Variations in Sexual Size Dimorphism in Two Anurans Along an Urbanization Gradient in Shanghai: Assessment of Rensch's Rule

Variations in sexual size dimorphism (SSD) have important consequences for animal ecology, behavior, population dynamics, and the evolution of life-history traits, and can be explained by many intraspecific and interpopulation hypotheses. Urbanization is an important factor that affects anuran phenotypic characteristics (including morphology), but how it impacts anuran SSD has been explored less widely. In this study, we aimed to investigate the differences in SSD in two dominant amphibian species populations; that is, the Hong Kong rice-paddy frog (Fejervarya multistriata) and Beijing gold-striped pond frog (Pelophylax plancyi), across an urban–rural gradient in Shanghai (including urban, suburban, and rural areas). We also tested whether these variations in SSD supported Rensch's rule or not. Our results showed that in both sexes, the snout-vent length (SVL) in F. multistriata was higher in urban areas compared with suburban and rural areas. In addition, only male P. plancyi exhibited a higher SVL in urban areas compared with those in rural areas. Furthermore, we found that the SSD in SVL was significantly higher for urban populations than suburban and rural populations of F. multistriata. All SSD results in terms of SVL, head width, forelimb length, and hindlimb length in these two frog species supported the inverse Rensch's rule based on reduced major axis regression. Moreover, the size scaling rates in these two frog species were higher in urban and suburban populations than rural populations. Thus, we concluded that urbanization shapes SSD in these two anurans according to the inverse of Rensch's rule, and that variations in morphological characteristics, SSD, and size scaling rates differed between these two anuran species regarded as an “urban-associated species” (P. plancyi) and “urban-sensitive species” (F. multistriata).