Associations Among Brain Size and Social-Modulating Hormones with Aggression and Cognitive Performance in a Monogamous Cichlid Fish

Abstract Social behavior is shaped by neural and endocrine mechanisms. In this framework, the social brain hypothesis assumes that bigger brains are selected to cope with complex social environments. On the other hand, hormones can act through organizational and activational effects, modulating brain function during ontogeny and in a fast-changing environment. Here, we test correlations among social-modulating hormones – cortisol, testosterone (T), and 11-ketotestosterone (11KT) – and the volume of specific brain areas with individuals’ social aggression and cognitive abilities in both males and females of the pearl cichlid Geophagus brasiliensis. We observed that, although both parents present similar roles during parental care – biparental brood care – reflecting in similar brain volumes, besides social and cognitive behaviors, the correlation between these factors and hormone levels are different comparing sexes. Females are associated with cortisol, which possibly enhances their cognitive performance, and is linked to bigger telencephalons and diencephalons. Males are influenced by testosterone and its conversion into 11KT, which seems to modulate their behavior. Thus, males’ and females’ social behaviors seem to be evolutionarily driven by similar brains, yet different hormonal activation for coping with the social environment.