Prior exposure to visual cues affecting dominance in the jewel fish, Hemichromis bimaculatus Gill 1862 (Pisces, Cichlidae).

Abstract

A jewel fish (Hemichromis bimaculatus that is reintroduced into a familiar aquarium compartment where it had remained before during two hours, is dominant over a conspecific of similar size that had been placed in the mean time into another similar aquarium compartment (Exp. I). Such an effect on dominance is also present if in a similar situation the dominance test compartment has an altered bottom topography, and if, in addition, the fish that comes back into its own isolation area, has now the disadvantage of a familiar visual cue being absent (or an unfamiliar one being present) in contrast to the other fish (Exp. 2). If the difference between the exposure compartments is restricted to mere visual cues, a similar advantage in dominance position in an equivalent new aquarium compartment, is again demonstrated for the fish having the familiar cues present (Exp. 4). This also holds for differential stimulation from a small glass jar being added to visual cues during one or two days (Exp. 5). However, in another experimental arrangement where both kinds of fishes are successively isolated (for 2 hours) in the same aquarium compartment, each fish receiving its own specific visual cues, the effect is not demonstrated (Exp. 3). Isolation during I6 hours in an aquarium compartment with two vertical black plastic strips attached to the outside of an aquarium wall and touching the bottom level, makes a fish dominant over a conspecific that has been isolated in the mean time in a similar aquarium compartment without those visual cues, when both fishes are allowed to meet in the connected isolation areas (Exp. 6). A positive relationship between initiating aggressive behaviour and subsequent dominance in an encounter is shown in several experiments. When only encounters with a fight are considered, the fishes with more familiar cues present, sometimes still had an advantage over their opponents, although not as strong an advantage as in encounters without fighting. Latency times before dominance was settled did not last as long when the fish with the more familiar cues present dominated, as when its opponent did, but this did not occur in Exp. 3 where no positive effect of the familiar cues on the dominance position was shown. For the fight durations, however, such a difference between dominant fishes in the two conditions, was significant in none of the experiments. The prior exposure effect on the dominance relation between two fishes as shown in several experiments, is regarded as an argument for the contention that even stable dominance relationships need not necessarily be dependent on physical strength relationships between individuals, but are probably determined by the continuous stimulation from their environment and by the mutual perception of each other's overt aggression and/or flight behaviour. The prior exposure effect possibly facilitates territory establishment in natural fish communities. Further experiments dealing with differential dominance effect caused by different qualities of (familiar) cues (as in Exp. 6) should be undertaken. The advantages and limitations of the experimental procedure used are briefly discussed.