Lineage-specific decoupling of arginine vasotocin-evoked aggression in male darters (Percidae: Etheostomatinae).

The repeated evolution of parental care is often accompanied by shifts in the neural circuits and hormonal pathways that regulate motivation and aggression. Nonapeptides of the oxytocin/vasopressin family, including arginine vasotocin (AVT) in teleost fishes, modulate diverse social behaviors across vertebrates such as courtship, pair bonding, parental care, and territoriality. The mechanism behind conservation of these neuromodulators during behavioral diversification remains unclear. We used a comparative pharmacological approach across six darter species (Percidae: Etheostomatinae) by injecting either AVT, an AVT receptor antagonist (Manning's compound), or a vehicle control (saline), followed by physiological and behavioral monitoring in standardized intruder assays. AVT administration significantly increased respiration rate, quantifiable as opercular beats per minute (OBPM) across species, indicating conserved AVT-responsive physiology. In contrast, AVT-evoked aggression varied sharply among lineages: AVT robustly elevated fin flare displays in caregiving species (Etheostoma podostemone, E. nigrum, E. flabellare, and Nothonotus starnesi) and in one non-care species with intense male-male competition (N. rufilineatus), but had no effect in E. vitreum, which has secondarily lost paternal care and spawns communally. This implies that AVT signaling remains physiologically intact, but its behavioral coupling has been selectively strengthened or lost across evolutionary transitions in parental care. These results identify lineage-specific rewiring of nonapeptide control of aggression and establish a framework for cellular and molecular dissection of AVT receptor expression and circuit remodeling in the vertebrate social brain.