Phenotypic plasticity is a core concept of evolutionary biology and there is ample evidence for adjustments to environmental conditions in a row of life history traits in a wide variety of living organisms. Also, environmentally induced changes in chemical defences form the basis of a large research field in plants. However, we know startlingly little about similar plasticity in chemical defences of animals, despite its potential significance for ecology, evolution and conservation, through effects on individual fitness, interspecific interactions, demography and speciation.We will test whether tadpoles of the common toad (Bufo bufo) adjust the production of toxins to the presence and abundance of predators, competitors or pathogens. Furthermore, we will examine whether environmentally induced changes in the production of toxic skin gland secretions lead to elevated fitness of tadpoles and, hence, these plastic responses can be considered adaptive. Finally, we will investigate if the expression of inducible defences carries costs that are immediately visible or appear in the long term.We will perform outdoors- and indoors experiments and validate experimental results by sampling natural populations in collaboration with world leading experts.Our results will have implications for evolutionary biology, chemical ecology, behavioural ecology and conservation biology. Because our project targets biogenic and bioactive chemicals, results may also provide new impulses towards agriculture, medicine or pharmacology. Thus, our project is likely to spark basic and applied research, potentially also resulting in economic revenues.