<p>Tijuca National Park, in the mountains above Rio de Janeiro, holds a distinction that most people don't know about: it is the world's largest urban rainforest. More than 3,900 hectares of Atlantic Forest — one of the world's most biodiverse and endangered ecosystems — sit within a major city, surrounded by apartment blocks, highways, and favelas. It is, in every sense, a forest that survived despite everything humans did to it.</p>
<p>Now, scientists are working to put back a piece of what was lost.</p>
<h2>The Yellow Fever Catastrophe</h2>
<p>Brown howler monkeys (<em>Alouatta guariba</em>) once ranged widely through the Atlantic Forest, their distinctive deep roars echoing through the canopy at dawn. They are crucial ecosystem engineers: enormous amounts of forest seed dispersal depend on them swallowing fruit and depositing seeds far from the parent tree in their dung.</p>
<p>Then came the yellow fever outbreaks. Monkeys are highly susceptible to yellow fever — far more so than humans — and waves of the virus struck Brazilian primate populations between 2016 and 2019 with devastating effect. In some areas, 90% of local populations were lost. Tijuca had already seen its howler monkey population reintroduced once, beginning in 2015. The fever set the programme back dramatically.</p>
<h2>The Vaccine Protocol</h2>
<p>The turning point was a breakthrough from Brazil's Fiocruz research institute: the human yellow fever vaccine, properly dosed, proved safe and immunogenic in brown howler monkeys. Scientists tested the vaccine carefully, monitoring for side effects, confirming immunity, and establishing the dosing protocols needed to protect animals before release.</p>
<p>Now, all howler monkeys destined for reintroduction or translocation in Brazil are vaccinated before going into the forest. It is a world first — a wildlife reintroduction programme with a vaccination component against a viral pathogen — and it appears to be working.</p>
<h2>Back in the Canopy</h2>
<p>Since reintroductions resumed in 2023 with vaccinated animals, the Tijuca population has been rebuilding. Groups have been established in multiple zones of the park. Researchers monitoring the animals report normal social behaviour, territorial vocalisation, and — crucially — feeding and seed dispersal activity across the forest.</p>
<p>The Atlantic Forest has lost more than 85% of its original extent. What remains is fragmented and isolated. But fragments like Tijuca, where conservation work is active and rigorous, are being used as models for what Atlantic Forest restoration can look like at scale. Bringing back the animals is as important as planting the trees.</p>
<p>"Without howler monkeys, the forest doesn't regenerate properly," explained one of the researchers involved in the project. "They're not just charismatic — they're functional. They move seeds that nothing else moves, to places nothing else reaches."</p>
<h2>Expanding the Model</h2>
<p>The vaccinated-release protocol developed in Rio de Janeiro is now being applied to reintroduction efforts in São Paulo state and Santa Catarina, with six groups planned for new release sites. Brazil's nationwide population management plan for the species now incorporates vaccination as standard procedure.</p>
<p>As the Atlantic Forest Alliance — formed in late 2025 — moves forward with large-scale restoration of river basins in São Paulo and Rio, providing habitat for expanding howler populations is part of the plan. The forest is coming back. The monkeys are coming with it.</p>
<p><em>Sources: Mongabay · Fiocruz · Current Conservation · Frontiers in Conservation Science, 2025-2026</em></p>
