Evaluating the physical condition of Hyla japonica using radiographic techniques

Amphibian populations are declining throughout the world, which has negatively impacted various organisms and ecosystems. The goal of this study was to obtain a better understanding of the physiological status and condition of individuals in amphibians is needed to preserve these groups. We used radiography to assess physical conditions in Hyla japonica frogs, which are widely distributed in East Asia. Bone images were examined to determine fractures and metabolic bone disease of frogs in a wild population. The bone-mineral density (BMD), stable nitrogen isotope ratio (δ¹⁵N), and physical conditions (body length, age, and body weight) and body compositions (fat mass and lean mass) were measured to identify changes correlating with the food-intake status. To analyze the physiological and morphological effects of predation pressure in H. japonica, we compared the BMD, food-intake status, physical conditions, body compositions and skeletal shape in H. japonica between populations with Lithobates catesbeianus (paddy fields with bullfrogs; PB) and without L. catesbeianus (no bullfrogs; NB). Only five of 101 H. japonica frogs had fractures, and no bone disease due to malnutrition was found. The food-intake status correlated with the BMD, but not to the other physical conditions and body compositions measured. Similarly, the BMD and δ¹⁵N of the NB group were higher than those of the PB group, but the other physical conditions and body compositions were not different. The skeletal shapes of the head and lower body were not significantly different between the two groups. Our findings revealed that the BMD can be used to determine the food-intake status and be changed in by predation pressure. We suggest using the BMD of anurans to estimate physiological changes that occur in response to the habitat or biological interactions.