In 2008, biologist Alexandra Pires had just completed her doctoral thesis, which described how agoutis, a large guinea pig-like rodent, were important for the regeneration of plant species in Brazil’s Atlantic Forest. When she told this to Ivandy Castro‑Astor, a researcher at Tijuca National Park, in the hills outside Rio de Janeiro, she learned that the rodents no longer existed there. Proof of this were the abundance of seeds from a tree known in Brazil as cutieira or “agouti tree” (Joannesia princeps), which were rotting on the forest floor.
“How can there be no agoutis in Tijuca National Park?” Pires recalls thinking at the time, to which Castro‑Astor replied: ‘I think you should release some agoutis there!’”
Eighteen years later, visitors to Tijuca can now observe red-rumped agoutis (Dasyprocta leporina), along with brown howler monkeys (Alouatta guariba) and yellow‑footed tortoises (Chelonoidis denticulata). Their presence in the forest is the result of the reintroduction program carried out by Refauna, an initiative of which Pires is the scientific director, with support from the Brazilian government’s Chico Mendes Institute for Biodiversity Conservation (ICMBio).
In early January, it was the turn of blue‑and‑yellow macaws (Ara ararauna) to make a comeback. Extinct in Rio de Janeiro for 200 years, today they’re once again flying in the skies over the city.
Refauna’s goal is to put an end to what’s known as empty forest syndrome, a concept identified by U.S. conservationist Kent Redford in 1992. In such forests, while the trees and other vegetation appear intact, the animals essential for ensuring their future through seed dispersal are missing.
“One of the main symptoms of the syndrome is precisely fruits rotting on the forest floor,” says Marcelo Rheingantz, executive director of Refauna and a biologist at the Federal University of Rio de Janeiro. “In the Atlantic Forest, for example, nine out of 10 plant species depend on animals to disperse their seeds.”
Before the rewilding work started, Tijuca was on the path to becoming an empty forest: lush, but without animals. Reintroducing them was the missing step after a long restoration effort to bring the forest back to life.
This process took more than a century and a half. It began in 1861, when Brazil was still an empire. The emperor at the time, Pedro II, ordered the expropriation of small farms and estates in the Tijuca mountains just outside Rio, which had been completely devastated to make way for coffee plantations. The next step was planting thousands of plants native to the Atlantic Forest to restore the original vegetation.
Tijuca National Park would be created a century later, in 1961, encompassing several forests restored in the Tijuca mountains. Animals, however, were still rare.
The first reintroduction of a species that had disappeared from the area was carried out in 1970 by biologist Adelmar Coimbra Filho, who released 46 channel-billed toucans (Ramphastos vitellinus). While it was an isolated action, it literally bore fruit: a study published in February 2026 found that the birds had reprised their important role as seed dispersers, contributing to forest recovery.
According to the study, the toucans interacted with at least 76% of the plants that were part of their original diet, many with hard seeds that only their powerful beaks could open. These included threatened species such as the juçara palm (Euterpe edulis) and bicuíba (Virola bicuhyba).
This pioneering initiative paved the way for the work now carried out by Refauna, which has refined the reintroduction methodology. In addition to a larger number of species, the selection is thoroughly planned and followed by a long acclimatization process in which the animals stay in enclosures within the forest. After their much-anticipated release, the work’s still not done; there’s still monitoring to do to ensure the animals thrive.
“One of the first actions is to determine whether the chosen species will adapt to an urban forest, and whether the park will support a population in the future, providing shelter and food,” says biologist and Refauna vice president Joana Macedo. “After selecting a species, we have to find individuals, a good number of healthy ones, because the sanitary issue is very delicate. If only few are available, then it’s not worth it because the chances of success are very small.”
Red-rumped agoutis are already a success story. More than 60 are estimated to live in the park, including fourth-generation descendants of the first ones released in 2009.
In 2015, brown howler monkeys were reintroduced. Dispersers of large seeds, these primates with powerful vocalizations that can be heard kilometers away had not been observed in the Tijuca forests for more than a century.
Initially, five howlers were to be released, but one was expelled by the group’s alpha male while they were still at the Rio de Janeiro Primatology Center. This left two males and two females, but because they were animals seized from captivity by law enforcement, some were deemed not ready for reintroduction back into the wild.
“We had howlers walking on the ground and looking for picnic areas inside the park,” Macedo says. And new training didn’t help. “Animals are very complex. Some adapt quickly, others take longer, and there are those that will never adapt. Fortunately, one of the howler couples that remained in the forest adapted extremely well and produced offspring.”
The challenges didn’t stop there. the howlers also faced a severe yellow fever outbreak starting at the end of 2016. Because of the epidemic, which killed thousands of wild primates across several regions of Brazil, Refauna didn’t make any more reintroductions until a vaccine was developed specifically for the animals and they could be immunized. Only after that were more howlers released.
In January 2026, the howlers and agoutis were joined by blue‑and‑yellow macaws. On Jan. 7, female macaws Fernanda, Fátima and Sueli began. It was the culmination of years of planning, with the selection of nondomesticated birds and seven months of intense training and preparation, including exercises to strengthen flight muscles. “It’s incredibly beautiful to see a macaw flying over the city. It’s a stunning, colorful animal that screams loudly,” Macedo says.
However, as further evidence that rewilding efforts require great determination and persistence, Sueli, Fátima and Fernanda had to be recaptured. People living near the park reported seeing them (each bird had a numbered ID neck tag) as they strayed farther away from the acclimatization enclosure.
