Adaptive Organ Size Variation in Nanorana quadranus Across Altitudinal and Climatic Gradients: Contradicting Hesse’s Rule and Digestion Theory

Understanding phenotypic variation in the morphological and physiological characteristics of amphibians and the underlying evolutionary mechanisms is a central topic of broad significance in evolutionary biology. To explore adaptive variations in the relative sizes of key organs (heart, kidneys, lungs, and digestive tract), we surveyed 11 populations of Nanorana quadranus distributed in the Qinling-Daba Mountains at altitudes ranging from 586 to 1702 m. Our results revealed distinct adaptive responses of the organs of N. quadranus to heterogeneous environmental pressures. Specifically, the relative heart size was negatively correlated with altitude, while the relative length of the digestive tract exhibited no significant correlation with either altitude or latitude. Notably, the environmental stress-induced changes in organ size did not conform to Hesse’s rule or classic digestive theory. On the basis of these findings, we speculate that the relative sizes of the heart and lungs in this species may be weakly influenced by ambient oxygen availability. Moreover, the abundant food resources in these habitats likely alleviate the physiological and energetic constraints on the digestive system, thereby reducing its metabolic burden. Further correlation analysis demonstrated that both relative heart and kidney sizes were positively associated with the mean temperature of coldest quarter. Collectively, these results provide novel insights into the adaptive evolution of amphibians in heterogeneous environments and highlight the regulatory roles of climate change and environmental gradients in shaping the physiological adaptations of amphibian species.