Abstract
The signals that mediate mate choice can be complex, comprising multiple components, and understanding how complex signals evolve under sexual selection has been the focus of much study. However, open questions still remain about the role of the female's sensory and perceptual processes in shaping the evolution of complex signals. Male green swordtails Xiphophorus hellerii have an elongated caudal fin that comprises colour, length and a black melanic margin; females prefer males with larger bodies, longer swords and complete black sword margins. Here, we used a two-choice assay to quantify female preferences for animations of courting males of different sizes with or without sword margin coloration, and found that, when a black melanic margin was present, females exhibited preferences for larger males. However, when the margin was absent, females did not show size-based mate preference, though females spent equal time assessing males in both treatments. Our results suggest that the presence/absence of the black sword margin is an important predictor of female preference, specifically a female's ability to discriminate between potential mates of different sizes, pointing to a novel size discrimination function of black margins in animal signals, which in many species involve patterns or structures with dark edges.