Animals establish social hierarchies through agonistic behavior. The recognition 
of the own and others social ranks is crucial for animals that live in groups to 
avoid costly constant conflicts. Weakly electric fish are valuable model systems 
for the study of agonistic behavior and its neuromodulation, given that they 
display conspicuous electrocommunication signals that are generated by a very 
well-known electromotor circuit. Brachyhypopomus gauderio is a gregarious 
electric fish, presents a polygynous breeding system, morphological and 
electrophysiological sexual dimorphism during the breeding season, and displays 
a typical intrasexual reproduction-related aggression. Dominants signal their 
social status by increasing their electric organ discharge (EOD) rate after an 
agonistic encounter (electric dominance). Subordinates only occasionally produce 
transient electric signals (chirps and offs). The hypothalamic neuropeptide 
arginine-vasotocin (AVT) and its mammalian homologue, arginine- vasopressin 
(AVP) are key modulators of social behavior across vertebrates. In this study, 
we focus on the role of AVT on dominance establishment in Brachyhypopomus 
gauderio by analyzing the effects of pharmacological manipulations of the AVT 
system in potential dominants. AVT exerts a very specific direct effect 
restricted only to EOD rate, and is responsible for the electric dominance. 
Unexpectedly, AVT did not affect the intensity of aggression in either 
contender. Nor was the time structure affected by AVT administration. We also 
present two interesting examples of the interplay between contenders by 
evaluating how AVT modulations, even when directed to one individual, affect the 
behavior of the dyad as a unit. First, we found that V1a AVT receptor antagonist 
Manning Compound (MC) induces a reversion in the positive correlation between 
dominants' and subordinates' attack rates, observed in both control and AVT 
treated dyads, suggesting that an endogenous AVT tone modulates aggressive 
interactions. Second, we confirmed that AVT administered to dominants induces an 
increase in the submissive transient electric signals in subordinates.