Changes in the chemical defenses of an invasive toad indicate drivers and limitations of adaptation

Chemical defenses of invasive species provide protection against predators, competitors, and parasites enabling successful colonization of novel environments and can be highly detrimental to native biota. Here, we study how the chemical defenses of a successful invasive toad have changed by comparing the native source population with the introduced populations on Mauritius, Réunion, and in Cape Town. In line with the enemy release hypothesis, on Mauritius and Réunion where the community of predator species is less diverse, the toads show marked reductions in toxin gland size as well as changes in the composition of the toxins. However, in Cape Town, where the introduction happened more recently and the predator community is more diverse, only the composition of the toad toxins changed. Our results indicate drivers and constraints of adaptations of chemical defenses and provide evidence for the role of enemy release in the success of an invasive toad.