Several small monkeys with fiery, orange manes and long furry tails leap from tree branches and rocks on the National Zoo’s forested campus in Washington, D.C. Occasionally, they dart into a shelter box that resembles a pet carrier and serves as their sanctuary. Meanwhile, a group of the same gray-faced monkeys, some sporting radio collars, scurry up vines and flowering plants in search of insects and tree resin in Brazil’s Atlantic coastal rainforest.

Though separated by thousands of miles, the two groups of golden lion tamarins are part of the same conservation program led by the National Zoo and Brazil’s Golden Lion Tamarin Association, an organization founded in 1992 to protect the species. The program breeds the endangered animals in zoos worldwide and then releases them into the wild, a strategy known as reintroduction. Now in its 24th year, the tamarin program, unlike most reintroductions, is producing some encouraging results.

Golden lion tamarins live exclusively in the Brazilian rainforest, and scientists estimate that a century ago they numbered in the tens of thousands. But in the early ’70s, deforestation, poaching, and people taking the monkeys as pets caused the population to dip to fewer than 200. Now there are about 1,500, just 500 short of the reintroduction program’s goal of 2,000. Scientists expect to hit this target well before the original goal of 2025, so long as advocates can secure enough land for the creatures.

Tamarins are one of the few success stories when it comes to reintroductions; others include bald eagles and Galapagos tortoises. People have been releasing once-captive animals into the wild for thousands of years, but reintroduction with the purpose of preserving biodiversity is a fairly new concept. Scientists are unsure when the first reintroduction took place, but it may have been in 1907 with the release of American bison into an Oklahoma reserve. In 1988, the World Conservation Union (IUCN) formed the Reintroduction Specialists Group (RSG) to provide guidance for the growing number of reintroduction programs.  Ten years later, it published its reintroduction guidelines. Though progress has been made, a 1994 study conducted by Ben Beck, coordinator for the golden lion tamarin reintroduction, found that only 11 percent of reintroductions establish a self-sustaining wild population. Those are daunting statistics considering that one in four mammals is in jeopardy and 16,119 species face extinction, according to a 2006 IUCN report. But experts say lessons learned from successful reintroductions may aid scientists in creating models for reintroducing other species.

Nobody can pinpoint why most reintroductions fail to establish self-sustaining populations, but researchers do know that it’s a long, costly, complex process. “The science of reintroducing species is a new one. Anytime you’re dealing with a new science, there are going to be more questions than answers,” says Dan Wharton, director of the Central Park Zoo in New York, who is not involved with the project. “We not only have to deal with taking captive animals and putting them in the wild,” says Devra Kleiman, previous coordinator for the program and North American section chair for the RSG. “We need to think about local communities, education, governments, and local regulations.”

And they need to be flexible. Those working on the tamarin project—who range from community educators to ecologists—rely on trial and error. When they find a strategy that works—or doesn’t—they adjust the program accordingly, says Beck. For instance, he and other researchers initially thought they could teach captive tamarins skills they would need in the wild, such as how to break open fruits and eggs. “It didn’t change the survival stats at all,” Beck says.