Cleaner fish punish cheats who offend their customers

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If someone at ⁞your workplace offends a client or a customer, they’d probably get an earful from their colleagues or boss. If someone annoys a friend of yours, you’d probably have a go at them. This capacity to punish those who behave badly, even if they haven’t wronged us personally, pervades all aspects of human society. And we’re not the only ones – fish punish bad behaviour too.

The bluestreak cleaner wrasse (Labroides dimidiatus) operates an underwater health spa for larger fish. It advertises its services with bright colours and distinctive dances. When customers arrive, the cleaner eats parasites and dead tissue lurking in any hard-to-reach places. Males and females will sometimes operate a joint business, working together to clean their clients. The clients, in return, dutifully pay the cleaners by not eating them.

That’s the basic idea, but cleaners sometimes violate their contracts. Rather than picking off parasites, they’ll take a bite of the mucus that lines their clients’ skin. That’s an offensive act – it’s like a masseuse having an inappropriate grope between strokes. The affronted client will often leave. That’s particularly bad news if the cleaners are working as a pair because the other fish, who didn’t do anything wrong, still loses out on future parasite meals.

Males don’t take this sort of behaviour lightly. Nichola Raihani from the Zoological Society of London has found that males will punish their female partners by chasing them aggressively, if their mucus-snatching antics cause a client to storm out.  This punishment is why pairs of cleaners offer better services to customers than singletons.

Raihani set up fake cleaning stations where clients were represented by Plexiglas plates containing two types of foods – tasty fish flakes to represent parasites, and even tastier prawns to represent mucus. Cleaner wrasses would far rather eat prawns than fish flakes but if they did so, Raihani played the role of a huffy client by immediately removing the plate.

When this happened, males were more likely to chase the females. After such a chase, females were less likely to eat the prawns again. Because they learned their lesson, the male got a fuller meal the next time a ‘client’ turned up.  If Raihani used a glass partition to prevent the males from starting their punishing pursuits, the females were just as likely to cheat the next time round and the male would go hungrier.

At first glance, the male cleaner wrasse behaves oddly for an animal, in punishing an offender on behalf of a third party, even though he hasn’t been wronged himself. That’s common practice in human societies but much rarer in the animal world. But Raihani’s experiments clearly show that the males are actually doing themselves a favour by punishing females on behalf of a third party. Their act of apparent altruism means they get more food in the long run.

It’s interesting to consider human punishment in this light. I’ve blogged about this topic several times before. Most previous studies have looked at punishment from society’s point of view, testing the idea that groups of cooperating humans are glued together by the capacity to penalise cheats and freeloaders.

Often, such punishments are essentially altruistic because they come at a personal cost – we expend a lot of time and effort to castigate those who we believe have acted poorly (just look at the entire sceptical blogosphere for an example). But the cleaner wrasse, in demonstrating how individuals as well as groups can reap benefits from third-party punishments, shows us that such punishments can evolve through selfish tendencies rather than altruistic ones.