A further examination of a pre-existing bias favouring a sword in the genusXiphophorus

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 genusXiphophorus, 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, femaleX.maculatuswere 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 toX.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. FemaleX.variatuswere found to prefer conspecific males with swords to those without. This work suggests thatX.variatusandX.maculatusshare a preference favouring a sword, despite the lack of a sword in these species.