Female preference for male sword length in the green swordtail, Xiphophorus helleri (Pisces: Poeciliidae)

Swordtail fish exhibit a striking sexual dimorphism in the structure of the caudal fin; males have a colourful, elongated caudal appendage, termed a sword, that is absent in females. In laboratory tests of female preference for male sword length in the green swordtai, Xiphophorus helleri, females were given the choice between a pair of males differing naturally or experimentally in sword length to test the hypothesis that the elaboration of this trait is partially a consequence of female choice. Five tests with different pairs of males were conducted in which sword differences between paired males varied from 6 to 24 mm. Female preferences were measured by scoring the amount of time a female spent in association with each male. In all five tests, females preferred males with longer swords. Female preference was shown to be an increasing function of difference in sword length: as the difference in sword length increased, the strength of the preference increased. Characters correlated with sword length were eliminated as sources of preference by experimentally altering sword lengths; preferences were reversed when the relative sword lengths were reversed. These results suggest that sexual selection by female choice may have played a role in the evolution of the sword and that female preference is based on sword length itself, and not on a character correlated with sword length.