SEX DIFFERENCES IN THE NEURAL CIRCUITS DRIVING VISUALLY EVOKED AGGRESSION IN SIAMESE FIGHTING FISH

Aggression is a fundamental component of social behavior across species, shaping social interactions and hierarchies. While aggression in humans and primates is largely driven by visual cues, most commonly used animal models, such as rodents and flies, rely primarily on olfactory signals, leaving the neural mechanisms of visually evoked aggression (vea) poorly understood. The Siamese fighting fish (Betta splendens), selectively bred for strong visually driven aggressive behavior, provides an ideal system to address this gap. In this study, we aimed to (i) develop behavioral assays that reliably elicit aggressive displays in adult males and females and (ii) identify the neural circuits underlying vea. Using machine learning–based markerless tracking, we quantified key behavioral features such as opercular flaring. Neural activity was assessed using neuroanatomical tracing, phosphorylation of ribosomal protein S6 (pS6) immunolabeling, and RNA-sequencing. We found that males displayed significantly higher levels of aggression than females when visually exposed to a male conspecific. pS6-labeling revealed increased neuronal activation in the dorsomedial pallium and other telencephalic regions during aggressive interactions. Transcriptomic analyses further identified differentially expressed genes between males and females in these regions, suggesting molecular mechanisms underlying sex-specific aggression. In addition, tracing revealed projections from retinal ganglion cells to the lateral thalamic nucleus and optic tectum, but not to the dorsomedial pallium, indicating a multi-step pathway linking visual input to aggression-related circuits. Together, these findings provide new insight into the neural and molecular basis of vea and establish Betta as a valuable model for studying sex differences in social behavior.