Supplementary Material for: Population density drives concerted increase in whole brain volume in a wrasse species Coris batuensis

Introduction: The factors shaping vertebrate brain evolution and cognition are broadly categorized as being either social or environmental. Yet their relative importance is debated, partly due to the limitations associated with standard interspecific evolutionary comparisons. Here, we adopt a complementary strategy leveraging within-population variation in fish brain size to ask how variation in social and environmental factors correlate with individual brain size. Methods: We investigated how overall brain size and brain part sizes varied between demes of the same population in the coral reef associated batu coris Coris batuensis. This species is ideal for our approach because its local population densities are dissociated from both interspecific densities and habitat complexity. Results: We found that individuals from demes with higher population densities possess larger overall brain volumes than those from lower population density environments, caused by an enlargement of all five main brain regions. Brain anatomical measures show no correlation with interspecific density or habitat complexity. Conclusion: Our results suggest that variation in intraspecific social challenges select on individual batu coris brain size, either through phenotypic plasticity, differential survival, or habitat choice. These results conform with a broader version of the social brain hypothesis, emphasizing the importance of the entire brain over specific regions like the neocortex in mammals or the telencephalon in fishes.