Life on the Edge: A Comparative Study of Ecophysiological Adaptations of Frogs to Tropical Semiarid Environments

A key goal of ecology and evolution is to understand the relative contributions of environment and history in determining the geographic distribution of organisms. For the Neotropical lowlands, where temperatures are similar across landscapes, we hypothesize that water balance may be a critical but understudied factor in determining the distribution of species. Amphibians are especially sensitive to variation in precipitation due to their permeable skin. Here we focused on lowland frogs of northwestern South America and investigated variation among 17 species in potentially important ecologically relevant performance variables related to water balance, testing for possible adaptations to semiarid conditions within species. We studied frogs from coastal xeric, savannah, and wet forest biomes under common laboratory conditions and quantified rates of evaporative water loss, rates of water uptake, and variation in water-searching behavior and performance. We found significant differences in all three performance variables among species even after accounting for shared evolutionary history. A phylogenetic ANCOVA showed that categorizing species by ecological habit (terrestrial vs. arboreal) explained much of the ecoperformance trait variation among species. Secondarily, environment explained additional variation; for example, coastal xeric species showed reduced rates of water loss, and terrestrial savannah amphibians showed lower rates of water uptake. Conspecific frog populations from different biomes exhibited similar performance. We compare our results with previous studies and conclude that ecological habit is the principle factor that predicts ecophysiological trait variation and the possible geographic distribution of lowland Neotropical frogs.