From the article: When one of these frogs is kept in captivity, however, their diet is often changed. The toxin filled beetles and ants are replaced by flies, mealworms and crickets – all of which are completely non-toxic. Due to not absorbing the toxins through their diet the frogs lose the ability to secrete it through their skin.

In the wild, the frogs eat ants, millipedes, mites and other arthropods that have toxic alkaloids naturally occurring in their skin. These smaller creatures biosynthesize the toxic compounds from things they eat on the forest floor, such as decaying plant material. These compounds are then transferred into the frog’s skin glands where they are stored and secreted when threatened.