Keeping Cleaner Fish in Check

Have you ever caught someone just before they say something embarrassing?  Did you give them a playful elbow?  Well, it turns out that cleaner fish do something quite similar...

Cleaner fish are the little hangers-on you see on larger fish.  And their name is self-explanatory, they clean the larger fish of parasites and dead skin cells.  This 'dirt' is the cleaner fish's food and it keeps the host fish happy, or at least prevents them from eating their followers.

Now Nicola Raihani and her team have found that male cleaner fish will punish the female cleaners if they step over the line and start munching on the tastier host fish, instead.  Because the host fish has a much more nutrient-rich mucus on their skin, and cleaner fish would much rather eat that. But this risks offending the host fish, which might mean the cleaner fish lose their food supply altogether.

In the journal Science this week they tested this by offering the cleaner fish some fish flake feed and some more extravagant prawns. They trained the cleaner fish so that, if one took a bite from the prawns, all the food would be removed from the tank.  Very quickly, the researchers saw that whenever a female cleaner took a bite from the prawns the males would punish her by chasing her away.  And afterwards the females were much less likely to give into their prawny temptation again.

I'm not sure what it says about male-female relationships.  I know I get a telling-off if I reach for the chocolate.  Perhaps I'm offending the god of good female figures?  Raihani said "the males are less well behaved than the females a lot of the time but perhaps part of the reason the males are so likely to cheat is that females never punish males,"

But it might tell us something about the evolution of human behaviour and how we came to monitor each other's behaviour for an overall benefit to the society.  Raihani suggests that, as the male fish are essentially looking after their own stomachs first, this is how behaviour which benefits the group as a whole might have evolved.