The role of sexual selection in maintaining a colour polymorphism in the pygmy swordtail, Xiphophorus pygmaeus

The forces responsible for maintaining genetic polymorphisms within populations are often unclear. Male pygmy swordtails are polymorphic at a single Y-linked locus that regulates body colour. The visually conspicuous, sex-limited polymorphism suggests that sexual selection is important for maintaining genetic variation for these conspicuous phenotypes. We performed a series of behavioural experiments that decoupled body colour from possible correlates, to detect female mating preferences for each morph and differences in male reproductive behaviour. Male body colour, male mating behaviour and male agonistic behaviour all appeared to influence male reproductive success. Females in two of three populations showed mating preferences for blue males, but gold males dominated in agonistic interactions and were more aggressive in pursuing females. Females in those populations preferred blue to gold males under optical filters that masked blue colour, but not under filters that masked gold colour, suggesting that female behaviour reflects an aversion to gold males. Females from one population characterized by high predation risk, the Nacimiento of the Río Huichihuayán, Mexico, showed no mating preferences for blue males, a significantly different result from those for females from two populations with a lower predation risk. Both female mating preferences and male competitive behaviour thus appeared to be important for maintaining the colour polymorphism. The lack of preference in the Nacimiento population is consistent with previous studies showing reduced preferences for conspicuous traits in this population.