“Perhaps part of the reason the males are so likely to cheat is that females never punish males”, marine scientist says.

Male Labroides dimidiatus cleaner fish punish females that bite instead of clean (thus driving off the bigger fish) but females never punish males for doing exactly the same thing.

The Bluestreak cleaner wrasse (Labroides dimidiatus) lives on coral reefs where it feeds by removing parasites and dead tissue from the skin of larger fish. Most of the time the wrasse provides bigger fish with a valuable service, but sometimes the tasty mucus in front of the cleaner turns into an irresistible temptation, prompting the wrasse to bite off a mouthful. This is naturally not appreciated by the bigger fish and a cleaner wrasse who can’t control his or her urges will have to watch the big fish take off in a jiffy, taking all the nutritious parasites with it.

When a male fish notice a female fish scaring off the big fish they are cleaning together he will promptly punish her for her injudiciousness. This might seem altruistic, but the male fish is actually pissed off at her for making his dinner swim away.

“The male’s dinner leaves if the female cheats,” says Nichola Raihani from the The Zoological Society of London who has been studying Labroides dimidiatus together with research partner Redouan Bshary.

“By punishing cheating females, the males are not really sticking up for the clients but are making sure that they get a decent meal,” Raihani explains.

Raihani believes true altruism is rare.

“When you see something that looks like it’s altruistic, if you look hard enough, there’s normally going to be a benefit somewhere down the line for the person that’s doing that supposedly altruistic act,” she says.
Interestingly enough, a female fish that has to watch her dinner swim away because a male wrasse couldn’t leave the mucus alone never punishes the culprit.

“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,” Dr Raihani told the Science podcast.

Males tend to be larger than females and this might be why the female finds it safer not to discipline him. All Bluestreak cleaner wrasses start out as females and in a group of 6-8 wrasses you will never find more than one male. If the male dies or is removed from the group, the strongest female will change into a male and take his place.

The wrasse study has been published in the journal Science.
You can download the podcast here: http://podcasts.aaas.org/science_podcast/SciencePodcast_100108.mp3