Environmental conditions can vary dramatically across spatial scales, spanning both regions that encompass geographically distinct ecological communities and entire species distributions. Evidence indicates that such variation is usually accompanied by adaptive variation in physiological traits, which has been shown to have environmental (i.e. plastic) and genetic components. Here we evaluate the potential influence of environmental conditions on physiological traits, particularly water loss and standard metabolic rates, among five species of salamanders in the genus Ambystoma from populations located across an ~200 km latitudinal gradient in Missouri. We reared larvae to metamorphosis under common aquatic conditions and maintained juveniles under common terrestrial conditions. We then used a flow through respirometry system and water loss analyzer to collect physiological data. If environmental conditions, particularly those experienced by larvae and juveniles, are a major factor shaping physiological phenotypes, we expected that population of the same species—regardless of their geographic origin—would exhibit similar rates of water loss and metabolism after being subject to the same conditions. We also expected comparable phenotypes among sympatric species. Alternatively, we anticipated that populations and species would maintain phenotypic differences that are likely genetic and adaptive, regardless of the environmental conditions experienced by larvae and juveniles.
Results/Conclusions
After correcting for body shape and size, we found water loss and metabolic rates to differ across both species and populations. Only 34% of the variation in water loss was explained by metabolic rate, suggesting that other mechanisms by which salamanders avoid desiccation, such as skin properties, contributed to the observed differences in water loss rates among the study species. Considering all species and populations, water loss rates increased with latitude, whereas there was no relationship between metabolic rate and latitude. However, phenotypic differences between populations of individual species did not always correlate with latitude. We found that even when the environmental conditions of the developing larvae and juveniles were the same, species and populations of Ambystoma salamanders exhibited differences in rates of water loss and metabolism, rejecting the role of plasticity as the primary mechanism responsible for the observed phenotypes. Moreover, maintained phenotypic variation across both species and populations strongly suggests that selection has favored genetically based differences in physiological traits. Differences in physiological phenotypes at the end of metamorphosis, when the terrestrial environment was novel, provide further support for genetically based variation. Additional studies are needed to evaluate the potential adaptive value of those differences.