Data on dyadic male contests

We focused on three frequent aggressive behaviors: (1) S-position: this threat display usually initiates a fight. Males swim in a parallel or anti-parallel position and bend their bodies in an S-shaped manner with all unpaired fins erect; (2) tail-beats: S-positions are often followed or superimposed by tail-beats, which are fast movements of head and tail in opposing directions that either touch the opponent’s body or send shock waves to the opponent; and (3) bites – we defined all incidences of ramming and bite-like attacks as bites, because both these behaviors occur extremely quickly and thus are indistinguishable to the human eye.

We also recorded fight duration until dominance was established. If no dominance was established within 10 minutes of the first interaction, we terminated the fight; those trials were discarded from the analysis of fighting durations, while fight durations were scored as “0” when no aggressive behavior occurred at all (those trials were terminated after a total of 15 minutes of observation). SL of both contestants was taken after a contest by laying the fish flat on plastic foil-covered millimeter paper.