Prior residency and the stability of dominance relationships in pairs of green swordtail fish Xiphophorus helleri (Pisces, Poeciliidae).

The stability of dominance relationships between pairs of male green swordtail fish was followed daily for 20 consecutive days. In one experimental sample composed of 21 pairs, dominance of one of the fish had been favoured on the first day by giving the fish prior familiarity (prior residency) with the aquarium where it was to meet an intruder. In a control sample composed of 12 pairs, two intruders met in an unfamiliar aquarium. It was expected that the advantage given to the dominant by familiarity with the aquarium on the first day would disappear as the subordinate acquired in turn familiarity with the milieu. In comparison with pairs composed of two intruders, this would show up by more frequent inversions of the initial dominance relationship in pairs composed of a prior resident and an intruder. Only two inversions occured over the 20 days of follow up and they occured equally in the experimental (5%) and control (8%) samples. These results confirm the great stability of dominance relationships in dyads and invalidate the hypothesis that the prior residency advantage would decay as the subordinate became familiar with the aquarium. Unexpectedly, 13 of the 66 (20%) fish died over the 20 days. Death equally occurred in both samples but 12 (92%) cases implied initial subordinates. The exception was an initial dominant which had become the subordinate pair member three days before death. Various hypotheses are suggested to account for the selective deaths of subordinates.