Weak Coupling Between Gas Exchange and Water Loss in Lungless Salamanders

Gas exchange requires a permeable respiratory surface, yet the same permeability also creates a pathway for water loss, posing a particular challenge for organisms whose respiratory surfaces are directly exposed to the terrestrial environment. Woodland salamanders (Plethodon) are a diverse radiation of lungless, terrestrial amphibians that rely entirely on cutaneous respiration, coupling gas exchange and water loss through a shared surface. Whether this shared surface produces a consistent relationship between water loss and metabolic rate across species, or whether the two traits can diverge despite their functional link, remains unresolved. Here, we quantified metabolic rate, water loss rate, and the ratio between these traits across 30 Plethodon species and related these traits to elevational distributions, geographic range size, and climatic conditions across species' ranges. We also compared the relationship between gas exchange and water loss in Plethodon to that observed across a broader set of amphibian taxa. Metabolic rate and water loss were not tightly correlated across Plethodon, despite a positive association across amphibians more broadly. Instead, physiological traits correlated with different aspects of species' elevational limits, range sizes, and climatic niches. Metabolic rate was most consistently associated with elevation and climate, with lower values observed in species occupying higher, cooler environments. Water loss alone showed weaker and less consistent environmental associations, but the transpiration ratio varied with elevational limits such that species extending into lower-elevation environments exhibited greater hydric efficiency. These patterns were also reflected in phylogenetic path analyses, which supported lower elevational limit as a mediator linking physiology to geographic range size. These results suggest that although cutaneous respiration imposes broad hydric costs across amphibians, Plethodon species do not show a single, shared pattern of physiological coupling between gas exchange and water loss.