated that female cleaner fish are indeed more likely to cheat when their male partners are out of view. The team also found that females paired with more punitive males cheated more strategically by moving behind the opaque barriers.

This sensitivity suggests that cleaner wrasse have evolved cognitive abilities that allow them to find solutions to their problems on a par with other animals, such as corvids and primates.

“It’s controversial because in many people’s scheme of the natural world primates can do things that are impossible for other animals, in particular fishes,” says at the Max Planck Institute of Animal Behavior in Germany. “The greatest message of this paper is that there is no ladder which humans sit at the top of and then there’s primates and then there’s something else.”

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