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Abstract

In recent years evidence has accumulated that at least some animals can remember the ‘what’, ‘where’ and ‘when’ of personal experiences. Currently, evidence for such ability is taxonomically restricted to birds and mammals. Here, we demonstrate for the first time that cleaner wrasses Labroides dimidiatus are able to remember when they interacted with what after a single event. In nature, cleaners remove ectoparasites from other reef fishes, so-called clients. Clients are depleted, non-stationary food patches at the end of an interaction and replenished only after a delay. In our experiments, we presented twelve cleaners every 2.5 min, a choice between two of a total of four plates with different colours and patterns. One plate was always accessible but contained a non-preferred food item while the other three contained a preferred food item, but allowed a next feeding event only after 5, 10 or 15 min. Thus, to maximise food intake, cleaners had to remember for each choice when they had last interacted with which plate. When confronted with two plates offering preferred food, cleaners showed an overall significant preference for the plate that allowed access during the trial. For six cleaners, the preference was significant. Also, on trials involving the always accessible plate, cleaners discriminated between trials in which they had to eat the non-preferred food and trials on which they could eat the preferred food. In conclusion, cleaners are able to track the ‘when’ and ‘what’ (or possibly ‘who’) within a biologically meaningful time period.