INSIDE THE STEEL WIRE CAGE, four orange-brown howler monkeys moved about, peering out of their temporary enclosure at the commotion around them. Fitted with fancy GPS and radio signal transmitters around their necks or ankles, the primates were surrounded by an emotionally-charged group of researchers, park officials, and environmentalists, all waiting for the climax of a years-long project. After a morning-long build-up, the door separating the animals from their new home — a rainforest contained in the core of one of Latin America’s largest metropolitan areas — was finally opened.

The monkeys advanced quietly. They had all been rescued from illegal keepers or selected from rehabilitation centers, and their reluctance to step outside their cage was a symptom of years spent in captivity. Kala, a young female, was the first to approach the door, venture off, and, before a score of watery-eyed humans, climb a nearby tree to explore the canopy above. The others soon followed suit, evoking an immediate air of celebration among onlookers, and a sense of accomplishment among those directly responsible for the translocation.

Important as their newly-acquired freedom was, the events of that September 2015 morning represented more than the release of a small group of captive animals. These monkeys were part of the much bigger story of a forest that had not seen a howler in more than a century, and the bold efforts of a small team of researchers to fill an emptied and broken woodland back up with life.

AT CENTRO DE TRIAGEM de Animais Silvestres (CETAS), Rio de Janeiro’s wildlife triage center, a cacophony erupts as I approach the spacious aviary where a peculiar crowd is taking flight lessons. In a blur of blue-and-yellow feathers, macaws traverse back and forth between perches strategically placed next to bowls full of tasty tidbits meant to entice them.

“We have to encourage them to fly around so they can get used to it again,” explains Marcelo Rheingantz, a biologist at Universidade Federal do Rio de Janeiro (UFRJ), one of the researchers preparing the motley birds for their impending release. “These animals arrived at CETAS after being seized from wildlife traffickers or were wounded, and many spent a long time in cages too small for flight. They need to regain their strength to fly long distances again.”

“We will also help them to relearn how to nest properly and which fruits they can eat in the wild.” Rheingantz adds. There are 26 macaws in the enclosure, but a few of them will never be fully rehabilitated — previous owners have clipped their wings and they can no longer fly. Those who can relearn their natural behaviors will be released in the coming months to fulfill a vital role in their soon-to-be home: Tijuca National Park.

Tijuca is a 4,000-hectare green patch in the heart of the concrete jungle that is Rio de Janeiro. Surrounded by the city’s nearly seven million inhabitants, it is the most visited national park in Brazil, and home to the globally renowned Christ the Redeemer, a statue that rises above the city from its perch atop Corcovado Mountain. Far from pristine, this small fragment of the Atlantic Forest — a biodiversity hotspot that once stretched across Brazil’s Atlantic coastline and extended as far inland as Paraguay — is a shadow of its former self.

The Atlantic Forest as a whole has been decimated by deforestation, and has lost some 85 percent of its original forest area. The fragment that is now Tijuca National Park was ravaged by logging, charcoal production, and coffee monocultures beginning in the seventeenth century, all of which transformed it into a mosaic of croplands and degraded woods. Unsurprisingly, this decimation quickly led to land erosion and a decline in rainfall that jeopardized Rio’s water supply. In 1861, responding to the looming threat of drought, then-monarch Pedro II ordered the extirpation of coffee plantations from the area and the reforestation of agricultural lands. A large section of the landscape was replanted with native flora, and what remained of the original forest was legally protected from harmful enterprises. The effort culminated in the creation of the largest reforested urban park in the modern world, and Tijuca was elevated to National Park status in 1967. Unfortunately, many native fauna that had once inhabited the forest were long gone by then, leaving behind a biological vacuum.

The absence of native species was a serious ecological problem. Every living being in an ecosystem interacts with and depends upon many others. Granted, a forest can often handle the loss of a few species without falling apart. But if medium-to-large vertebrates begin to vanish, things are likely to unravel. Piles of fruit begin to accumulate and deteriorate on the ground because there are no creatures large enough to eat them. Inside this rotting mass, seeds lie undispersed, and the waste attracts rats, insects, and pathogens. Any fortunate seedling survivor that manages to sprout will be instantly locked in a detrimental competition for resources with other undispersed seedlings growing nearby, and plants that can rely on really small animals — or no animals at all — for seed dispersal may become more dominant across the landscape.

“The absence of some of our native animals can facilitate a retrogressive succession — the conversion of a mature forest environment into an immature one, filled with vegetation that should only be present when an open area is still on its way to becoming a woodland,” explains Alexandra Pires, a researcher from Universidade Federal Rural do Rio de Janeiro (UFRRJ) and specialist in animal-plant interactions. “This transformation can have serious consequences in terms of carbon stock, soil quality, water regimes, and much more.”

History had left Tijuca in this strange state. The urban park was a much-preferred alternative to monocultures, favelas, or luxury condos, yes, but a far cry from the exuberant woodland it once was. Inadvertently, this condition also made Tijuca the perfect laboratory for a unique rewilding experiment.

IT STARTED SERENDIPITOUSLY with a carpool that Pires was part of back in 2008. At the wheel was Ivandy Castro-Astor, another ecologist and Tijuca National Park’s research coordinator at the time. Both researchers were returning from a meeting and were chatting about Pires’ doctoral thesis and how she had found agoutis, a rodent native to northern and central South America, to be highly efficient seed dispersers for certain native tree species, especially the Joannesia princeps, which is listed as vulnerable under the International Union for Conservation of Nature’s Red List of Threatened Species. Locally known as cutieira, this medium-sized tree with a spread canopy, whose name can be literally translated as “agouti tree,” depends almost exclusively on these cousins of the capybara to spread its seeds.

In wide-eyed astonishment, Ivandy promptly replied that they should probably do something about the animal’s absence from Tijuca, and quickly: A cluster of beautiful cutieiras grew near the old park administration headquarters. “Wait,” Pires interjected. “What do you mean? There are no agoutis in Tijuca?”

Pires struggled to believe that could be possible. These rodents should be common in this region of the Atlantic Forest. But the weight of evidence was overwhelming: Not a single park visitor or staff member had registered any agoutis in decades, despite the fact that pacas — much less conspicuous rodents — were regularly observed in Tijuca. Park staff and researchers believe illegal hunters extirpated agoutis from the urban forest sometime in the twentieth century.

This grave loss had been noted before. In the 1970s, conservationist Adelmar Coimbra-Filho tried to return agoutis to Tijuca, along with many other extirpated animals, including Ariel toucans, bare-throated bellbirds, and rufous-bellied thrushes. A few of the birds that were reintroduced to the park at that time were able to reestablish themselves, but the rodents didn’t manage the same. Simply loading a bunch of agoutis onto a truck and casting them free in the wild apparently wasn’t enough to secure a viable population. But Pires was vaguely familiar with a method that might have better results. That very day, she mentioned the park’s plight to her husband, UFRJ researcher Fernando Fernandez, who just so happens to be one of the most prominent conservation biologists in Brazil.

“She asked me if I was interested in joining a project to return agoutis to Tijuca,” recalled Fernandez. “And, of course, I was very interested!” It was the beginning of a long and fruitful initiative. Instead of merely translocating the animals, the plan was to use the management tools endorsed by the emerging discipline of reintroduction biology to prepare and closely monitor the reddish rodents and see how they fared in the months and years that followed. This involved quarantining them at Rio de Janeiro’s Zoo for veterinary evaluation ahead of their release, fitting them with ear tags and radio transmitters as well as marking their fur, and distributing automatic cameras and traps throughout the forest.

Successfully establishing new animal populations is remarkably difficult. Even with modern scientific developments, a large proportion of reintroduction projects end up in failure. The relationship between organisms and ecosystems is complex, and what looks prudent on paper may not work in real life. When managing a reintroduced species, problems tend to arise with some frequency. Monitoring equipment can affect animals in unpredictable ways, food supplementation may prove to be too scarce or too abundant, or individuals may simply react to their new environment in unexpected ways. Fortunately, Fernandez and Pires’ teams of students were up to the challenge. From 2010 to 2014, 31 agoutis were translocated to the national park, mostly from a gardened plaza in downtown Rio. Today, every agouti in the forest is wild born, and their numbers appear strong and stable.

Even more importantly, research soon confirmed what the biologists had been hoping for. There was hard evidence that the animals were contributing to the establishment of a new generation of at least two different native tree species — the aforementioned cutieira and Astrocaryum aculeatissimum, a spiny palm regionally known as iri — and probably many more. Agoutis, like some squirrels, are scatter hoarders, which means that they have the convenient habit of burying seeds and fruits in a variety of places for future consumption. Like every wise saver, they cache more than they are likely to need, but they also tend to lose track of the occasional hiding spot.

Even with modern scientific developments, a large proportion of reintroduction projects end up in failure.

The “lost” seeds, stashed underground, find themselves in perfect conditions for growth. “These animals manipulate large seeds in a very useful way,” Pires explains. “They deposit the ones they don’t eat in places that are favorable for their growth. But their contribution goes beyond even that. They consume the smaller seeds of plants that are characteristic of immature forests, helping other trees to outcompete them. Agoutis help keep a mature forest mature.”

The establishment of agoutis in Tijuca was a remarkable success, but at that point, the project had merely joined a growing number of wildlife management initiatives across the world that had reintroduced animals — like California condors, Iberian Lynxes, oryxes, and many more — to environments from which they had been previously lost. But most reintroduction projects had been focused on single species, and those working in Tijuca found this insufficient. In that small impacted forest was the opportunity for something more. They wanted a whole ecosystem back.

BRINGING THE ENTIRE TIJUCA ecosystem back to life was more complicated than simply relocating a handful of species. Many plants and animals missing from Tijuca were dependent on other missing species to thrive. The key to the process was identifying which animals could be safely returned to the forest as it was; which would demand prior translocation of a few former compatriots in order to establish a foothold; and which would require a much more species-rich environment in order to survive. As they envisioned how to structure the effort, the researchers termed their initiative a “refaunation” — the restoration of an entire faunal community. With that, project Refauna was born.

Right out of the gate, there were certain limitations regarding what Refauna could realistically accomplish. Tijuca simply wasn’t big enough to accommodate populations of some of the Atlantic Forest’s largest native dwellers. Also, as a park nested inside a city, animals that could pose a threat to residents — or at least the appearance of one — were scratched from the reintroduction list. Tapirs, jaguars, pumas, and more were sadly eliminated on these prerequisites. Limitations aside, the team still had to decide how to build upon the agoutis’ reintroduction.

“I had just read, coincidentally, the diary of Darwin’s journey aboard the Beagle,” says Ernesto Viveiros de Castro, who assumed the position of Tijuca’s director in 2012, referring to naturalist Charles Darwin’s 1830s trip around the world on the United Kingdom Royal Navy’s HMS Beagle. A couple years earlier, while working at Serra dos Órgãos National Park, de Castro had created a plan to reintroduce tapirs, one that was never implemented. “In his journey, Darwin visited Tijuca and collected a couple of howler monkeys.” It was solid evidence that the animals were native to the region, even if there had been no signs of them since the early 1800s.

“We were hoping to find a species that was charismatic, ecologically interesting, and available to be translocated from different sources,” says de Castro, “and howlers seemed to fit the profile. Finding individuals that could be reintroduced turned out to be harder than expected, though.”

Much thought went into the return of the primates to Tijuca. Unlike agoutis, howler monkeys are deeply social animals, so gathering random individuals and putting them together would not guarantee a cohesive group. If the right combination of monkeys could be assembled, however, Fernandez knew the animals would be a good fit for the park. “Even eating mostly leaves, these animals have a diverse fruit diet, and one that complements the preferences of the agoutis. This means that they can disperse seeds that agoutis cannot, and vice-versa. And, ecology aside, they are very docile and likable, and could attract a good deal of positive attention to the park and the project.”

Most reintroduction projects have been focused on single species, but researchers working in Tijuca found this insufficient.

With help from the Centro de Primatologia do Rio de Janeiro (CPRJ), a primate research center, four howler monkeys were gathered. They spent time together ahead of their release in order to form bonds that would help them survive in the wild. They also received constant veterinary care, were introduced to a diet of native leaves, and were fitted with tracking equipment as they awaited their September 2015 release. Three others would soon join them at the park.

This reintroduction did not go as smoothly as that of the agoutis. Some of the animals, having spent many years in captivity, failed to adapt to their new environment in Tijuca. They sought constant contact with park visitors and were removed back to CPRJ facilities after a few months. Meanwhile, tracking equipment for those monkeys who remained in Tijuca malfunctioned, turning monitoring efforts into the proverbial search for needles in a haystack. On top of it all, 2017 brought a Yellow Fever outbreak of unprecedented spread to Brazil. As howler monkeys are particularly vulnerable hosts to the disease, translocating additional monkeys to the park quickly became out of the question.

Despite these difficulties, two of the animals managed not only to hang on, but to reproduce, and a new infant was born in 2016, a sign that Refauna, even after stumbling, was on the right track. In June of 2018, the team lost track of the new howler family, but researchers are now waiting for an opportunity to translocate additional howlers to the park from forest fragments too small for them.

As they continue efforts with howler monkeys, the Refauna crew is also planning to introduce new species this year. In a rustic enclosure at CETAS, yellow-footed tortoises await relocation. Not 200-feet from their transitory home are the boisterous blue-and-yellow macaws. “We expect to translocate 59 tortoises and 20 macaws to Tijuca in the coming months,” says Marcelo. “The animals should be adapted and ready by then.”

Tortoises are effective seed dispersers, and they can also help to maintain local plant diversity by treading upon all seedlings and preferentially eating the more palatable and fast-growing ones, keeping any single tree species from dominating the landscape. Macaws should also make unique contributions to Tijuca, spreading large seeds across great distances as they fly across and beyond the mountains of the park.

The future teems with possibility. As Fernandez never tires of voicing, “Refauna’s objective is to restore, as far as possible, the fauna and ecological interactions of an Atlantic Forest fragment.” With that aim in mind, the team is studying the possibility of reintroducing iguanas in the near future. Researchers have suggested golden lion tamarins — an endangered monkey native to the region — as another potential addition for the coming years. To Refauna staff, Tijuca is now a half-empty stage waiting for more species to join the cast.

ONE RATHER UNEXPECTED outcome of this project, given Tijuca’s prime location in the middle of Rio’s concrete world, is that it is helping rewild the lives of local residents while refilling the park.

“We have produced quality science for over two decades in our laboratory and no one ever showed up to our door to ask us about what we were doing,” says Fernandez. “Now, with Refauna, journalists and magazines approach us by the dozens. Everyone seems fascinated. This kind of attention can attract lots of resources and public support to conservation. Refauna is amassing an enormous amount of goodwill.”

“This is an excellent opportunity to show the population how it is possible to restore the Atlantic Forest,” de Castro adds. “People usually recognize that deforestation is a problem, which is absolutely correct, because its effects are readily apparent.” Defaunation, or loss of animals, however, is nearly invisible to the untrained eye. An empty and decaying forest is still a standing one. “In Tijuca there is now the opportunity for them to learn about the hazard of defaunation, and how it can be solved.”

Locals are becoming invested in the Tijuca effort. Park rangers constantly hear from visitors excited by animal sightings. So frequent are these reports that de Castro is considering putting them to good use. “A visitor recently saw an agouti at Serra da Carioca, a sector of the park that is distant from the areas where these animals were reintroduced, and that was new to us,” he says. “We are now thinking about how to use such information to help monitor the reintroduced species, and about creating a citizen science initiative in the park.”

Refauna is also showing that reintroductions can be both successful and cost effective, and is clearing the way for others to follow their path. The team is creating refaunation protocols that might be applied to the hundreds of other Atlantic Forest fragments scattered across Brazil’s Atlantic coast.

By trying different methods in the field — and inevitably bumping into failures — Refauna is identifying which management practices are useful for each species, and making sure that future initiatives will cost less time, money, and human effort to succeed.

Something similar can be said about the Brazilian legal system. As reintroduction is a fairly new concept in the South American nation, there is still some confusion regarding which licenses are necessary and which government agencies should be consulted regarding each part of the process. By navigating these bureaucratic issues, Refauna is creating precedents that will facilitate future efforts.

Refauna’s work also seems to be inspiring other reintroductions across Brazil, though so far these efforts have focused primarily on single species. A group partially coordinated by former Refauna member Bruno Cid, for example, is preparing to bring scarlet ibises back to a southern region of Rio de Janeiro State. A red-footed tortoise translocation is already under way across the country at a forested landscape near Universidade Federal do Mato Grosso do Sul. And in 2018, another branch of Refauna coordinated by researcher Maron Galliez reintroduced tapirs to Reserva Ecológica de Guapiaçu, also in the State of Rio de Janeiro.

IN THE EARLY HOURS of a scorching morning last year, a team of three eager undergrads walked up and down Tijuca’s main trails. It is a Friday in late December — summer in the southern hemisphere, and approaching Christmas — and the park is quiet, mainly devoid of visitors. For six months, the students’ mission had been frustrated by defective equipment, but they carry on, diligently scanning the branches above them for the three lost howler monkeys. Suddenly, a flash of movement catches their attention. They are used to the anti-climactic sight of capuchin monkeys, which also inhabit the park, but still they crane their necks hoping for a clear view.

Backlit by the sun, an orange-lined silhouette appears, drawing gasps from the team. As the students ready their cameras, another monkey appears. Eagerly snapping pictures, they squint, straining to see the final missing howler. A new rustle of leaves directs them to a branch far above. There, Kala carries her newborn calmly through the trees.