Prior residence, body size and the dynamics of territorial disputes between male freshwater angelfish

Territorial interactions between pails of size mis-matched, sexually mature male angelfish Pterophyllum scalare were investigated in three different conditions: with the larger fish resident (the large resident condition), with the smaller fish resident (the small resident condition) and in a neutral territory (the neutral condition). In the two resident conditions, approximately half of the intruders had previously held territories and half had not. In all categories of fight, one fish showed submissive postures and lost the fight; eventual losers performed both attack and threat at a lower rate than eventual winners. Attack rate declined as the encounter progressed, while rate of performance of threat postures increased. In fights on neutral territories, the larger fish won all fights. In all fights with a resident-intruder asymmetry, the resident fish won the encounter, regardless of relative size. In eventual winners but not in eventual losers, levels of attack were lowest in the neutral encounters. In the small resident condition, levels of attack (corrected For activity of the resident) were lower in intruders that had previously held a breeding territory. Relative size influenced behaviour shown during fights, in that overall intensity was correlated negatively with size differential in all conditions. Thus although prior residence is the primary determinant of the outcome of territorial encounters in this species, both relative body size and prior possession of territory also influence the nature of the interaction. (C) 1999 The Fisheries Society of the British Isles.