A monkey that purrs and a piranha that prefers a vegetarian diet are among 400 new species discovered in the rainforests of the Amazon, scientists say. Discovered by scientific expeditions since 2010, the 441 new species include 258 plants, 84 fish, 58 amphibians, 22 reptiles, 18 birds and one mammal, wildlife group WWF-UK, which compiled the list, reported. \"The more scientists look, the more they find,\" Damian Fleming, head of programs for Brazil and the Amazon at WWF-UK, told The Guardian. \"With an average of two new species identified every week for the past four years, it\'s clear that the extraordinary Amazon remains one of the most important centers of global biodiversity.\" A \"purring\" Caqueta titi monkey of the Colombian Amazon was one of the discoveries. Babies of the species have the endearing trait of making purring sounds when happy, scientists said. \"All of the babies purr like cats,\" Thomas Defler, who helped discover the species, said. \"When they feel very content they purr towards each other, and the ones we raised would purr to us.\" Also described was a strictly herbivorous \"vegetarian piranha\" (Tometes camunani), which lives in rocky rapids in the Brazilian Amazon and feeds on aquatic herbs. The Amazon ecosystem, the world\'s largest tropical rainforest and river system at 6.7 million square miles, is shared by Brazil, Bolivia, Peru, Ecuador, Colombia, Venezuela, Guyana, Suriname and French Guiana. \"The richness of the Amazon\'s forests and freshwater habitats continues to amaze the world,\" Fleming said.