Power and temptation cause shifts between exploitation and cooperation in a cleaner wrasse mutualism

Abstract

In many instances of cooperation, only one individual has both the potential and the incentive to 'cheat' and exploit its partner. Under these asymmetric conditions, a simple model predicts that variation in the temptation to cheat and in the potential victim's capacity for partner control leads to shifts between exploitation and cooperation. Here, we show that the threat of early termination of an interaction was sufficient to induce cleaner wrasse Labroides dimidiatus to feed selectively against their preference (which corresponds to cooperatively eating client fish ectoparasites), provided that their preference for alternative food was weak. Under opposite conditions, cleaners fed selectively according to their own preference (which corresponds to cheating by eating client mucus). By contrast, a non-cleaning fish species, Halichoeres melanurus, failed to adjust its foraging behaviour under these same conditions. Thus, cleaners appear to have evolved the power to strategically adjust their levels of cooperation according to the circumstances.

Figure 1.

Box-and-whisker plots of mean number of less-preferred items eaten per round by (a) 16 cleaner wrasse Labroides dimidiatus and (b) 14 non-cleaner pinstripe wrasse Halichoeres melanurus in four different treatment combinations. Temptation to cheat was either ‘low’ because of a large proportion of the preferred food type in the less-preferred food or ‘high’ because of a smaller proportion of the preferred food type in the less-preferred food, and after a fish ate a preferred food item the plate was removed either quickly (no more feeding possible = high victim control over duration) or slowly (some more feeding possible = low victim control over duration). For L. dimidiatus, letters indicate significant differences among treatment groups. There were no significant differences between treatments for H. melanurus. The asterisks indicate significant differences with the value expected from random foraging (dashed line, calculated by assuming that at each bite, the probability of eating a less-preferred item corresponds to its relative abundance). One asterisk indicates a p-value between 0.05 and 0.01, two asterisks indicate a p-value lower than 0.01. Outliers are represented as circles.