Spatial aspects of nest defence by pumpkinseed sunfish ( Lepomis gibbosus): Stimulus features and an application of catastrophe theory

Abstract

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.