Cleaner wrasse can reach Piagetian stage 4 of object permanence when primarily exposed to stage 3

Abstract

Object permanence (OP) is the ability to retain a stable representation of an object, even if it is partially or completely out of sight. Despite its seeming simplicity, this concept has posed challenges for many species, with factors like life history and development being proposed as potential explanations. We tested for OP in the cleaner wrasse Labroides dimidiatus, a species that exhibits complex ecological strategies in the wild, who has been tested in a variety of cognitive experiments, and who could help shed light on differences in brain size and cognitive abilities between ectotherms and endotherms. In two separate series of experiments, we investigated the potential role of ecological relevancy and the complexity of the condition. To test for OP, we used a food reward, a conspecific, or a predator that we presented to cleaner wrasse Piagetian stage 3 or 4 and added a delay or a supplementary cognitive charge with an "A-not-B error" paradigm. Our findings suggest that cleaner wrasse possess some primary forms of OP that depend on training and are thus comparable in performance to some larger-brained endotherm species. Furthermore, ecological relevancy did not clearly influence cleaner wrasse performance. Our findings provide the first evidence of OP in cleaner wrasse; however, we discuss methodological improvements that could help minimize potential biases in future studies.