- In restinga shrub areas of the Atlantic Forest on the coast of Paraná state, researchers have carried out the first yellow fever vaccinations of black-faced lion tamarins.
- The initiative took place in July 2026 and aims to protect the species, known locally as the “caissara lion tamarin,” from disease outbreaks that could further reduce its small wild population.
- The primate is listed as endangered on the IUCN Red List, with a population estimated at fewer than 400 individuals in the wild and none in captivity, increasing the risk of sudden extinction.
On a Wednesday in July, deep in a coastal restinga area of Brazil’s Paraná state, muffled footsteps broke through the chirping of manakins. Three field researchers from the conservation NGO Society for Wildlife Research and Environmental Education (SPVS) moved carefully along a trail, each carrying a small cage covered in a dark cloth.
Each cage held a black-faced lion tamarin, known locally as the caissara lion tamarin (Leontopithecus caissara). Captured safely from the canopy, the three small primates — two males and one female — were carried to a makeshift tent in a forest clearing, where they would make history. Within hours, they would become the first of their species to be vaccinated against yellow fever.
A team of researchers and veterinarians was waiting, quickly transferring each tamarin into a cotton bag to be weighed, starting a race against the clock to protect one of Brazil’s most threatened mammals.
L. caissara is one of four lion tamarin species endemic to the Atlantic Forest; it ranks among the world’s rarest primates. First found and described by science in 1990, its wild population was estimated at just 392 individuals in a 2011 study. Most tamarins are confined to coastal restinga areas along the São Paulo-Paraná state border.
The species’ already restricted range has also been fragmented for decades. Part of the problems started with the Varadouro Canal, a 6-kilometer (3.7-mile)-long, 50-meter (164-feet)-wide artificial waterway opened in the early 1950s to facilitate interstate trade and cargo shipping. By severing a peninsula from the mainland, the canal created an artificial territory that became Superagui Island.
Connecting the bays of Cananéia, in São Paulo, and Paranaguá and Pinheiros, in Paraná, the waterway split the tamarin population in two. Today, isolated groups survive between the Lagamar de Cananéia State Park on the São Paulo coast and Superagui National Park on the northern coast of Paraná.
Experts say this separation cuts off natural gene flow, leaving small subpopulations vulnerable to inbreeding depression — a drop in survival and reproductive fitness caused by mating among close relatives. Over time, that isolation also undermines the species’ ability to adapt to environmental changes.
There is some controversy over the best name for L. caissara, said SPVS veterinarian Paula Mangini, who took part in the unprecedented July 2026 vaccination effort. “Black-faced lion tamarin” is the most widely used term, mirroring the physical descriptions given to its close relatives, the golden lion tamarin (Leontopithecus rosalia), golden-headed lion tamarin (Leontopithecus chrysomelas), and black lion tamarin (Leontopithecus chrysopygus). Yet popular culture and several researchers favor the term “caissara lion tamarin” to forge an explicit link between the primate and its habitat.
The name caissara comes from “caiçara,” an Indigenous term used to describe the traditional coastal communities of southern and southeastern Brazil, whose livelihoods and culture are closely tied to the sea and Atlantic Forest.
“It’s the same thing with the golden-headed lion tamarin, which is often called Bahian lion tamarin,” [a reference to Bahia state], Mangini told Mongabay. “I’m in favor of identifying it with the area; I think it helps with conservation as you make the population identify with the animal. I vote for ‘caissara lion tamarin.’ And it’s in the scientific name, right? It is Leontopithecus caissara.”
The primate is listed as endangered on the International Union for Conservation of Nature (IUCN) Red List. Still based on the 2011 study, the IUCN estimates the number of adult individuals at around 250.
Unlike other lion tamarins, Mangini noted that L. caissara has no insurance population in a controlled environment, whether in primatology centers or zoos. If disease strikes its small wild population, the species could be suddenly wiped out with no captive individuals to be included in repopulation projects.
Yellow fever poses one of the most serious threats to Brazil’s primates as Mongabay reported in a recent story. And while the lack of continuous monitoring of the black-faced species on Superagui Island — home to Superagui National Park, the site of the initial vaccinations — has prevented researchers from finding evidence of black-faced lion tamarins that died during recent disease outbreaks, science points to several risks.
“We know that yellow fever has passed through Superagui Island [in recent years] because howler monkeys vanished. Howler monkeys are extremely sensitive to yellow fever, with huge casualties [numbers of confirmed fatal cases],” Mangini said.
Vaccination aims to conserve the species
She explains that this is how the idea of vaccinating black-faced lion tamarins emerged as part of a mission to provide the animals with a line of biological defense. According to Mangini, in addition to being the rarest of lion tamarins, L. caissara is also the least studied.
After researchers Vanessa Persson and Maria Lúcia Lorini formally described the species in 1990, confirming rumors of a dark-faced lion tamarin in Superagui’s coastal forests, initial studies examined its diet, behavior and health. Fieldwork for population sampling began in the early 2000s.
But then monitoring stalled, experts say. While working to recover the red-tailed amazon (Amazona brasiliensis) in the same state border area in the 2010s, SPVS researchers realized that no one was actively studying or protecting the rare mammal.
In 2018, SPVS stepped in to coordinate the black-faced lion tamarin conservation program. The plan combined scientific research and population monitoring with habitat protection, conservation unit management, and community environmental education. Its priority was to reduce the risks of future epidemics.
From 2021 to 2023, SPVS led a series of multi-institutional workshops involving researchers from the Chico Mendes Institute for Biodiversity Conservation (ICMBio), the Oswaldo Cruz Foundation (Fiocruz), and the Immunobiological Institute of Technology (Bio-Manguinhos) to assess the health threats facing the species.
The assessment discussions led to a technical agreement between researchers and conservation organizations: Under guidelines set at an SPVS planning meeting in April 2025, every black-faced lion tamarin captured during field research must receive yellow fever vaccine. That protocol paved the way for Mangini and her team to vaccinate the three tamarins in July 2026.
“There is a lot of uncertainty about this species. But if the [scientific] community can make sure that [the animal] is vaccinated against yellow fever, the risk to its survival decreases,” Mangini said.
The vaccination day was a blend of stress, quick hospital procedures and strong emotion. As soon as the three tamarins arrived at the field clinic, Mangini said, the team found itself racing against “two clocks.”
The first countdown was the anesthesia, which kept the animals asleep for up to 50 minutes. During that brief window, the team had to collect blood, hair, and fecal samples, take body measurements, insert microchips, attach radio-tracking collars, and then administer the vaccines.
In addition to the narrow window for carrying out all the procedures on each animal, the yellow fever vaccine itself also faced a countdown.
Adapted from human yellow fever vaccines and produced by Bio-Manguinhos, the immunizer remains effective for only three hours once it is diluted. All three tamarins had to be injected within that interval.
The most stressful step was fitting the radio collars needed to track the vaccinated group. Made from flexible, silicone-coated steel wire, collars must be assembled on the spot to match each animal’s neck.
“It seems easy when we talk about it, but when you have to actually do it, it’s not. The cable doesn’t go through, it comes back; then the other wire comes back …,” Mangini said. Placing the collars took so long that the first two tamarins had to be given extra anesthetic to keep them under while the team finished.
Since they received their doses, the three vaccinated tamarins have been monitored daily through radio collars and camera traps, showing no side effects.
Researchers bank on environmental awareness
In late August, researchers met with local residents to present the results of the vaccination project and answer questions.
SPVS biologist Adrianne Pires leads the project’s community outreach work in Superagui, in Guaraqueçaba municipality. Born and raised on the island, Pires runs workshops, talks and educational activities with schools and community members, working to foster local pride in the black-faced lion tamarin as a shared symbol of identity, memory and cultural heritage.
Pires said residents have come to realize that “conservation and development can indeed go hand in hand, as long as there is dialogue with the community and respect for the territory.”
“The idea is to seek development in a way that values local culture, people and nature. Being from the territory, I see that conserving does not mean preventing development but rather finding ways to develop without losing what makes this place so special,” she said.
Elenise Sipinski is technical coordinator of the black-faced lion tamarin conservation program. She argues that safeguarding the wild population could also turn the primate into an asset for Superagui Island, drawing wildlife watchers, scientists and tourists interested in sustainable activities.
“The tamarin will only be there if this area remains well preserved and residents keep this caring perspective. In return, this ecological asset can enhance tourism as well as the boat, inn and craft services that are typical of the region,” she said.
The next step involved recapturing the vaccinated tamarins to collect blood samples and verify whether the vaccine was effective. The team is still awaiting laboratory results, but they told Mongabay that the recapture and subsequent release went off without a hitch.
Pending those results, Sipinski said, they plan to expand vaccination to additional groups of tamarins on Superagui and across the state line, on the São Paulo coast.
Over the long-term, researchers hope to establish a captive insurance population similar to those already maintained for other lion tamarin species. In the meantime, Sipinski said, the vaccination drive is providing vital biological data that will help biologists design safe control strategies.
“It’s a very sensitive process, and we have to work with the community so that they know we are not removing animals from the wild to put them in a zoo. We must identify which individuals are suitable so that each one is placed in the ideal spot. We have to bring nature to the place that feels as close as possible to its environment,” Sipinski said.
Recalling the moment the animals were returned to the wild, Mangini grew emotional. One field work scene stayed with her: As the tamarins shook off the last effects of the sedative, they grabbed branches and went right back into the forest.
Inside the tent, no one could hold back tears.
“It gives us more hope. We know that [vaccinating only] three animals means nothing for [the entire population of] a species. But it’s the first step.”
Banner image: A black-faced lion tamarin photographed during the yellow fever vaccination project conducted by SPVS. Image by Gabriel Marchi.
This story was reported by Mongabay’s Brasil team and first published here on Sept. 17, 2026.
Researchers find ‘priority zones’ for yellow fever vaccination of golden lion tamarins
Citation:
Nascimento, A. T. A., Prado, F., Valladares-Padua, C. B., … Júnior, P. M. (2011). Population density of black-faced lion tamarin (Leontopithecus caissara). Neotropical Primates, 18(1), 17–21. doi:10.1896/044.018.0103
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