"Finding the extremely rare Prolemur simus in a place where nobody expected it was probably more exciting than discovering a new lemur species," said Edward Louis, a US conservation geneticist who coordinated the joint research, in a statement.

Experts now believe 30 to 40 of the distinctive big-eyed lemurs -- which have jaws powerful enough to crack their favourite food, giant bamboo -- live in Torotorofotsy.

They say that habitat destruction caused by slash-and-burn agriculture and illegal logging threatens the previously known populations that total about 100 individuals, making the existence of the newly found lemurs particularly valuable.

"Our hope is that the presence of these critically threatened creatures will increase efforts to protect their habitat and keep them alive," said Rainer Dolch of MITSINJO, a Malagasy group that manages the Torotorofotsy site.

Biologists have long flocked to Madagascar, where about 90 percent of species are unique to the giant island.

Part of the reason for its profuse biodiversity is its varied terrain, including rainforest, dry forest, lowlands and mountains, and part is its geologic history.

Once part of the African mainland, Madagascar drifted off some 100 to 200 million years ago. It eventually drew colonist species, including lemurs, whose ancestors probably rafted over on floating vegetation, scientists say.

The latest research was carried out by MITSINJO and the Henry Doorly Zoo in Omaha, Nebraska, supported by the Margot Marsh Biodiversity Foundation and Conservation International.

The researchers will present their updated information on the greater bamboo lemur on August 3-8 at the International Primatological Society 2008 Congress in Edinburgh, Scotland. (Editing by Mariam Karouny) (For Reuters latest environment blogs click on: http://blogs.reuters.com/environment/) (For full Reuters Africa coverage and to have your say on the top issues, visit: http://africa.reuters.com/ )