Do gold Midas cichlid fish win fights because of their color, or because they lack normal coloration?

Summary

Gold morphs (G) of the polychromatic Midas cichlid, Cichlasoma citrinellum, dominate those of normal coloration (N). The adults vary greatly in weight because they continue to grow after reaching sexual maturity; relative weight is an important determinant of dominance. The first objective here was to characterize dominance in relation to color and to relative size by staging a large number of encounters between fish of various weights. Logistic curves were fitted to the data to analyze the relationship between relative size and winning. The baseline curves for N vs N and G vs G (Fig. 1, Table 1) did not differ (Table 2). The shape of the curve for N vs G also was not different from that of the baseline curves. But the curve for N vs G was shifted such that N must weigh 116% of G for the probability of winning to be 0.50.

The dominance effect of G stems directly from their color, not from the absence of normal markings or from genetically coupled aggressiveness. When G were matched against white morphs (W) the logistic curve was displaced such that W must be 117% the weight of G to enjoy a 50% chance of winning, just as with N vs G. In N vs W, the chance of either winning was 50% when they were equal in weight, thus just as in the baseline curves for N vs N and G vs G.

Absolute size played a role. Large mature fish had a higher logistic coefficient, indicating greater sensitivity to differences in relative weight, than did small immature fish (Table 3).

Data were also gathered from some of the contests on the consequences of winning and losing. Losers were often forced to the surface and suffered more splits in their fins. However, when G lost to N many G were tolerated near the bottom (Table 4). On average, G losers had fewer splits in their fins than did the N winners. W losers were tolerated near the bottom, and W winners and losers suffered litte damage, suggesting the colorless pattern W provokes fewer attacks (Tables 4–6).