The Establishment and Reversibility of Dominance Relationships in Jewel Fish, Hemichromis Bimaculatus Gill (Pisces, Cichlidae): Effects of Prior Exposure and Prior Residence Situations

Using adult jewel fish (Hemichromis bimaculatus), a highly territorial member of the family Cichlidae, three separate experiments were conducted to investigate environmental factors important in the establishment and maintenance of dominance relationships. In Experiment 1, fish were acclimated for 2 days, while individually housed in an aquarium containing either a ceramic pot or a clump of vegetation. Pairs of fish (one pair member from each treatment condition) met in a separate test aquarium containing one of the two possible cues. After a dominance relationship was established, the same pairs were rematched 24 hours later in the same test aquarium, but with the other cue substituted for the Day 1 cue (e.g., pot instead of vegetation or viceversa). In the Day 1 bouts, the fish with "prior exposure" to the cue present in the test aquarium dominated over the pair member unfamiliar with the cue. On Day 2, the Day I dominance relationship was maintained, even though the cues had been reversed. There was no resistance to being dominated on Day 2, while vigorous fighting typically occurred on Day 1. In Experiment 2, the procedure was the same as that of Experiment 1, except that the dominance encounters occurred in the actual "home" aquaria of the pair members. For example, the Day 1 encounter would take place in the home aquarium containing the vegetation, while the Day 2 rematch occurred in the home aquarium containing the pot. As in Experiment 1, the fish were put into their home aquaria for 24 hours after the Day 1 encounter and then retested for dominance in the other pair member's home aquarium. On Day 1, the resident (in its home aquarium) dominated significantly more often than the intruding pair member (prior residence effect). On Day 2, approximately 36% of the Day 1 dominance relationships reversed. Also, marked fighting usually occurred on both days of testing. Experiment 3 was the same in procedure as Experiment 2, except that the 2 day pre-test acclimation period was increased to 7 days with the expectation of producing more Day 2 reversals, or at least producing greater resistance to the continuation of Day 1 relationships on Day 2 (when the Day 1 loser refought in its own aquarium). The results of Experiment 3 were not significantly different from those of Experiment 2. The three experiments, taken together, show that familiarity with environmental cues produces a dominance advantage when encountering a strange fish not having had this "prior exposure" or "prior residence" experience (Day 1). However, this familiarity with environmental cues was not a strong enough factor to produce reversals on Day 2, when testing took place in a separate aquarium containing one familiar visual cue (prior exposure situation). In contrast, in the prior residence situations of Experiments 2 and 3, with the addition of many more familiar cues (e.g., water, light direction, etc.), significantly more dominance reversals occurred on Day 2. Also, Day 1 losers showed more resistance to being dominated again when they were tested in their own home aquarium on Day 2. In all three experiments, Day 2 dominance decisions were most influenced by the past experience between the pair members (Day 1 dominance decision). However, the additional environmental cues provided by the prior residence situation (Experiments 2 and 3) increased the probability of dominance reversals and fighting on Day 2. Another interesting finding in these experiments was that the pair member that bit first in an encounter usually became dominant. Dominance fighting in this species seems to be both an antecedant and consequence of establishing a territory, with environmental familiarity possibly faciliating these processes.