The five new species are found in Brazil, Peru, and Bolivia - three of them are endemic to Brazil and one to Peru.

Three species recognised were previously considered to be subspecies, and another three, already described, were previously thought to be just variants.

The study is the largest revision for any Neotropical primate genus in more than half a century, researchers said.

Dr Laura K Marsh, the director and co-founder of the Global Conservation Institute, carried out the study - 10 years of research involving the examination of specimens in 36 museums in 17 countries in North America, South America, Europe and Japan.

"I began to suspect there might be more species of saki monkeys when I was doing field research in Ecuador," Marsh said.

"The more I saw, the more I realised that scientists had been confused in their evaluation of the diversity of sakis for over two centuries," said Marsh.

"Saki monkeys, like many rain forest primates, are excellent indicators for the health of tropical forest systems," said Russell A Mittermeier, President of Conservation International, after whom one of the new species, Pithecia mittermeieri, is named.

"This revision of the genus shows clearly how little we still know about the diversity of the natural world that surrounds us and upon which we ourselves depend so much," said Mittermeier.

Saki monkeys are distributed throughout the Amazon Basin and the Guiana Shield. They are elusive and little studied in the wild, and our understanding of their conservation status is minimal, researchers said.

The research was published in the journal Neotropical Primates.

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