Interactive effects of road salt and leaf litter on wood frog sex ratios and sexual size dimorphism

Abstract

Myriad natural and anthropogenic chemicals alter aquatic vertebrate sex ratios, with implications for population dynamics. Despite 22 million metric tons of salt applied to US roads annually, with much of it entering aquatic environments, it is unknown whether salt impacts sex ratios. Moreover, changes in forest composition co-occur with increased road salt application, dramatically changing ecosystems. We explore how road salt (sodium chloride) and two leaf litter types might influence amphibian development. By examining wood frog (Rana sylvatica = Lithobates sylvaticus) metamorphs reared with different combinations of salt (114 and 867 mg Cl·L−1) and litter species (none, maple (Acer rubrum), oak (Quercus spp.)), we show that salt masculinizes tadpole sex ratios, whereas oak, but not maple, litter feminizes populations. Road salt addition eliminates sexual dimorphism in oak-reared tadpoles, but enhances sexual size dimorphisms in maple-reared tadpoles, producing larger females. We are the first to show that road salt and native tree leaf litter manipulates vertebrate sex ratios and sex-specific development. Human land use might therefore influence vertebrate development through direct effects of contamination and indirect effects of altered botanical composition.