Social fish have larger brains and greater relative telencephalon sizes: support for the social brain hypothesis from wild, intraspecific comparisons.

The social brain hypothesis (SBH) posits that complex social environments drive the evolution of larger brains and enlargement of specific brain regions. Among species comparisons often report contrasting relationships between social complexity and brain size, potentially due to confounding effects of phylogeny, morphology and ecology. Here, we explore this relationship in a single fish species, combining behavioural observations and brain measurements of two wild populations of the cichlid Neolamprologus brevis, which occupies similar ecological niches across its range but inhabits contrasting social environments depending on local shelter abundance. We quantified social behaviour and brain size to assess whether increased social interactions are associated with greater relative brain size or region-specific neuroanatomical adaptations. We found that individuals from the more socially complex population, which exhibited more frequent social interactions, had significantly larger total brain volumes compared to the less social population. We also found that the more social population exhibited relatively larger telencephalon and smaller hypothalamus volumes, suggesting mosaic adaptation to social demands. Feeding behaviour did not differ between populations, suggesting that differences in energy intake are unlikely to account for brain size variation. By integrating behavioural and neuroanatomical data, our study provides empirical support for the SBH in a natural, within-species comparison.