Avoiding a compromise between sexual selection and species recognition: female swordtail fish assess multiple species-specific cues

Females increase their risk of mating with heterospecifics when they prefer the traits of conspecifics that overlap with traits found in heterospecifics. Xiphophorus pygmaeus females have a strong preference for larger males, which could lead to females preferring to mate with heterospecific males; almost all sympatric X. cortezi males are larger than X. pygmaeus males. In this study, we show that X. pygmaeus females preferred the chemical cues from conspecifics over those of X. cortezi males. However, preference for the chemical cues of conspecifics could not reverse the preference for larger heterospecific males. Only when females were presented with two species-specific cues (vertical bars and chemical cues) did more females spend more time on average with the smaller conspecific males. These results support the ‘‘backup signal’’ hypothesis for the evolution of multiple preferences; together, the two species-specific cues increased the accuracy with which females were able to avoid heterospecific males. In addition, the results suggest that in those situations in which the traits of conspecifics overlap with traits found in heterospecifics, females can use the assessment of multiple cues to avoid mating with heterospecifics without compromising their preference for the highest-quality conspecific.