Competition induces increased toxin production in toad larvae without allelopathic effects on heterospecific tadpoles

Inducible defences are a form of phenotypic plasticity by which organisms respond to and mitigate the threat posed by predators, parasites and competitors. While anti‐predatory defences are often in trade‐off with anti‐competitor responses, chemicals that deter predators may have negative effects on competitors as well. Allelopathy is well known in plants and plant‐like animals, but whether the toxins of mobile, behaviourally and morphologically complex animals are induced by and exert allelopathic effects on competitors is poorly known. Common toads Bufo bufo synthesize bufadienolides which make them unpalatable or toxic to many predators. However, bufadienolide content of toad tadpoles correlates positively with the density of competitors in natural populations, suggesting that they may upregulate their toxin production to inhibit their competitors, such as heterospecific tadpoles that may be vulnerable to toad toxins. We conducted a microcosm experiment with tadpoles of common toads and agile frogs Rana dalmatina, in which we manipulated the density of conspecific and heterospecific competitors. We measured the bufadienolide content of toad tadpoles to test for competitor‐induced changes in toxin production, and we assessed the growth and development of agile frog tadpoles to test for allelopathy. We found that toad tadpoles contained higher amounts of bufadienolides at higher densities; however, heterospecific competitors did not have a stronger effect than conspecifics. Furthermore, the presence or density of toad tadpoles had no effect on the body mass and development rate of agile frog tadpoles. Our results demonstrate competitor‐induced plasticity in toxin production, but we found no support for an allelopathic function of bufadienolides. Instead, we suggest that inducible changes in bufadienolide production may serve to mitigate risks posed by competitors, including aggression, cannibalism or disease. Therefore, bufadienolides are intriguing candidates for multi‐purpose defences that may provide protection not only against predators but also against competitors. A plain language summary is available for this article.