RAPID EVOLUTION OF SOCIALITY DRIVES CHANGES IN BRAIN STRUCTURE AND FUNCTION

Sociality has long been considered a major driver of brain and cognitive evolution, yet most supporting evidence derives from comparative studies across species. Consequently, experimental approaches that establish causality between sociality and brain evolution are still missing in the field. Here, we experimentally tested the effects of the evolution of sociality on brain structure and function using an artificial selection experiment in zebrafish. After three generations, the highly social line was already divergent from the control line (random mated), and from the asocial line (selected for preference to associate with moving dots). We assessed whether the selected phenotypes differed in neuronal activation patterns in response to a social stimulus and in brain composition. Neuronal activation was quantified using pS6 as a molecular marker. Our results revealed line-specific differences in several brain nuclei, including a significant reduction in neuronal activation in the Vv (putative homologue of the lateral septum) in the social line. This nucleus contains a large population of inhibitory neurons, whose reduced activity has been linked to social fear contagion in zebrafish, suggesting that increased sociality is associated with decreased inhibitory neuronal activity. In addition, the social line exhibited changes in brain composition, with increased neuronal numbers and neuronal density across major brain regions. Together, these findings demonstrate that sociality is a selectable trait that drives rapid changes in brain structure and function.