Summary
Using fixed stimulus dummies as territory intruders, we studied the spatial distributions of the aggressive responses made by nesting male pumpkinseed sunfish. Results replicated over three field seasons indicate that males defend two territory boundaries concurrently, depending on whether an intruder is approaching (“the defence perimeter’) or withdrawing (‘the attack perimeter’). This finding supports Zeeman's (1976) ‘cusp catastrophe model’ of nest defence. In addition, the defence perimeter remains constant over the breeding cycle while the attack perimeter varies. We discuss the ecological costs and benefits of this variation and present a motivational interpretation of the cusp catastrophe model. By varying the speed at which dummies intruded into nests, we determined that males react with a fixed latency to intruders at a fixed distance from the nest. Finally, the spatial distributions of male defence responses were partially determined by the location of the nest rim, but were unaffected by dummy size (contrary to Zeeman's model) or posture.
Methodology
Data were collected from three colonies of fish during the 1976, 1977, and 1978 field seasons (May to June) in Lake Opinicon, the site of the Queen's University Biology Station, 40 km NE of Kingston, Ontario. A pictorial representation of the 1977 colony is presented in Gross & Nowell (1980). Nests in less than a metre of water were marked as soon as they were established by anchoring a floating wooden tag attached with a monofilament line to a stake planted 15 cm from the rim of the nest. The marker had no apparent effect on the nesting fish, and since fish exhibit nest fidelity, nest marking allows individual recognition. Data on nest features, especially radius (the distance from nest centre to rim), were collected.
The use of painted-silhouette stimulus dummies (illustrated in Colgan & Gross (1977) and described under Dummy Posture and Size below) permitted the presentation of a standardized stimulus to different fish and on different occasions. Throughout the reproductive phase each fish was tested daily with two dummies. One dummy was used in the morning test (between 0700 and 1200 hours)and the second in the afternoon (between 1400 and 1800 hours), weather permitting, with the temporal order of dummy tests 'switched on successive days. Generally an interval of from 4 to 5 h elapsed between the two daily tests, which was known to be ample time to avoid any possible carry-over effects between tests.
During each test, the dummy was presented six times by an observer standing quietly in the water about 1 m away. Each presentation involved moving the dummy horizontally and broadside into the nest, pausing for 3 s over the nest centre, and then withdrawing the dummy at the same speed. The dummy was then removed from the view of the test fish for 10 to 15 s before the next presentation.