Spatial variation in sexual size dimorphism and mating associations in the color polymorphic Eastern Red-Backed Salamander (Plethodon cinereus)

Abstract

Sexual selection due to mate preference for certain traits can maintain phenotypic diversity within populations and species. In taxa with discrete phenotypes, such as color polymorphic species, assortative mating may lead to disruptive selection and sympatric divergence, yet how such interactions vary over species' ranges remains poorly understood. To address this shortcoming, we examined spatial patterns of sexual size dimorphism and presumptive male–female mating pairs based on body size and coloration in the color polymorphic Eastern Red-backed Salamander, Plethodon cinereus . This species exhibits two common color morphs, striped and unstriped, that also differ in other elements of their biology, including ecology, behavior, and morphology. Across six populations that vary in color morph frequency, we predicted each site would show female-biased sexual size dimorphism and a positive body size relationship between presumptive mating pairs. In polymorphic sites, we also predicted that morphs would demonstrate color-based assortative mating, with larger body size as an associated trait. We found female-biased sexual size dimorphism in three of six study sites, while a positive body size relationship between male and female pairs was documented in four populations, and no evidence of male–female associations by color and body size was found in polymorphic populations. The spatial variation across all of our study axes demonstrates the importance of geographic context in shaping sexual selection dynamics and patterns of local adaptation.

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