A newly discovered species of monkey that purrs like a cat is among more than 400 new species of plants and animals that have been found in the under-explored Amazon rainforest.

In the past four years, researchers working the in the Amazon have discovered a wide array of new species, including the purring monkey, a flame-patterned lizard, a vegetarian piranha and a frog the size of a thumbnail, according to the World Wildlife Fund (WWF), which recently compiled a list of the new species discoveries in the Amazon between 2010 and 2013.

The purring monkey (Callicebus caquetensis), also known as the caqueta titi monkey, is one of about 20 species of titi monkey, all of which are endemic to the Amazon forest basin. When baby caqueta titi monkeys are content, they tend to purr towards each other like cats.

A total of 441 new species have been described, most of which are thought to be endemic to the Amazon rainforest. The WWF reports 258 plants, 84 fish, 58 amphibians, 22 reptiles, 18 birds and one mammal. The number of new species found does not reflect the many new insect in invertebrate discoveries.

"These species form a unique natural heritage that we need to conserve," said Claudio Maretti, of the WWF's Living Amazon Initiative. "This means protecting their home -- the amazing Amazon rainforest -- which is under threat from deforestation and dam development."

The elusive flame-patterned lizard (Cercosaura hypnoides) has proven difficult to keep track of. Researchers found the remarkable lizards from the hatchlings of eggs, but the lizards have not been seen in the wild since their initial discovery, which could indicate that the lizards are endangered.

Another Amazonian oddity, the vegetarian piranha, can weigh up to 9 pounds, feeding on the seedlings of river plants for its main food source.

Most of these new species are thought to be found nowhere else in the world but the Amazon rainforests. And some of them may already be in danger of going extinct.

The thumbnail-sized frog is already thought to be highly endangered. Its Latin name Allobates amissibilis, means "that may be lost" and alludes to the name of the region of Guyana where it is found, which may soon be opened for tourism, which could prove to be the bane of the tiny frog's existence.

"Compiling and updating data on new species discovered in the vast extension of the Amazon over the last four years has shown us just how important the region is for humanity and how fundamentally important it is to research it, understand it and conserve it. The destruction of these ecosystems is threatening biodiversity and the services it provides to societies and economies. We cannot allow this natural heritage to be lost forever," Maretti said.