Skin toxin production of toads changes during early ontogeny but is not adjusted to the microbiota of the aquatic environment

Many animals display spectacular adjustments of their phenotype to changing environmental conditions in terms of altered behaviour, physiology and morphology, but whether similar plasticity in chemical defences is taxonomically widespread as well has remained unknown. Bufadienolides, toxins produced by some anuran amphibians, are known to be effective against many predators, but they can also contribute to defences against microorganisms. We experimentally investigated if qualitative and/or quantitative changes in the chemical defences of Bufo bufo tadpoles are induced by differences in the microbiota of their environment. We reared animals in purified tap water, or in pond water containing lowered or natural quantities of the natural microbiota, and measured the number of compounds and total quantity of bufadienolides at three ontogenetic stages during larval development, as well as 10 days after metamorphosis. We observed a significant decrease in the number of compounds, and a parallel increase in the total quantity of bufadienolides after metamorphosis, independent from treatments. The quantity of individual bufadienolide compounds also changed during ontogeny independently from treatments. Our results, thus, suggest that although bufadienolide production changes during early ontogeny, it is not adjusted to the microbiota present in the environment.