Fungal disease does not affect skin carotenoid concentrations in dietary-supplemented strawberry poison frogs

Carotenoids are colored molecules that are sequestered through diet and provide health benefits and affect reproductive success in certain organisms. Batrachochytrium dendrobatidis (Bd) is a worldwide amphibian infection that invades skin, disrupts vital regulatory processes and can cause death. We hypothesized Bd skin infection affects sequestration of important skin carotenoids in carotenoid-supplemented strawberry poison frogs (Oophaga pumilio). We predicted that infection would deplete skin carotenoid concentrations due to the energetic or immunological trade-off required when fighting infection. We experimentally infected a group of adult male and female O. pumilio with Bd and used high-performance liquid chromatography (HPLC) to compare types and concentrations of 17 unique skin carotenoids between infected and control (mock-infected) animals that were all supplemented with dietary carotenoids. We achieved a range of Bd infection (from around 1.0 × 10¹ to above 1.0 × 10⁷ zoospore equivalents) in all infected frogs. We found no effect of Bd infection on skin carotenoid concentrations, regardless of sex. This suggests Bd infection is not sufficient to alter skin carotenoids in O. pumilio with non-limited dietary carotenoids, perhaps due to sufficient energy available both for combating infection and maintaining skin carotenoids in captive animals. We suggest further study of the energetics of sequestering and metabolizing carotenoids in amphibians and its relationship to energy needed when combating Bd infection to better understand how skin carotenoids are maintained when Bd infection takes hold. Studying the effect of pathogen infection on organism carotenoid sequestration will continue to be important as emerging infectious diseases surface worldwide.