Anim. Behav., 1995,50,365-375 A furtherexamination of a pre-existingbias favouringa sword in the genus

Abstract. Models of sexual selection have recently been broadened to include the pre-existing bias model. This model suggests that female preferences that result in the evolution of male traits can be established prior to the appearance of a male trait. Previous work suggests that in the genus Xiphophorus, which consists of swordless platyfish and sworded swordtails, female southern platyfish, X maculatus, prefer conspecific males with surgically attached swords, despite phylogenetic evidence that the common ancestor of platyfish and swordtails was swordless. In the present study, female X maculatus were found to prefer conspecific males with a naturally occurring component of the sword, an abbreviated lower caudal stripe. To investigate the possibility that the previously suggested pre-existing bias favouring a sword was merely an extension of a preference for this stripe specific to X maculatus, whether female variable platyfish, X variatus, also demonstrate a preference for males with swords was tested; this species has neither a sword nor any component of the sword. Female X variatus were found to prefer conspecificmales with swords to those without. This work suggests that X variatus and X maculatus share a preference favouring a sword, despite the lack of a sword in these species..:jJ1995 The Association for the Study of Animal Behaviour The hypothesis that female preferences can evolve prior to the appearance of male traits favoured by the preference (Barlow 1977; West-Eberhar