Scientists have found that male cleaner fish will punish their female partners if they do something that offends the bigger client fish they are cleaning.This "third-party punishment" is shocking because the males don't appear to be directly harmed by the females' behaviour.

Nichola Raihani of the Zoological Society of London and colleagues at the University of Queensland, Australia, and the University of Neuchâtel, Switzerland, say their study could clarify the evolution of apparently altruistic behaviour in other species, including humans.

The bluestreak cleaner wrasse (Labroides dimidiatus) is a cleaner fish that lives in coral reefs and eats parasites that breed on larger "client" fish.The fish will intermittently try to sneak a larger meal by eating some of the mucus from the skin of the bigger fish, but this upsets the client fish.
"Clients will go away if they are cheated at a cleaning station. That means the male's dinner goes away if the female cheats," said Raihani in a statement.

Males will insistently chase female cleaners who try to cheat the system by sneaking a bite of skin mucus."By punishing dishonest females, the males are not actually sticking up for the clients but are making sure that they get a decent meal," said Raihani.Raihani and her colleagues tested this behaviour by putting two cleaner fish, a male and a female, in a tank and providing them a plate of fish flakes and prawns.

The fish will eat both foods, but much prefers the prawns. Whenever a fish ate a prawn, the researchers removed the plate of food. Ultimately, the males started punishing the females whenever the females ate a prawn. The researchers observed that the females were less likely to eat the prawns following the punishment.

Punishing the females permitted the males to eat more fish flakes, just as punishing the females in nature for eating the client fish's skin mucus allows the males to eat more parasites.