Scientists from Switzerland, Australia and Sweden have found that cleaner wrasse (Labroides dimidiatus) provide better service to client fish when working in pairs.
Cleaner wrasse typically eat ectoparasites off the client fish, but are prone to cheating and eat client mucus instead, a move which causes the client fish to leave (therefore the benefits of cheating can be gained by only one cleaner when a pair of them work on a client fish).
Redouan Bshary and coauthors first used mathematical models to predict the behaviour of cleaner wrasses working in pairs (which typically consist of a male and a female fish): they found that ...cooperative pairs provide higher service quality than singletons as long as there is added search efficiency for a pair, whereas service quality would decrease or stay the same if each cleaner in the pair uses its singleton strategy.
The authors then compared the results of their modelling to field observations made in Egypt and experiments conducted in aquaria and found both to yield consistent results that support the cooperative solution predicted by the model.
The results also explain the observation that client fish appear to preferentially visit stations with pairs of cleaner fish.
The authors also observed that the increased service quality during pair inspection was mainly due to the smaller females behaving significantly more cooperatively than their larger male partners, and that the males regularly chase the females if the female takes a bite of mucus (indicating a situation where there may be a threat of punishment for the females if they cheat).
The study is published in a recent issue of the journal Nature.
For more information, see the paper: Bshary, R AS Grutter, AST Willener and O Leimar (2008) Pairs of cooperating cleaner fish provide better service quality than singletons. Nature 455, pp. 964"966.