Evolution of sexual dimorphism and cryptic signals in three viviparous Sceloporus lizard species (#809)
Human vision often fails to distinguish sexual dichromatism (i.e. sexual dimorphism in color), and “cryptic” signals often go undetected. Here, we analyze the potential for cryptic (i.e. not visible to the human eye) sexual and interspecific differences in chromatism across three Sceloporus lizard species, that differ in ventral signaling colors. Because viviparous lizard species tend to be the most colorful, we focus on a viviparous clade that varies in degree of sexual dichromatism. One species (S. grammicus)is highly sexually dimorphic, exhibiting the ancestral male-blue and female-white bellies. The other two species appear to be sexually monomorphic; in one species (S. jarrovii), there is a loss of dimorphism through female gain of blue color, and in the other species (S. megalepidurus), there is a loss of dimorphism through male loss of blue color. We measured the spectral reflectance of male and female bellies from these three species in the field (N=27-30 per sex, per species). To estimate the conspicuousness of these colors as viewed by conspecifics, we filtered the data through a Vorobyev-Osorio visual model for tetrachromats, using published photoreceptor sensitivities from Crotaphytus lizards. We found that all lizards, regardless of belly color, were conspicuous (Just Noticeable Differences >1), but gaining blue color in S. jarrovii females did not make them more conspicuous than heterospecific white-bellied lizards. S. megalepidurus males, despite losing blue patches, were still quite conspicuous, perhaps exhibiting a cryptic ultraviolet signal. Interestingly, S. megalepidurus males and females were the most similar in color. Sexual dichromatism in polygamous taxa is often hypothesized to be an indicator of the strength of sexual selection, and perhaps this lack of dichromatism in S. megalepidurus is due to relaxed sexual selection due to their shift to a more pair-bonded social structure. Implications for the evolution of visual signal design will be discussed.