Animals across species compete for limited resources. While in some species competition behavior is solely based on own abilities, others assess their opponents to facilitate these interactions. Using cues and communication signals, contestants gather information about their opponent, adjust their behavior accordingly, and can thereby avoid high costs of escalating fights. We tracked electrocommunication signals, in particular “rises”, and agonistic behaviors of the gymnotiform electric fish Apteronotus leptorhynchus instaged competition experiments. A larger body-size relative to the opponent was the sole significant predictor for winners. Sex and the frequency of the continuously emitted electric field were only mildly influencing competition outcome. In males, correlations of body-size and winning were stronger than in females and, especially when losing against females, communication and agonistic interactions were enhanced, hinting towards males being more motivated to compete. Fish that lost competitions emitted the majority of rises, whereby their quantity depended on the competitors’ relative size and sex. The emission of rises was costly since it provoked ritualized biting or chasing behaviors by the other fish. Despite winners being accurately predictable based on rise numbers already after the initial 25 minutes, losers continued to emit rises. The number of rises emitted by losers and the duration of chasing behaviors depended in similar ways on physical attributes of contestants. The detailed evaluation of these correlations hint towards A. leptorhynchus adjusting their competition behavior according to mutual assessment, where rises could signal a loser's motivation to continue assessment through ritualized fighting.

Social animals establish dominance hierarchies to regulate access to resources. Although communication signals could reduce costs in negotiating dominance, their detailed role and emergence in non-mammalian vertebrates is not well researched. We tracked electrocommunication signals and agonistic behaviors of the gymnotiform fish Apteronotus leptorhynchus in staged competition experiments. Subordinates emitted the majority of so called "rises" in dependence on the competitor's relative size and sex. Rises provoked ritualized biting or chasing behaviors by dominant fish. Already after 25 minutes losers were accurately predictable based on rise numbers, but they continued to emit rises. We suggest the interplay between communication and aggression to fine tune relative dominance without questioning dominance rank. This communication system regulates the skewness of access to resources within a dominance hierarchy and allows A. leptorhynchus to populate neotropical rivers with high abundances.