From parasitism to mutualism

What prevents an exploitative individual from taking advantage of a cooperative partner? Most attempts to answer this question focus on reciprocity - exploitation may not pay, if it triggers retaliation.

But in many encounters, only one individual can exploit the other. For example, when coral reef fish approach a 'cleaner' wrasse to have their parasites removed, the cleaner can take advantage of the client by biting living tissue as well. Reciprocity cannot explain why most cleaners refrain from biting, because non-predatory client fish are unable to bite back.

In the September issue of Ecology Letters, Johnstone and Bshary demonstrate that the threat of leaving by the client may nevertheless be sufficient to keep the cleaner in line, and we show that the predictions of this model match existing data on cleaner and client behaviour. Cooperation degenerates into exploitation only as the client loses the ability to escape.

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