Skin pH and buffering ability vary between two co-occurring semi-aquatic frog species.

Amphibians face global declines linked to anthropogenic environmental change, including modifications to freshwater habitats. Human impacts on water chemistry, including acid rain and run-off of road salt into wetlands, may affect the physiology of amphibians with aquatic life stages. Specifically, water pH varies among freshwater habitats and affects amphibian development, behaviour, and physiology. For example, changes in skin pH affect the activity of enzymes on the skin, including those involved in antimicrobial functions. In this study, we explored the ability of free-ranging amphibians to maintain homeostasis across a range of naturally occurring water pH and salinity. We sampled two species of frogs at 19 wetlands around Peterborough, Ontario, measuring water pH, water salinity, and the skin pH of northern leopard frogs (Lithobates pipiens; n = 141) and green frogs (Lithobates clamitans; n = 329). We found that water pH increased with salinity, and was weakly related to the proportion of built-up habitat around wetlands. Frog skin pH was significantly associated with water pH, but both species showed a strong ability to buffer their skin pH across a range of conditions. On average, the ventral skin pH of L. pipiens increased by 0.37 units for each 1 unit increase in water pH, while skin pH of L. clamitans increased by 0.12. Specific responses to water chemistry differed between the two species: skin pH of L. pipiens varied with demographic group and body size, but skin pH of L. clamitans did not. As human effects on wetland habitats increase, these amphibians' ability to buffer skin pH may provide some protection against anthropogenic changes in wetland water chemistry.