Zegni Triki , Xaing yi Li Richter, Ana Pinto, Antoine Baud, Sandra Ann Binning, Melisande Aellen, Yasmin Emery, Virginie Staubli, Nichola Raihani, Redouan Bshary

Supply and demand affect the values of goods exchanged in cooperative trades where high demand typically leads to a higher price. An exception has been described in the marine cleaning mutualism involving the cleaner fish Labroides dimidiatus and its variety of ‘client’ coral reef fishes. Cleaner fish feed on clients’ ectoparasites but prefer eating clients’ mucus instead, which constitutes cheating. Here, we provide field observations, followed by a set of laboratory experiments with real clients and Plexiglas feeding plates as surrogates for clients. In the field and in three experiments with real clients, we found that satiated cleaner fish were more cooperative, even though low hunger levels should make them less dependent on cleaning interactions. Similarly, the more abstract version of the experiments using Plexiglas plates offering two food types mimicking client ectoparasites and mucus showed that satiation led cleaner fish to feed more against their preferences – an indicator of cooperative behaviour. However, this outcome occurred only if the temptation to eat the preferred food was low. When the temptation to cheat was high, cleaners did so. We provide further general support to these findings with a game-theoretic model. Many mutualisms involve food as a commodity. Thus, identifying foraging decision rules will enhance our understanding of how individuals adjust to real-time market conditions rather than playing evolved strategies adapted to the average market conditions.