Visual Assessment of Relative Body Size in a Cichlid Fish, the Oscar, Astronotus ocellatus

Abstract

In cichlid fishes, fighting ability, success at nest defense, and fecundity are all positively correlated with body size. Selection should therefore favor the evolution of strategies for the assessment of relative body size during intraspecific interactions. The effect of relative body size on agonistic behavior in the oscar (Astronotus ocellatus) was tested using dummies. As predicted by game theory models, attack was strongly correlated with the size of the dummy relative to the subject. The most intense aggression occurred when dummy size was approximately 75% of subject size. These data support the prediciton that perceived body size asymmetries are important determinants of agonistic behavior in these fishes. Further, the results suggest that oscars can visually assess relative body size, and may estimate relative fighting ability without tests of strength. Subject size was positively correlated with intensity of aggressive behavior. Attack and display frequencies were positively correlated with a subject's body size, while latencies to approach and attack dummies were negatively correlated with subject size.

The effect of increased light intensity on the aggressive behavior of the Nile tilapia, Oreochromis niloticus (Teleostei: Cichlidae)

“…The groups were formed using animals with similar standard length (one-way ANOVA, low light: df = 2, F = 0.02, p = 0.98; high light: df = 2, F = 0.03, p = 0.97) and body weight (one-way ANOVA, low light: df = 2, F = 0.12, p = 0.89; high light: df = 2, F = 0.39, p = 0.68) because size is a factor that affects agonistic behavior (BEECHING 1992). The animals were identified by different cuts in the tail fin (FERNANDES & VOLPATO 1993, HÖGLUND et al 2005.…”

Social Behavior and Welfare in Nile Tilapia

“…An important cue related to social information is the access to the opponent's fighting abilities [38]. In this sense, body size constitutes crucial information, as several cichlid species can visually access this characteristic in their opponents (see [39], for Melanochromis auratus, and [40] for Astronotus ocellatus). The more similar the size, the longer and harder the fighting, as shown for Nannacara anomala [41].…”

Fighting and Aggression

“…For example, the size of a potential opponent relative to a focal fish is a critical determinant of what happens when rivals meet. Oscars (cichlid fish, Astronotus ocellatus) chase and bite fish that are smaller than them, but avoid larger opponents (Beeching 1992). As a consequence of this kind of effect, fights between evenly matched fish are commonly longer and fiercer than those where there is a clear size difference between potential opponents (Huntingford & Turner 1987).…”