MAROJEJY NATIONAL PARK, Madagascar -- Since daybreak he has been scanning the treetops for the creatures that move as if by pogo stick and look as if they wear white fur coats and big, black, round sunglasses.
It is after 2 p.m., and the dense, hilly rain forest has yet to give primatologist Erik Patel a glimpse of Propithecus candidus, the rare lemur known as the silky sifaka. It is one of the world's 25 most-endangered primates. Fewer than 1,000 silky sifakas are thought to exist, all of them in this rugged patch of northeast Madagascar.
Finally, a guide working with him spots a flash of white deep in a ravine. Patel skitters and slides down the slope, where he hears a familiar sound: the sneeze-like ''zzuss" call the animals emit when alarmed.
''It's Pink Face," he says, wiping sweat from his forehead. ''I know it's him."
Patel, 35, is the planet's foremost specialist on the silky sifaka. Until 2001, when he began work on a doctorate at Cornell University, the sum of knowledge about these animals was as fleeting as their ghostly visage. He chose this path because he would have to beat it.
Over the past four years, Patel has spent 14 months camping in this forest. He is back for a few weeks to observe Pink Face and the five others in this lemur community. He hopes new data will shed more light on how the animals communicate, information that may one day yield clues about how speech evolved in humans, the lemurs' distant cousins.
And he feels a sense of urgency. Despite an increase in conservation efforts, no one knows how much longer the silky sifakas can survive persistent hunting and deforestation.
No one can mistake these animals for any other kind of lemur. Only the silky sifakas have fluffy white fur covering their body except for the face, which is slate-gray or pinkish. Their eyes are reddish-orange.
They have been dubbed ''flying angels" for the way they appear to soar from tree to tree. They use their powerful legs and finger-like toes to grasp a tree trunk or branch and quickly push off to the next one, up to 10 feet away.
Taxonomists group them into 70 species and subspecies (humans are known to have driven 17 species to extinction), and all occur naturally only in this country.
Scientists generally consider lemurs, which have the smallest brains among primates, to be less intelligent than relatives such as baboons, chimpanzees, apes, and various monkeys.
Yet their parallel evolution has made lemurs different in key ways and therefore interesting to scientists looking for broad-based models to explain the evolution of primates.
The study of lemurs is relatively young, particularly when it comes to species in forbidding terrain, and few live in tougher territory for researchers than the mountain-dwelling silky sifaka.
No one had done a full-fledged study until Patel's arrival in 2001.
For the first six weeks, the silky sifakas hurried away from Patel and his research assistants: Until then, most humans who approached probably meant to kill them.
Gradually, the animals got used to Patel and his team, which included up to four American or British field assistants, plus Malagasy guides hired from the village below the park.
Sifakas are a large category of lemurs. They resemble a cross between a bear, a monkey, and a raccoon. To Patel, they call to mind cats. The exception is Pink Face, the twentysomething male. Patel came to think of Pink Face as a fellow man.
Patel's team documented how sifakas spent their time (resting nearly half of it, then foraging for food or playing and grooming) and the home range of the study group (the equivalent of 83 football fields), and the distance they typically covered in a day (four-10ths of a mile). Scent markings left by males and females were painstakingly recorded. The result was the first documented behavior known as ''totem-tree" scent marking in primates -- the repeated marking of particular trees mainly by males apparently competing for female attention.
Patel also made the most complete audio recordings of lemurs in the wild.
They are ''talkative," Patel says, and he has amassed a catalog of 600 zzuss calls alone.
Patel documented as well the threats to silky sifakas, mainly from people. From villagers outside the park he learned that Madagascar's well-to-do enjoyed the taste of lemurs as a delicacy or ''picnic food" and hired unemployed villagers to hunt them.