Ecogeographical Adaptation Revisited: Morphological Variations in the Plateau Brown Frog along an Elevation Gradient on the Qinghai–Tibetan Plateau

SIMPLE SUMMARY: A number of studies have explored how the body size and extremities of frogs vary in response to the changing environmental conditions across different geographical gradients, but the outcomes remain controversial. Here, we studied the morphological variations of the plateau brown frog (Rana kukunoris) along an elevation gradient (~1800–3500 m) on the eastern margin of the Qinghai–Tibetan Plateau to understand how environmental and biological factors affect them, and to assess whether these variations help to improve thermoregulation. Although we found that male and female frogs showed different variations in body size and extremities along the elevational gradient, both of them showed a significant decrease in the ratio of extremities to body size with increasing elevation. The decreasing ratio implies a gain of thermoregulatory benefits based on the ecogeographical rules. Moreover, the morphological variations were found to be strongly related to both environmental and biological factors. These results suggest that ecogeographical adaptation in frogs may be more complicated than other terrestrial vertebrate species. Most importantly, the adaptation should be viewed as a result of both environmental and biological factors, while it may also appear as an interactive change between body size and extremities. ABSTRACT: Several anurans have broad elevational and latitudinal distribution ranges; distinct species and populations may face various environmental and selection stresses. Due to their environmental sensitivity, adaptation is critical for the long-term persistence of anurans. Previous studies have tried to identify the ecogeographical pattern and its mechanism in anurans, suggesting different patterns, but the related explanatory mechanisms are yet to be generally supported and are suggested to be complicated. To explore the elusive mechanisms, we studied the morphological variation of the plateau brown frog (Rana kukunoris) along an elevational gradient on the eastern margin of the Qinghai–Tibetan Plateau. Using body size, extremity length, and the ratio between them (extremities/body size) as testing indicators, we examined potential ecogeographical adaptations and investigated how environmental and biological factors could shape the morphological development in R. kukunoris. We found that males and females showed different variations in body size and extremities along the elevational gradient, whereas both of them showed a decreasing extremities/body size ratio along elevation. Together with the strong correlations between environmental and biological factors and the morphometrics, we identified ecogeographical adaptation and a sexual difference in the selective pressures on the extremities and body size of the plateau brown frog. Our results imply that geographic variations in anuran morphological traits should be understood as an outcome of environmental and biological factors. Furthermore, ecogeographical adaptation in anurans can manifest as an interactive change between body size and extremities.