Vertebrates can also biosynthesize defensive chemicals or sequester them from plants or prey. Sequestered compounds have been observed in frogs, natricine snakes, and two genera of birds, Pitohui and Ifrita. It is suspected that some well-known compounds such as tetrodotoxin produced by newts and pufferfish are derived from invertebrate prey. Bufadienolides, defensive chemicals produced by toads, have been found in glands of natricine snakes used for defense.