Poison dart frogs are so named because their poisonous secretions have been used by Amerindian tribes to poison blow darts for hunting. Only three species are dangerous to humans. One of those - the golden poison frog - is considered the most toxic vertebrate on Earth. The beautifully bright colours and patterns of these frogs are a warning to potential predators that they don't make good eating. There could be over 175 species in this family of amphibians, all found in the rainforests of Central and South America. Many species of poison dart frogs are classified as critically endangered.