Seasonal plasticity in neuroendocrine mechanisms relevant to year-round territorial aggression in a wild teleost fish

Animals experience cyclical environmental changes, such as seasons, that require physiological adjustments to support different behaviors. Although many behaviors occur only during specific periods, some species, like Gymnotus omarorum, display territorial aggression year-round, making them valuable models to study seasonal plasticity in the mechanisms maintaining stable behavioral outputs. G. omarorum is a teleost fish in which neuroestrogens have been shown to play a key role in non-breeding aggression. Here, we quantified circulating hormone levels and gene expression in the social behavior network of wild breeding and non-breeding individuals. During the non-breeding season, both sexes exhibited elevated circulating androgen levels, providing potential substrates for local estrogen synthesis. Consistently, brain aromatase and estrogen receptor expression were also upregulated, suggesting an increased capacity for local estrogen synthesis and signaling. Our findings provide the first evidence in a teleost of seasonal plasticity in the mechanisms underlying territorial aggression. Comparisons with birds and mammals reveal both shared and lineage-specific strategies, highlighting common endocrine principles while revealing the evolutionary diversity of solutions to maintain a stable behavioral phenotype across changing seasonal contexts.