Brazil takes pioneering action — and a vaccine — to rewild howler monkeys

  • Brown howler monkeys (Alouatta guariba), endemic to the Atlantic Forest in Brazil and Argentina, became one of the 25 most threatened primate species following a yellow fever outbreak in late 2016.
  • In response, Brazilian government agencies and other conservation organizations launched a nationwide population management plan, the first of its kind in the country, focused on coordinating captive facilities with experts who could relocate animals to areas where populations have vanished or declined.
  • Nationwide management of howler monkeys was made possible by the adaptation of a vaccine — originally developed for humans — against the yellow fever virus.
  • Howler reintroduction initiatives in Brazil have already begun showing signs of success.

It was Hope who first dared to approach Juvenal. They had been on the opposing sides of a quarrel ever since Hope’s family wrapped their tails around the branches of that 10,000-acre urban forest in the heart of Rio de Janeiro. The arrival of her clan contested the eight-year rule of Juvenal’s group in Tijuca National Park.

Brown howler monkeys (Alouatta guariba) are, by primate standards, peaceful denizens of the Atlantic Forest — gentle leaf-eaters that also enjoy the occasional fruit. But that doesn’t put them above skirmishing for territory. Around two weeks ago, Juvenal and another male had attempted to attack the leader of Hope’s group, Max. The howlers, true to their name, kept exchanging threatening howls since the incident.

On that day, though, Hope seemed to reach out for peace. She touched Juvenal, and — to the delight of the researchers spectating the scene — he accepted the gesture. Just like that, the howler groups appeared to reach an understanding.

The human part of the drama, however, is more extensive.

Just a few years back, brown howlers were considered a vulnerable species by the Chico Mendes Institute for Biodiversity Conservation (ICMBio) — the Brazilian agency responsible for managing protected areas and biodiversity — but of least concern according to the IUCN Red List. Everything changed in December 2016, when brown howler numbers started to plummet; they became rare in many forests and disappeared altogether from others. The species is now among the 25 most threatened primates on the planet.

The reintroduction of howlers — including Juvenal, Hope and Max — conducted by the NGO Refauna in Tijuca is part of a monumental effort to rescue the species. Wildlife managers from all over Brazil have organized the nation’s first population management program — an initiative that connects captive facilities with experts able to release them back into forests. It’s an ambitious project that was only made possible by the production of a vaccine against the monster that killed howlers by the thousands: yellow fever.

From ‘bucketloads’ to almost extinct

When Zelinda Hirano, professor at the University of Blumenau, started working with howlers in 1991, they weren’t endangered.

“People here used to say we had a bucketload of monkeys,” she recalls. “They were everywhere. I worked in this beautiful forest area with 59 howlers, and I worked with them for 32 years. Then came the yellow fever outbreak. Not a single one survived.”

Hirano is the creator of Project Bugio — the Brazilian name for howlers — in the state of Santa Catarina. She didn’t start out as a conservationist. Originally from São Paulo, she began her career as a medical biochemist. After getting married, she moved to the small city of Indaial, where she kept hearing these loud, guttural howls coming from the woods around town.

“I would ask people ‘what is this animal?’ and ‘why does it yell so much?’” she remembers.

One morning, she decided to follow those strange sounds up a nearby hill. The call brought her face-to-face with a “fabulous reddish thing!” she recalls. Although dubbed “brown howlers,” their coloration actually ranges from brown to dark red.

Few Brazilian researchers paid any attention to brown howler monkeys when Hirano started working with them. Back then, the howler’s biggest problems were habitat destruction, fragmentation and the occasional bump against the human world. Constantly on forest borders, howlers are vulnerable to all sorts of mishaps, such as being hit by vehicles, electrocuted in power lines or attacked by dogs.

In 2008, however, a serious threat reared its head in Rio Grande do Sul, Brazil’s southernmost state. Yellow fever is a viral hemorrhagic disease transmitted by mosquitoes of the Aedes genus. It can be fatal for humans, but it is absolutely devastating for many nonhuman primates. Especially howlers.

The 2008-09 epizootic — as animal epidemics are called — killed thousands of brown howlers. According to an ICMBio report, researchers and officials cataloged 1,183 dead howlers, though there were probably many more. Some populations, such as in the Santa Maria region, declined by 75%, and the species completely disappeared from half its forest fragments.

But it wasn’t until 2016 that another yellow fever epizootic would spread across the species’ entire range. While it is difficult to assess the full extent of this second outbreak — there were simply not enough data on howler populations prior to the outbreak — the few places with data paint a grim picture. The private reserve Feliciano Miguel Abdala in Caratinga, Minas Gerais, for example, estimated a population decline of 86.6%.

“The forest became silent,” Hirano says. “It is the saddest thing to hear the human population tell you that they can’t hear a single howler anymore.” She says the upcoming ICMBio assessment will classify the species as endangered.

A coordinated plan

The duty of ICMBio goes beyond providing risk assessments. The agency has put forward guidelines for many endangered species in Brazil, called national action plans for the conservation of endangered species (PANs). Atlantic Forest primates have their own PAN, and within it hid the proverbial ace for potential howler recovery.

As conservationists met to discuss the howler’s plight, they turned to the idea of implementing a nationwide population management program.

“[This] is considered a last resort,” says Rafael Rossato of the National Center for Research and Conservation of Brazilian Primates (CPB). The possibility of large-scale population management is part of the PAN, but it had never been done before.

The program would connect captive facilities across the country with experts capable of relocating the animals to areas where populations had vanished or declined.

Still, as Rossato notes, such close management “carries a series of risks and a chance of failure.”

To justify such risks, a species has to face the prospect of severe reduction or extinction within 30 years. In 2022, a panel of experts, including ICMbio representatives, found that howlers fulfilled that requirement.

Launched in 2023, the Alouatta guariba population management program is the first of its kind in Brazil. Supervised by a committee coordinated by Hirano, it includes representatives from CPB, wildlife managers, veterinarians, geneticists and representatives of all eight Brazilian states where the species occurs.

Brown howlers are a genetically diverse species — they even used to be split into two subspecies. To respect regional genetic differences, the program divided the howler’s range into five distinct management areas.

The far south management area also includes the single brown howler population in Argentina, where only 20-50 animals are left. The program plans to relocate monkeys from Rio Grande do Sul and Santa Catarina to reinforce this population.

But before conservationists could attempt any translocations, animals had to first be protected against yellow fever.

“The virus persists in forests [in southern Brazil], occasionally killing animals, but it hasn’t reached the same proportions as before. It would need a large vulnerable population to become an epidemic again, and we lost too many primates for that,” says Dr. Silvia Bahadian, a veterinarian at the Rio de Janeiro Primatology Center (CPRJ).

In 2018, Bahadian, together with Dr. Alcides Pissinatti from CPRJ and Dr. Marcos Freire from Fundação Oswaldo Cruz, started to research a way to adapt the yellow fever vaccine — developed for human use — to howler monkeys.

In July 2022, Hirano herself coordinated one of the tests of this adaptation on living howlers. All of the 77 vaccinated animals living in captivity in Santa Catarina successfully developed antibodies against the virus.

Today, the program’s protocol determines that every brown howler must be vaccinated before translocation.

Left: Dr. Silvia Bahadian vaccinating a monkey against the yellow fever virus at CPRJ. Image courtesy of Marcelo Rheingantz. Right: Vaccination of a howler monkey in Santa Catarina, in 2022. Image courtesy of Projeto Bugio/CEPESBI.

In the battlegrounds

In September 2015, before the second yellow fever outbreak, the NGO Refauna was already busy reintroducing brown howlers to Tijuca — the first howlers the urban forest had seen in more than a century. Refauna had already brought red-rumped agoutis (Dasyprocta leporina) back to the park, but the primates were proving a bigger challenge than their rodent predecessors.

Some of the monkeys came from captivity or semi-domesticated backgrounds and ended up having to be moved back to captivity after seeking constant contact with visitors. The researchers also had problems with tracking equipment, which would malfunction or, in one case, wound an animal’s leg, leading to a costly veterinary intervention.

“We ended up with an initial population based solely on one reproductive couple: Kala and Juvenal,” says Dr. Marcelo Rheingantz, Refauna’s executive director.

In 2016, Kala would birth and raise her first baby, a sign that even with problems, Refauna’s efforts were paying off. But a family of three is far from a reliable seed for a sustainable population.

Refauna’s plan was always to gather and release multiple groups. The yellow fever outbreak in 2016, however, forced the initiative into a pause.

Luckily, Kala and Juvenal did more than just survive the following years. Tucked into the urban matrix of Rio de Janeiro, Tijuca dodged the outbreak, and what started as a couple and a baby grew to six individuals.

In 2023, conservationists finally brought more howlers to the park, including Max and Hope, freshly vaccinated after their stay in CPRJ. They were first placed in an acclimation enclosure — a large wire cage in the forest.

“We use the enclosure to accustom the animals to the forest environment,” says Matheus Sette e Camara, coordinator of the howler reintroduction. “They needed to get used to the new smells, sounds and to other animals — including the other howlers.”

Even with prior experience, the second relocation proved complicated. This time, the team abandoned radio tracking equipment and settled for finding howlers the old fashion way: with patience and binoculars.

The new group also suffered two losses before their release into the wild. One infant died from pneumonia during the acclimation period, and a young female was returned to CPRJ after being ostracized from the group just a week before the enclosure doors opened.

Conservationists finally released the remaining six howlers in January. A moment of celebration for everyone involved in the project — except for Juvenal’s crowd.

“They came across Juvenal’s group outside the enclosure, and everyone seemed surprised and cautious at first,” Camara says. “But three days later, Juvenal, along with another male, silently approached the new group to attack Max. Max was forced to run, leaving the females to hide back in the acclimation enclosure, which was still open at the time.”

The researchers would follow these altercations for a couple weeks, until Hope’s peaceful gesture seemed to mark the end of hostilities. Still, risks remained. Mel, another female, fell from a tree and broke her arm. The team removed her and her infant from the forest. Today, four animals compose Tijuca’s second brown howler group.

Despite all the challenges, howlers have lived in the park for nearly a decade now, and Tijuca’s experience has helped instruct new initiatives.

In Florianópolis, capital of Santa Catarina, Dr. Vanessa Kanaan — technical director of the Wild Space Institute — is conducting another howler reintroduction. She has already released three groups in Rio Vermelho State Park and will soon translocate more howlers to another location: Lagoa do Peri. A small population in Serra da Cantareira, in São Paulo state, is also receiving new howlers.

A hope for the future

“It is very rewarding to see an animal reintroduced,” Bahadian says, adding that even the best captive circumstances “can’t come close to offering howlers what should be their normal life, with tall branches and big trees.”

Reintroductions — this defiant conservation practice that integrates so many different skills — brings hope for more than individual animals. It can also help restore whole ecosystems.

“[This is] an opportunity to unite the captivity management experience — and the possibility of giving a destination to rescued animals — with the necessity of restoring wild populations which have been reduced or extinct,” Rheingantz says.

Possamai, C. B., Rodrigues de Melo, F., Mendes, S. L., & Strier, K. B. (2022). Demographic changes in an Atlantic Forest primate community following a yellow fever outbreak. American Journal of Primatology, 84(9). doi:10.1002/ajp.23425

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