An experiment was designed to test the hypothesis that the effects of prior residence on aggressive behaviour and on dominance might not be found if both the resident and the intruder were handled before the test; in Braddock's experiment (1949) demonstrating these effects, the intruders were the only handled fishes.

All our fishes, ♂♂ of the genus Xiphophorus, were isolated for a pre-experimental period of exactly 22 h; then two fishes were randomly selected and handled as follows for the dominance test (phase 1): — In the Experimental condition (23 E-pairs), fishes were taken out of their isolation tank and were then introduced simultaneously into the tank that had been occupied by one of them, randomly selected (the “resident”). — In the Control condition (23 C-pairs), the resident was taken out of its tank 5 min before the end of the 22 h-isolation period, and then it was again introduced into its area for the remaining 5 min residence-period; thereafter, the “intruder” was taken out of its isolation tank and transferred into the tank occupied by the resident. As soon as the dominance of a fish was definitely recorded in the resident‘ area, the members of each pair were handled again according to their previous E or C-condition; for this second transfer (phase 2) both fishes were introduced into the isolation tank that had been occupied by the intruder of the first test prior to its initial transfer.

The results of the first test demonstrated that the two handling conditions had very different effects on the behaviour of the pair-members. Only in the C-pairs did prior residence affect positively initiation of aggressive behaviour and final dominance; reciprocal aggression was much more frequent between members of the E-pairs. The dominance-subordination relationships that had established during the first test remained highly stable (90% of the cases) after the second transfer, indicating that true territoriality, i.e. dominance reversal when each fish becomes resident in its turn, was mostly absent in our pairs of Poeciliids.