Pairs of cooperating cleaner fish provide better service quality than singletons

The evolution of cooperation between unrelated individuals is a puzzle for researchers in both the social and natural sciences. One possible obstacle to a greater understanding of the phenomenon is the tendency for theorists and experimentalists to work independently. Bshary et al. avoid that charge by combining game theory modelling, field observations and experimental testing in a study of a hitherto unexplored problem of cooperation, the cleaning mutualism between stable male–female pairs of the cleaner wrasse Labroides dimidiatus and their client fish. Theory predicts that two providers should offer a higher service quality to clients than single providers, as long as they cooperate with each other. Field observations and experiments confirm the model prediction. Key to the success of a pair of cleaner wrasse is the fact that while one cleaner eats ectoparasites off the client the other can carry on with its preferred cheating behaviour, eating the client's mucus, while client satisfaction is still guaranteed.