Are cleaner fish, Labroides dimidiatus, inequity averse?

Inequity aversion (IA), a willingness to incur temporary costs to prevent unequal outcomes, is common in humans and thought to be beneficial in the context of cooperative relationships with nonkin, since it might allow individuals to regulate contributions to cooperative activities. Attempts to address whether nonhuman animals also show IA have produced mixed results: some studies found that cooperative species are more likely to show IA while others did not. This ambiguity may arise because animals are typically tested for an aversion to working for differential food rewards, even though most tested species do not regularly cooperate to access food. We used the interspecific mutualism between cleaner fish and their reef-fish ‘clients’ to investigate whether IA exists in a species that regularly cooperates with unrelated individuals in the food domain. Cleaners were tested in pairs of actors and recipients. Actors had to perform a task to provide a food reward to both actor and recipient. Cleaners show consistent food preferences in the wild and under laboratory conditions, allowing us to vary the value of the food reward offered to actor and recipient to test whether actors were less likely to work when recipients received higher value rewards. We performed two experiments: actors worked either for their opposite-sex partner or for a same-sex competitor. We found no evidence that cleaners were sensitive to inequity: actors were equally likely to perform the task in all experimental conditions. We discuss these results in light of theories of the evolution of IA.