Full scientific descriptions of the monkeys, Callicebus bernhardi and Callicebus stephennashi, were published by the peer-review journal Neotropical Primates.
"This once again demonstrates how little we know about biodiversity, these are the 37th and 38th new primate species described since 1990," said Conservation International's President Russell Mittermeier, a co-author of the scientific descriptions.
Van Roosmalen discovered the bernhardi monkey - named after Holland's Prince Bernhard - between the east bank of Madeira river and the lower reaches of the Aripuana river.
He found it accidentally while traveling in the region searching for another new species, the dwarf marmoset, which a local fisherman brought to his door in 1996 because he knew Van Roosmalen cared for orphaned monkeys.
The bernhardi monkey is remarkable for its dark orange sideburns, chest and inner sides of its limbs. It has a reddish-brown back and a white-tipped black tail. [side by side images at end of text]
The bernhardi's head and body measure about 38 cm and the tail another 56 cm. An average individual weighs about 940 grams.
Van Roosmalen found the second monkey last year while conducting a scientific survey near the Purus river with his son, Tomas.
The stephennashi is a silver monkey with a black forehead and red sideburns, chest and red on the inner sides of its limbs. Its head and body measures about 28 cm and the tail another 43 cm. An average individual weighs about 680 grams. [images above left, and at very bottom of page]
The new monkeys are the fourth and fifth new species described by Van Roosmalen, who says he has discovered another 20 unnamed species in the same central-Amazon region.
There are 310 known monkey species in the world and Brazil has the largest
number of them - 95 endemic species.
"It's not that I'm so special. The fact is that since the 1800s nobody did any work like I'm doing the last five years," Van Roosmalen said from Amsterdam, Netherlands, where he will present a portrait of the bernhardi monkey to the Dutch prince later this week.
The flood plain region is similar to a landlocked archipelago where dozens
of smaller, crisscrossing rivers serve as natural barriers, allowing once-similar
species to evolve differently over thousands of years.
"It's a Terra Incognita and at the same time an El Dorado for biologists, full of new species and not visited by biologists or botanists," Van Roosmalen said. El Dorado refers to a mythical land of gold and plenty sought by centuries of adventurers in South America.
The newly discovered monkeys are very important. They give us a better understanding, although still an incomplete understanding, of this group and of primates in the Amazon basin in general.
Though the monkeys are in a little-disturbed part of the Amazon and are not threatened with extinction, Van Roosmalen is working to guarantee their survival.
To that end, he is offering to name the other discovered species after people who pay to help create nature reserves in the region.
Under Brazilian law, citizens can create private nature reserves on their land in exchange for an exemption from property taxes. So far, Van Roosmalen, a naturalized Brazilian, has created two reserves covering about 600 square kilometers of unspoiled forest.