Nearing extinction in the wild only a few decades ago, the golden lion tamarin (Leontopithecus rosalia) has become an international symbol of success in conservation. Many of the squirrel-size monkeys had been captured as pets or for zoo exhibits. Those remaining in the treetops—about 200 in 1983—hung on for dear life in increasingly isolated patches of forest. Since then, more than 140 captive-bred tamarins have been released in and around the Poço das Antas Biological Reserve in the state of Rio de Janeiro. They are now mating with the wild population, which has rebounded to about a thousand.

"It's always like this," says Adriano Chiarello. "You know they're here, but you can't see them." The Brazilian conservation biologist bends his neck backward like a yoga master to peer at a tree's uppermost branches a hundred feet (30.5 meters) above us. Somewhere in the leafy canopy, a female maned sloth and her eight-month-old infant are hidden from view. A steadily beeping radio signal from the mother's collar has brought Chiarello to the base of the tree, but even technology has its limits. The biologist must now spot the pair the old-fashioned way: with his eyes alone.

"If they don't move, we may never see them," Chiarello sighs. "And you know, they really are sloths. They spend hours sitting, sleeping, never moving. That's what they do 80 to 90 percent of the day: nothing."

He wipes his eyes, shakes his head, then returns to his craning yoga pose. "Wait. . . . Maybe my insult has worked. Look there—right over your head. She's braced against a branch."

I follow Chiarello's pointed finger and spy the mother's dark brown face among the leaves. She buries her face under her arm and looks instantly like a large, furred coconut or bees' nest.

"Do you see that? How she can vanish?" Chiarello asks. "For their size, they are so well camouflaged. And . . . wow! Now she's moving!"

For Chiarello, such a sloth-on-the-move sighting is a peak experience, the ultimate biological moment that holds the promise of new insights.

The baby sloth, looking like a Teletubby wearing a curly lambskin coat, emerges from its mother's arms. It climbs over her and then playfully—lazily—slaps at its mother's face. The mother does nothing in return. "They never respond to their babies," whispers Chiarello, adding that mother sloths neither play nor get angry with their offspring. Instead, with all the speed of a desert tortoise, the mother reaches an arm out to a nearby branch and nibbles the leaves.

Chiarello's graduate students—at the Catholic University of Minas Gerais, where he's a professor—busily take notes. We all stretch our necks, craning this way and that, to keep the sloths in view as the pair move like sleepwalking high-wire artists along the branches to the freshest leaves. Astonishingly, given the mother's 15-pound (6.8 kilograms) build, she and her baby hang from the pencil-thin twigs like strange, half-animated fruits.

Chiarello's "main actress," as he fondly refers to the mother sloth, is the star in his study, funded in part by the National Geographic Society, of the endangered mammals of the São Lourenço Municipal Park, a small fragment of Brazil's Atlantic forest, or Mata Atlântica as the Brazilians call it. Like many mammals here, the maned sloth has lost huge tracts of its original habitat since the first Portuguese mariners stepped ashore in April 1500. At that time the Mata Atlântica is believed to have covered about 520,000 square miles (1,346,794 square kilometers), making it about one-fifth the size of the present Amazon forest 500 miles (805 kilometers) to the northwest. The rain forest hugged the coastline from the country's snout-like protuberance of what is today the state of Rio Grande do Norte to its border with Uruguay. In some places it extended inland for 300 miles (483 kilometers) or more, covering a range of habitats from coastal mangrove thickets to mountain massifs averaging 3,000 feet (914.4 meters) high, blanketed with broad-leaved evergreens and conifers.

Forebodingly, one of the first things the Portuguese seafarers did was to chop down a tree. They fashioned a cross from it and celebrated a Mass, claiming the land and its forest for their God and king. Over the next 500 years many more trees were felled, and the forest was transformed into cities, mines, and fields planted with sugarcane, coffee, cacao, and eucalyptus—all introduced species. Now, some 70 percent of Brazil's population lives in what was once the Atlantic forest, with most people concentrated in two of the three largest cities in South America, São Paulo and Rio de Janeiro.