Do Midas cichlids win through prowess or daring? It depends

Summary

Individual Midas cichlids (Cichlasoma citrinellum) show persistent differences in aggressive behavior toward dummy fish. The starting question was whether the level of such behavior can be used to predict the winner of a fight. In the first (long-term) experiment two fish were matched for size, color, and sex. After 24 h the opaque barrier separating the 2 fish was removed; they immediately behaved aggressively. Scores for aggression toward dummies did not predict the winners, nor did taking the initiative in escalation. But weight did foretell the winners, who averaged only 2% heavier than their opponents. The conventional display phase of such fights was brief (20% of total duration), and escalation was rapid. Both winners and losers sustained damage, but losers accumulated damage faster than winners. In the subsequent (short-term) experiment the fish were separated only 1–2 h. Now aggression scores predicted winners, and winners were the fish who escalated. Weight of fish had no effect. The conventional phase was relatively much longer, about half the length of the fight. Losers accumulated damage at the same rate as the losers in the long-term experiment, but the fights were shorter; winners suffered little damage. The fish had difficulty assessing one another. Fighting prowess was remarkably uniform when weight was factored out. Daring to escalate, in contrast, varied among individuals and correlated with aggression scores. Prowess (=weight) determined the outcome in the long-term experiment, which may find its parallel in nature in intraterritorial disputes. Daring to escalate determined the winner in the short-term experiment; this may be comparable to establishing a territory in nature.

Prowess probably results from strong directional selection because it has low costs and high benefits. In contrast, daring is subject to bi-directional selection because both costs and benefits are high. Resource holding potential is conventionally viewed as deriving from prowess of self and opponent and value of resource; to that one must add the individual's inherent aggressiveness.