Evolution of female mating preferences in stalk-eyed flies

Sensory exploitation predicts that female mate preferences exist before the evolution of exaggerated male ornaments. We tested this prediction by estimating female preference functions, remating intervals, and copulation durations for three species of stalk-eyed flies. Two species, Cyrtodiopsis whitei and C dahnanni, exhibit extreme sexual dimorphism in eye span, with eye stalks exceeding body length in large males. In contrast, C qiunqutguttata of both sexes possess short eye stalks. Maximum parsimony analysis of 437 basepairs of the 16S mitochondrial ribosomal RNA gene from 6 Malaysian diopsids reveals that short, sexually monomorphic eye stalks are plesiomorphic in Cyrtodiopsis. Observations of multiple copulations by females in paired-choice mating chambers indicated that female C whitei and C. dalmanm exhibit relative preferences for longer eye stalks such that preference intensity increases linearly with the difference in eye stalk length between males. Females from the sexually monomorphic species showed no detectable preference for male eye stalk length. Female mating preferences of bodi sexually dimorphic species exhibited significant repeatability, as expected if genetic variation underlies the preference. In addition, female C whitei and C. dalmanm exhibited shorter copulations, mated more frequently, and rejected fewer mating attempts than female C quinqueguttata. Thus, opportunities for sperm competition have increased with acquisition of female preferences. We conclude that female sensory bias for males with long eye span did not exist in a common ancestor to these species. Instead, female preference and remating propensity either coevolved with eye span dimorphism or evolved after male eye stalks elongated.