Territorial prior residence,size asymmetry,and escalation of aggression in convict cichlids (Cichlasoma nigrofasciatum Günther)

The present study was designed to assess the effects of resident versus intruder size differences upon the territorial prior-residence effect and level of territorial aggression in convict cichlids. Prior to a direct territorial dominance encounter, pairs of fish were randomly composed for one of three experimental treatment conditions: 1) the resident having a body-length 20–30% larger than that of the intruder, 2) the intruder having a body-length 20–30% larger than the resident, or 3) the combatants' body-lengths differing by no more than 5%. After a 3-day territorial acclimation period in their individual territories, the subject designated as the intruder was introduced to the resident's territory. For each encounter the pairmember that attacked first and the one that ultimately established dominance were recorded. Also measured during the encounter were the total number of bites, resident bites, intruder bites, and jaw-locking frequency and duration. The results revealed that there was a significant resident advantage in the resident-larger group. The intruder-larger group resulted in a significant intruder dominance advantage. However, no significant dominance advantage occurred in the same-size group. As predicted by game theory, there was significantly less escalation of aggression in contests in which one combatant held both designated asymmetric cues (prior residence, size) than in contests in which one combatant had prior residence, or when these two asymmetric cues were divided between the pairmembers. The size asymmetry is more important in determining dominance than the prior-residence asymmetry, for the particular size-difference range selected in the present study.