Phenotypic trait variations in the frog Nanorana parkeri: differing adaptive strategies to altitude between sexes

In many animals, changes in altitude drive adaptive variation in body size. However, how other phenotypic traits change when faced with different environments has been little studied in ectotherms. In this study, we selected the high Himalaya frog Nanorana parkeri as a model species for investigating the adaptive evolution of phenotypic traits that respond to altitude in both sexes. First, we found that body mass in populations at higher altitudes was lower than at lower altitudes in females, with no difference observed in males. Second, we found significant differences in fresh liver mass, fresh heart mass, and the ratio of liver mass to body mass with increasing altitude, while hindlimb length decreased with altitude in both sexes. Third, snout-urostyle length, hindlimb length, fresh heart mass and the ratio of heart mass to body mass showed significant negative correlations with increasing altitude in both sexes. In contrast, body mass showed a significant correlation with altitude in females but not males. On the other hand, the ratio of liver mass to body mass showed a significant correlation with altitude in males but not in females. Thus, the species displayed sex-specific organ-size variation along elevation gradients, which may trade-off in life history strategies among populations. We speculate that selection favours a larger heart and liver mass to maintain a higher respiratory rate and energy consumption as an adaptation to high-altitude environments.