Cheaters may not prosper—but punishers do, according to a new study.



Male cleaner fish will chase and pester female fish if they interfere with the male's mealtime—the first evidence of a species benefiting from third-party punishment.



If you're a cleaner fish, it's bad table manners to nibble on the mucous layer of "client" fish, which are generally bigger than the cleaners. Clients stop by multifish cleaning "stations" to get rid of their parasites, which become food for the cleaners.



(Related: "Cleaner Fish Wear 'Uniforms' to Advertise, Avoid Danger.")



But biting off a chunk of tasty mucous means the larger fish may flee—so one mischievous cleaner can deprive another from a meal.



The male "loses something if the female cheats the client, and that's why he corrects the behavior," said study co-author Redouan Bshary, a behavioral ecologist at Switzerland's Université de Neuchâtel.



Not that males are always respectable: They'll cheat, too, but females endure most of the punishment simply because they're weaker, he added.



"Imagine you are collaborating with Mike Tyson," Bshary said. "If you cheat he will punish you, but if he cheats you probably won't do anything."



Prawn Punishment



Scientists had observed male cleaner fish chasing mucous-eating females in the wild.



But to determine if the males were punishing females, Bshary and colleagues created an experiment. They provided aquarium-dwelling bluestreak cleaner wrasse with a plate of fish flakes—their boring, everyday diet—and prawns, which are about as delectable as fish mucous. (See a wrasse picture.)



Each time a female ate a prawn, scientists removed all the food from the tank.



The team observed that the males chastised prawn-eating females—and that the females obeyed by stopping the behavior.