Impacts of <scp>UVB</scp> provision and dietary calcium content on serum vitamin D₃, growth rates, skeletal structure and coloration in captive oriental fire‐bellied toads (<i>Bombina orientalis</i>)

<jats:title>Summary</jats:title><jats:p>Many amphibian species are dependent on <jats:italic>ex situ</jats:italic> conservation interventions for their long‐term persistence. However, projects have been jeopardised by husbandry issues involving poor calcium metabolism and nutritional metabolic bone disease (<jats:styled-content style="fixed-case">NMBD</jats:styled-content>). Healthy calcium metabolism requires appropriate dietary calcium content and access to vitamin D<jats:sub>3</jats:sub>. In many animals, vitamin D<jats:sub>3</jats:sub> can be photobiosynthesised in skin exposed to <jats:styled-content style="fixed-case">UVB</jats:styled-content> radiation, as well as extracted from the diet, but the extent of vitamin D<jats:sub>3</jats:sub> photobiosynthesis in amphibians is poorly known. Additionally, prey insects for captive amphibians are deficient in calcium and calcium content must be artificially increased, but the effects of different levels of augmentation and their interaction with <jats:styled-content style="fixed-case">UVB</jats:styled-content> exposure are also little understood. We fed captive fire‐bellied toads (<jats:italic>Bombina orientalis</jats:italic>) with crickets augmented to contain 5% and 10% calcium and housed them with and without <jats:styled-content style="fixed-case">UVB</jats:styled-content> exposure. Despite additional dietary vitamin D<jats:sub>3</jats:sub> supplementation, we found that toads exposed to <jats:styled-content style="fixed-case">UVB</jats:styled-content> radiation exhibited significantly higher serum vitamin D<jats:sub>3</jats:sub> levels, indicating that this species may partly rely on photobiosynthesis sources of vitamin D<jats:sub>3</jats:sub>. These data are the first to show a direct link between <jats:styled-content style="fixed-case">UVB</jats:styled-content> exposure and serum vitamin D<jats:sub>3</jats:sub> in an amphibian. We found significant positive effects of <jats:styled-content style="fixed-case">UVB</jats:styled-content> exposure and 10% dietary calcium content on skeletal structure, as well as complex interactions between treatments. We also found <jats:styled-content style="fixed-case">UVB</jats:styled-content> radiation exposure resulted in more rapid natural coloration acquisition. Together, this indicates that standard calcium plus vitamin D<jats:sub>3</jats:sub> supplementation methods may not fully substitute for <jats:styled-content style="fixed-case">UVB</jats:styled-content> exposure and for increased feeder insect calcium content. This may have implications for the success of <jats:italic>ex situ</jats:italic> amphibian conservation, as well as for the welfare of captive amphibians in general. Our data lend support for the provision of <jats:styled-content style="fixed-case">UVB</jats:styled-content> radiation for captive, basking amphibians.</jats:p>