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."
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.
The study, published today in Science, "is really the first to show a direct benefit to the individual who does the punishing," according to Sarah Brosnan, a behavioral scientist at Georgia State University.
The discovery also offers a "potential explanation for how [punishing] might have evolved in other organisms as well—but this may not hold for other types of social [animals]," added Brosnan, who was not involved in the research.
For example most human studies of third-party punishment show that—unlike in cleaner fish—the individual doesn't benefit, according to Peter Richerson, an expert in human cultural evolution at the University of California, Davis.
Though controversial, some scientists say these experiments show that human do-gooders evolved to benefit the group, rather than the individual, Richerson said.
But he doesn't think the cleaner fish study will lend much insight into human behavior.
"No such issue arises in the [fish] experiment because the males benefit directly from punishing defector females."