Reproductive signals of female lizards: pattern of trait expression and male response

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Relative to the volume of studies concerning the function and evolution of male-biased sexually dimorphic traits, instances of female-biased sexual dimorphisms remain largely unstudied, especially in species with conventional sex roles. I investigated the signal function of a female-specific ornamental trait using the striped plateau lizard (Sceloporus virgatus, Phrynosomatidae) as a model system. During the reproductive season, female S. virgatus develop orange color on their throats that is absent in conspecific males. I established the relationship between color expression and female reproductive state, and determined male response to female color. I show that dynamic changes occurring within the color patch can potentially identify each stage of the female reproductive cycle, largely because of a lag in patch growth relative to color intensification. Sexual receptivity is associated with intense patches rapidly growing in size; ovulation occurs near peak color expression; and the unreceptive period is associated with large patches fading in intensity. Because females express orange color during both the receptive and unreceptive periods, the pattern of color expression is consistent with the courtship-stimulation and courtship-rejection hypotheses of signal function. Males may preferentially associate with females that have more highly developed color patches during the courtship season, and/or ignore such females when they are unreceptive. An examination of male behavior towards unfamiliar females indicates that female color has a role in courtship stimulation but has little, if any, role in courtship rejection. During the pre-mating season, males maintained significantly closer affiliation with, and tended to perform more social behavior towards females with more intense color. During the post-mating season, female color had no apparent effect on male behavior. The evolution and current function of female ornaments may vary among taxonomically-related species as a result of differences in ecology, social system, and life-history.