Geographical variation in body size and sexual size dimorphism of introduced American bullfrogs in southwestern China

Invasive species often exhibit geographical variations in life history traits that may allow them to successfully invade different environments. We investigated geographical variation in body size and sexual size dimorphism (SSD) of invasive bullfrogs in southwestern China, by sampling two breeding populations (descendants of a single source population) inhabiting sites at low (1,412 m, Shiping) and high (2,692 m, Luguhu) altitudes. Both populations exhibited significant SSD, with females larger than males. At high altitude, mean body size of both sexes and the degree of SSD were significantly reduced; the reduction in mean body size with increasing altitude was more pronounced in females, although not significantly so. Female bullfrogs also showed a significant decrease in average age at high altitude that may be a major factor related to this pattern; average age of male bullfrogs did not vary significantly with altitude. Growth rate of both sexes was also lower at high altitude. Our results provide the first evidence that introduced bullfrog's exhibit geographical variation in morphology in invaded areas in response to different environments, likely due to changes in climate. Additional research is required to determine the mechanism of this variation (i.e., physiological or developmental plasticity, mortality rate, selective pressure) and most importantly, to evaluate the potential for variation in the impacts of introduced bullfrogs on native ecosystems in China.