The tiny primates - their old-man faces framed by manes of golden hair - scampered to the window to investigate the woman gazing at them.
If only they knew what she had done on their behalf.
The woman, Denise Rambaldi, heads a program in Brazil that has rejuvenated the golden lion tamarin, a squirrel-size primate that lives along the Atlantic Coast near Rio de Janeiro. Experts say the effort provides a blueprint for how to rescue a species.
Rambaldi and international colleagues attacked the problem from every angle. They introduced captive-born animals into the wild. They used new technology to trace their movements and bring them aid when they needed it. And they planted corridors of forest and worked with ranchers to protect vital habitat.
The upshot is that the golden lion tamarin is the first primate species that has been turned around, experts say.
Tonight, at a Brazilian-themed gala, the Philadelphia Zoo is taking a step to ensure that the work, and the species, can continue.
As a finale to a gift-giving spree celebrating its 150th anniversary, the zoo is awarding a half-million dollars over the next 10 years to Brazil's Golden Lion Tamarin Association to help fund the executive-director position that Rambaldi holds.
Russell Mittermeier, president of the nonprofit Conservation International, said the gift was "something a lot of the zoo community should emulate: that recognizing and providing long-term support to some of the best conservation initiatives in the world is really the way to go."
Mittermeier also heads the primate group within the International Union for Conservation of Nature, which last month released its list of the 25 most endangered primate species on the planet.
At one time, the golden lion tamarin would have been on that list. In the 1980s, no one knew how many of the species remained, perhaps just a few hundred.
A small cadre of people got to work, studying the tamarin's biology and needs.
Rambaldi recalled the group having one car, and that rarely ran. The zoo's chief operating officer, Andy Baker, was a young researcher who slept in a box on the floor and kept tamarin hours - 4 a.m. to 4 p.m. - so he could study their behavior.
Golden lion tamarins occupied a tiny bit of habitat to begin with. But because it was situated within two hours of teeming Rio, the pressures on the species mounted.
Originally, the pet trade took a heavy toll. Then, the tamarins' habitat shrank as forests were felled for cattle farms and subdivisions. Today, less than 8 percent of the tamarins' richly diverse rain forest is left.
This is what conservationists say is so unusual: More than a quarter-century of effort ensued involving hundreds of people from many countries, untold dollars, and strict attention to every facet of conservation, said Rambaldi, who at one point was holding down a full-time job in tamarin conservation and studying for a law degree.
It took biologists learning more about the needs of the animals and how they interacted. It took forest experts to identify critical features of the habitat, and legal experts to find innovative ways to protect land. It took education on every level, from young students who have grown up to become decision-makers to landowners who now brag about how many tamarins live on their property.
Conservationists are now working to figure out how to give landowners a financial incentive - through carbon offsets - to protect forests, which will also benefit other species and protect the region's water supply.
With populations isolated and inbreeding in fragmented habitats, workers moved individual tamarins around so the gene pool would remain viable.
Zoos worldwide took part. In the beginning, the zoo population was not breeding very successfully.
But husbandry techniques improved and tamarin management became a model for today's species-survival programs.
Eventually, nearly 200 golden lion tamarins that had been captive-bred in zoos were released back into the wild - a go-for-broke tactic that has rarely worked among other species because the animals don't know how to survive or lose out in territorial challenges.
But researchers put zoo animals through "boot camps" in large enclosures, restricting their food so the animals would learn to forage.
The animals were released into the wild at an age when young tamarins separate from their family groups to find mates. They wore radio collars so humans could tail them, provide food or veterinary care if necessary, and return them if they became lost.
Now, about half of the tamarins in the wild are captive-born animals or their descendants, Rambaldi said.
In 2003, the IUCN took a unique step and upgraded the golden lion tamarin's status from "critically endangered" to "endangered."
All this took money, although as conservation goes, the tamarins came relatively cheap.
"This program was not anywhere near as expensive as some of the recovery programs in the States that have not been as successful or took a lot longer" - such as those for condors, whooping cranes and black-footed ferrets, said Devra Kleiman, a Smithsonian Institution senior scientist. She is also a zoo consultant and tamarin researcher of such longevity that she is often called "the matriarch of all things golden lion tamarin."
She credited Rambaldi's inventiveness and tenacity, in part, and the fact that the nongovernment association she heads "can be leaner and meaner."
Normally, Kleiman said, conservation efforts are "constantly on a precipice," unsure of where next year's money will come from. The zoo award "is going to provide unbelievable stability for a number of years," she said.
Baker said that when the zoo was looking for a program to fund, it wanted one that had proven progress but that still needed a boost "to make that long-term, tipping-point difference."
The tamarin program has continually evolved - another of its precedent-setting strengths, the experts say - and now has a specific goal: 2,000 tamarins occupying 25,000 hectares (nearly 100 square miles) of forest that is both permanently protected from development and, where it is in patches, connected by corridors of forest.
It is well on the way: Today, there are 1,600 tamarins on 10,600 hectares.
They have become an emblematic species for the rain forest, their image appearing on Brazil's 20-real banknotes.
"The tamarin has an important role to get the public attention," Rambaldi said. "Not only for the species itself, but for the entire ecosystem."