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Brain size and weight vary tremendously in the animal kingdom. It has been suggested that brain structural develop- ment must evolve balanced between the advantages of dealing with greater social challenges and the energetic costs of maintain- ing and developing larger brains. Here we ask if interspecific differences in cooperative behaviour (i.e. cleaning behaviour) are related to brain weight variations in four close-related species of Labrid fish: two are obligatory cleanerfish throughout their en- tire life (Labroides dimidiatus and L. bicolor), one facultative cleaner fish Labropsis australis and one last species that never en- gage in cleaning Labrichthys unilineatus. We first search for the link between the rate of species' cooperation and its relative brain weight, and finally, if the degree of social complexity and cooperation are reflected in the weight of its major brain sub- structures. Overall, no differences were found in relative brain weight (in relation to body weight) across species. Fine-scale dif- ferences were solely demonstrated for the facultative cleaner L. australis, at the brainstem level. Furthermore, data visual exami- nation indicates that the average cerebellum and brainstem weights appear to be larger for L. dimidiatus. Because variation was solely found at specific brain areas (such as cerebellum and brainstem) and not for the whole brain weight values, it suggests that species social-ecological and cognitive demands may be directly contributing to a selective investment in relevant brain areas. This study provides first preliminary evidence that links potential differences in cognitive ability in cooperative behaviour to how these may mediate the evolution of brain structural development in non-mammal vertebrate groups (Current Zoology 61 (2): 274-280, 2015). Keywords Brain weight, Body weight, Macro-area weight, Labridae, Cooperation, Cleaning behaviour

Preliminary notes on brain weight variation across labrid fish species with different levels of cooperative behaviour