Social Familiarity and Nip Dominance in Male Swordtails ( Xiphophorus Helleri ) and Platys ( Xiphophorus Maculatus )

Male swordtails and platys (ns =12) were tested for nip dominance among familiar (tankmate) and unfamiliar (nontankmate) con-specific pairs in four round robin contests conducted at weekly intervals. Stable dominance relationships were found for both species, with less aggression manifested between familiar swordtail pairs, and to a lesser extent, between familiar platy pairs, than between nonfamiliar pairs. Significant correlations between physical size and dominance status were also found for both species.