Cleaner wrasse failed in early testing stages of both visual and spatial working memory paradigms.

Working memory (WM), an attention-based short-term storage system responsible for the manipulation and integration of past knowledge with present information for goal-directed behavior, is a key executive function and a principal predictor of general intelligence. Despite its importance, WM has not been a major research topic in animal behavior. Here, we first summarize key ideas related to WM from the social sciences for interested colleagues. Given that past methodological inconsistencies have led to mixed results and conclusions across various species, we then designed experiments that incorporate the critical components of WM, facilitating cross-species comparisons and accounting for potential ecological influences. We present such experiments on WM in an ectothermic vertebrate, the cleaner wrasse (Labroides dimidiatus), which faces environmental challenges potentially requiring complex cognitive adaptations. Overcoming several experimental challenges, we consistently obtained negative results across multiple experimental paradigms. As our experiments were specifically designed to test WM, our negative results call into question previous studies in other fish species that provide evidence for WM using different paradigms. More specific tests for WM should be developed to confirm the presence or absence of this executive function in other ectotherm vertebrates. The absence of WM may be a key factor underlying the significant encephalization gap between ectotherm and endotherm vertebrate species.