Summary
Changes in aggressive responsiveness to 5 stimulus dummies (Normal, Aggressive, and Subordinate sunfish, Pike, and Minnow) over the 4 periods (Nesting, Spawning, Brooding, and Vacating) of the reproductive phase were studied in 49 ♂ ♂ over two seasons in the field by presenting the Normal sunfish dummy and one other dummy daily. The results showed large initial increases and subsequent decreases in responsiveness over this phase. The dummies were treated as conspecific or heterospecific and the fish discriminated among them in terms of placement of Bites and frequencies of different responses, especially Opercular Spreads. Positive correlations among the response frequencies indicated a unitary intraspecific aggression system. External and internal stimuli combine additively to elicit aggressive reactions. Energetic costs and sexual selection are likely operative in the evolution of aggressive responsiveness.
Methodology
The study was conducted during the breeding seasons of 1974 and 1975 at the Queen’s University Biology Station, approximately 40 km northeast of Kingston on Lake Opinicon. At this location the fish nest in colonies in shallow sandy coves (typical water depth 50 cm) from late May to mid July. Nests were randomly chosen and marked as they were established by anchoring a labelled wooden float a short distance from the rim of the nest. This enables individual recognition of fish, as nest fidelity has been shown by tagged individuals (unpubl. data). Each nest was visited daily during its existence, weather permitting, between either 0700 and 1100 or 1400 and 1800 h and its occupant was tested by presenting the Normal and one other of the painted plywood dummies (Fig. 1). Choice of size, shape, and colour of the dummies involved a compromise between the simulation of natural fish and the simplicity required for standardization across dummies. The following dummies reflected a choice of stimuli usually present for a nesting 8 to respond to and hence likely to evoke responses of interest.