Innate immunity and antioxidant costs of low temperatures in native green treefrogs (Hyla cinerea) and invasive tropical Cuban treefrogs (Osteopilus septentrionalis)

Abstract

Thermal tolerance plays a role in the geographic range over which different species can survive, and temperature fluctuations can affect amphibian physiology. We studied physiological costs associated with low temperatures in green treefrogs (Hyla cinerea), which are native to the United States, and tropical, non-native Cuban treefrogs (Osteopilus septentrionalis) that have invaded Florida. Specifically, we examined variation in innate immunity and skin antioxidant capacity. To complete the study, 107 treefrog tadpoles were raised through metamorphosis and, as young frogs, divided into a control group maintained at room temperature of approximately 24°C and an experimental group at approximately 8°C. Individuals in the low temperature group were placed in the cold temperatures for 6-8 hours per day for 14 days to simulate early fall nocturnal ambient temperatures in northern regions of states along the Gulf of Mexico in the United States. Cuban treefrogs had a significantly greater survival rate than green treefrogs in the control group, but there was no significant difference in survival between experimental groups. We also found that each species was negatively affected by low temperatures, with a decrease in bacterial killing ability and antioxidants on their skin, but the Cuban treefrogs were affected to a much greater extent. These findings suggest that the spread of Cuban treefrogs into areas north of their current invasive range may be slowed by physiological costs of low temperatures, but we do not expect significantly greater mortality rates associated with cold alone than treefrogs already occupying such a range.