Brain evolution in social insects: advocating for the comparative approach.

Sociality is classified as one of the major transitions in the evolution of complexity and much effort has been dedicated to understanding what traits predispose lineages to sociality. Conversely, studies addressing the role of sociality in brain evolution (e.g., the social brain hypothesis) have not focused on particular traits and instead relied largely on measurements of relative brain composition. Hymenoptera range from solitary to advanced social species, providing enticing comparisons for studying sociality and neural trait evolution. Here we argue that measuring the role of sociality in brain evolution will benefit from attending to recent advances in neuroethology and adopting existing phylogenetic comparative methods employed in analysis of non-neural traits. Such analyses should rely on traits we expect to vary at the taxonomic level used in comparative analyses and include phylogenetic structure. We outline the limits of brain size and volumetric interpretation and advocate closer attention to trait stability and plasticity at different levels of organization. We propose neural traits measured at the cellular, circuit, and molecular levels will serve as more robust variables for evolutionary analyses. We include examples of particular traits and specific clades that are well-suited to answer questions about the role of sociality in nervous system evolution.