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Image scoring and cooperation in a cleaner fish mutualism

2006; Nature Portfolio; Volume: 441; Issue: 7096 Linguagem: Inglês

10.1038/nature04755

1476-4687

Redouan Bshary, Alexandra S. Grutter,

Experimental Behavioral Economics Studies

If we see someone do a good turn to someone else we are more inclined to help the altruist at some point in the future, rather than someone with a reputation for being selfish. This ‘image scoring’ behaviour is thought to help maintain human cooperation. Redouan Bshary and Alexandra Grutter now provide experimental evidence that it can also stabilize cooperation between non-human animals. The cooperation in this case is displayed by cleaner fish that remove ectoparasites from their partners in a mutual relationship, the ‘client’ fish. The cleaners may cooperate in the task of removing parasites; or a cleaner fish may ‘cheat’ and simply feed on client mucus leaving the other cleaners to do the work. A series of foraging experiments shows that client fish engage in image scoring, and that the cleaners cooperate more in the presence of an image-scoring client. Humans are highly social animals and often help unrelated individuals that may never reciprocate the altruist's favour1,2,3,4,5. This apparent evolutionary puzzle may be explained by the altruist's gain in social image: image-scoring bystanders, also known as eavesdroppers, notice the altruistic act and therefore are more likely to help the altruist in the future5,6,7. Such complex indirect reciprocity based on altruistic acts may evolve only after simple indirect reciprocity has been established, which requires two steps. First, image scoring evolves when bystanders gain personal benefits from information gathered, for example, by finding cooperative partners8,9,10. Second, altruistic behaviour in the presence of such bystanders may evolve if altruists benefit from access to the bystanders. Here, we provide experimental evidence for both of the requirements in a cleaning mutualism involving the cleaner fish Labroides dimidiatus. These cleaners may cooperate and remove ectoparasites from clients or they may cheat by feeding on client mucus11,12. As mucus may be preferred over typical client ectoparasites13, clients must make cleaners feed against their preference to obtain a cooperative service. We found that eavesdropping clients spent more time next to ‘cooperative’ than ‘unknown cooperative level’ cleaners, which shows that clients engage in image-scoring behaviour. Furthermore, trained cleaners learned to feed more cooperatively when in an ‘image-scoring’ than in a ‘non-image-scoring’ situation.