On a rare sunny morning in the Amazon’s high rainy season, a two-story houseboat retrofitted as a floating research station docks on the shore of Deus é Pai. The small river community sits on the Eiru River in Brazil’s Upper Jurua watershed, near the Peruvian border. A smaller boat pulls up behind it, carrying 8,000 liters (2,100 gallons) of diesel fuel and the support crew needed to power a three-month expedition.

Villagers peer out through open windows and watch as three researchers hop off the boat and struggle up the steep, muddy bank. They shake hands with a mother and her children sitting on a porch, and introduce themselves: Felipe Ennes, a barefoot PhD student with a thick graying beard who studies bald-headed uakari monkeys; Lísley Lemos, flip-flop clad researcher who studies hunting pressure on howler monkeys; and Laura Marsh, the expedition leader and an expert on saki monkeys.

Lemos sweeps her arm toward the fifteen other people standing on the boats and explains, in Portuguese, that they are part of a scientific mission, dubbed Houseboat Amazon. The team—from the U.S., Brazil, Mexico, Colombia and the U.K.—includes conservation biologists, veterinarians, drone operators, and photographers, plus a boat captain, cooks, and mechanics. Lemos asks the locals for permission to park the boat along the shore, take photographs, and fly drones overhead. “We’re looking for a missing saki monkey with golden arms and legs,” she explains.

Forty kilometers from the isolated city of Eirunepé and on the border of the Kulina Indigenous Reserve, Deus é Pai is the first of many communities where the team will hire guides to help search for the “missing” primate: the bald-faced Vanzolini saki monkey (Pithecia vanzollinii). Over the next three months, they will explore four rivers in the Juruá River’s 43,000-square-mile watershed—where they think, based on historical records and other factors, the monkey might still live.

The Juruá River is one of the longest and most sinuous of the Amazon’s tributaries. From its headwaters near the Serra do Divisor National Park on the border of Peru, it snakes through dense jungle for roughly 3,200 kilometers (2,000 miles) until it merges with the Rio Salimões farther northeast. Since biodiversity naturally increases from east to west in the Amazon, the Juruá’s location deep in the west sets it up to be among the world’s top biodiversity hotspots.

Each year, this landscape transforms into the most extensive system of riverine flooded forests on Earth, known here as várzea. With the onset of the rainy season, sediment-loaded water from the Andes pulses into the low-lying plains. In the forests, that water mixes with the black-water swamps and fills up countless side channels. At high water, the forests flood to depths up to 50 meters. This not only confines the forest’s animals to the canopy, it brings the scientists closer to them, making even the forest’s most elusive creatures easier to spot.

Leaving on foot or in canoes from the houseboat each morning, the expedition team will survey the region, recording species of mammals and primates and their distribution across the watershed—something that has never been done before in this area. Their goal: to determine which species live where, how they’re faring, and what, if anything, should be done to protect them.

While scientists have identified 16 species of saki monkey endemic to South America, what they know about these animals is generally limited to what can be gleaned from the most fleeting of glimpses. Saki monkeys stand about 3 feet tall, smaller than howler and spider monkeys, but larger than the titis and tamarins that often share the same forests.

Sakis use their long fluffy tails, half again the length of their body, to help steer as they leap breathtaking distances through the treetops—earning them the moniker of macacos que voo, or monkeys that fly. When threatened by a predator—or simply to dry off after a downpour—sakis puff up their long hair.

Sakis are relatively docile, and quiet, in the wild. Their soft clicks, chirps, and cooing noises are barely audible above the cacophony of other forest sounds. Unlike the bolder spider monkeys that tend to scream and throw sticks at a threat, sakis are more likely to retreat or hide.

The Vanzolini saki, named after the Brazilian scientist and musician Paulo Emilio Vanzolini, is distinct: It alone among the sakis has golden arms, legs, and neck. Its face, black except for a white stripe around its dog-like snout, is framed by a shaggy mane of hair that resembles a loose toupee. Except for a bare chest, the rest of its body is covered in black-and-white speckled hair. Its shaggy form and thick tail makes it easy to recognize, even as a shadow in a distant tree.

Despite this, the last record of the Vanzolini Saki monkey along the Rio Eiru was made in 1936, when an Ecuadorian naturalist named Alfonso Olalla shot 35 specimens for museum collections. After that, science lost track of the primate. According to Marsh, the monkey has dropped off the map for over 80 years.

Finding a lost species such as this is no easy task. Locals who live in remote regions of the world—walking the trails, fishing the rivers, hunting the forests—may know its whereabouts, but that information rarely reaches scientists unless they get out into the field. Which is precisely why these conservation biologists have traveled to this remote corner of the Amazon. In order to protect a species, you first have to know where it is.

Much like bees and other pollinators, monkeys are essential to a healthy, functioning forest. As the animals move through the ecosystem, they feed on fruit, and then spread seeds in their droppings. They even distribute pollen along their path. Despite the importance of the ecological role they serve, however, primates are disappearing from their forest homes at an alarming rate. In January, a couple weeks before Marsh set off, a team of primatologists led by Anthony Rylands of Conservation International announced that three-quarters of the world’s primates are currently in decline, largely due to habitat loss and over-hunting. Several species have already vanished over the past few decades and are presumed extinct.

“This is exactly why we are doing a major survey in Brazil,” Marsh wrote on the Houseboat Amazon Facebook page before the trip began. Surveying the Juruá watershed is the first step to determining the heath of the forest and the conservation status of the species that live there. The surveys will give scientists a baseline of information vital to tracking population trends—and figuring out how to protect these animals into the future.

What they find may also help to shed light on the health of the Amazon’s forests in general. Rainforest conservation today, says Marsh, is “like the game Jenga. If you take out one species, the system is okay. Take out another one, and maybe it’s still okay. But take out that last one, and the whole thing collapses. The problem is, we don’t know which last thing that is.”

So when a monkey goes missing, scientists start to wonder: Is that the last piece? Have we now lost what’s necessary for the rainforest to function as a healthy ecosystem? For a region as vast as the Amazon, we may not know what the tipping point is until we reach it.
Back in the village of Deus é Pai, Alejandra Duarte, a researcher from Mexico, approaches a barefoot boy of about 7 years old sitting on his porch in a threadbare cotton t-shirt and blue shorts. Duarte opens a blank notebook and hands the boy a bag of colored pencils. She starts drawing the houseboat and asks him to draw his house while she chats with his mother about the family’s favorite foods. After a few minutes, Duarte pulls out laminated photographs of animals and asks the mom to pick out which ones the family hunts for food. Duarte then separates out the monkeys and asks the mom to sort them by most to least tasty. The mom places the larger spider monkeys on top, followed by howlers, capuchins and sakis towards the bottom. None of these monkeys are abundant in the forests, she tells Duarte.

A lecturer at the National School of Anthropology and History in Mexico City, Duarte says she studies humans “as one more primate in the ecosystem.” She is interested in how our use of natural resources impacts the other primates. Over the next several days, while her colleagues survey the forests for monkeys, Duarte helps villagers harvest and peel manioc, make açai, and boil pupunha (an edible palm fruit). By participating in the villagers’ daily lives, she collects data on how they use the forest’s resources. She also visits every household that has a pet monkey to collect hair for future DNA analysis, to take the animal’s body measurements, and to gather information about where the monkey came from and how much the family paid for it.

Duarte also works with the locals to make maps, which provide the team with vital information: locations of salt licks where the field team can set up camera traps, trails to survey, and, occasionally, drug trafficking activity to avoid.

Six days in, the boat has moved downriver to the next community along the Rio Eiru. Marsh and a small team follow a local guide on foot into the forest, as three other teams depart in canoes. The morning hadn’t started well. The just-hired guide showed up with a shotgun in hand, hoping to take advantage of the gig by hunting the same monkeys the scientists are trying to protect. When two guides in the previous community had shown up with shotguns, the scientists hoped it was an anomaly, but the approach now seems to be a trend. While the scientists acknowledge that hunting is how the local people feed their families, Marsh feels the need to draw a clear line.

“This can’t be a thing,” she says, with obvious frustration. “They can go and shoot everything they see with us after we leave. It’s their forest. We can’t stop them. But they can’t bring a gun when they are working with us.”

Unfortunately, it’s not unusual for hunters to target the animals that conservation biologists study. Outside of Manaus, researchers had to stop tagging and tracking endangered manatees because hunters were following them and killing the animals. In New Mexico and Arizona, scientists have started keeping the GPS and radio signals of collared Mexican wolves secret because poachers were using the signals to pinpoint the animals’ whereabouts. “It’s a problem everywhere,” says an exasperated Marsh. “This has happened in every community I’ve worked in, from Belize to Ecuador to Vietnam.”

The guide, having left his shotgun at home, now leads us down a trail faintly marked with snapped branches and saplings macheted to shin height. Shotgun shells hang on twigs to mark side trails. Heading for a salt lick to set up camera traps, the scientists keep their eyes and ears tuned for any sign of sakis.

During the first five hours of the hike, Marsh periodically bushwhacks off the trail, following the sound of fruit dropping or leaves rustling in the canopy—possible signs of monkeys. “When you’ve got everything turned on high, every little movement makes your heart skip in excitement,” she says.

So far, the team has encountered an anteater (Tamandua tetradactyla), a three-toed sloth (Bradypus variegatus), a tayra (Eira barbara), a red-headed manakin (Pipra rubrocapilla), and a black tarantula the size of the GPS in Marsh’s hand; but no monkeys.

Out of nowhere, an exceptionally loud bird exclaims whet-whew just overhead. It sounds more like a rude man’s cat-call than a bird. “That’s a screaming pia. It’s normally an indicator of undisturbed forest,” says Marsh. To her eyes—those of an experienced tropical ecologist—the forest looks healthy. The trees along the trail average two feet in diameter or more. Some rise from the ground straight as a pencil into the canopy; others prop themselves up with angled stilt roots above the seasonally flooded ground. Still others fan out in the iconic folds of buttress roots that impress every newcomer to a rainforest—and that give the tree enough support to reach heights of 130 feet or more. Small ferns and glossy pink and green plants blanket the forest floor.

For a healthy forest, though, it’s unusually quiet. Marsh picks up a handful of uneaten yellow fruit from among dozens fallen on the ground. She has walked through forests like this in national parks in Ecuador and Peru. “Those forests had monkey tornadoes,” Marsh says, meaning they contained a full slate of rainforest primates, everything from the larger howler and spider monkeys all the way down to tiny marmosets. Traveling through the forest in unison, each species typically gathers fruits from different heights in the trees and disperses seeds as it goes. “This is a pretty forest and there’s obviously plenty of fruit. So, where are all the monkeys?”

On the boat that evening, Ennes tells Marsh that their guide advised them never to walk off-trail around the community. The villagers, he said, set up gun-traps with wire triggers to shoot pacas and tapir at eye level.

Most primatologists have a story about the moment they fell in love with the creatures they study. For Marsh, it was walking the newly cut transects of Tiputini Biodiversity Station in Ecuador’s Yasuní Biosphere Reserve as a graduate student in 2001. She spent several seasons conducting the first primate surveys of the area. That generally meant staring up into the canopy, but one day she glanced down and locked eyes with a monkey sitting on a branch overhanging the trail. “I almost missed it,” she said, “and I didn’t know what it was.” Using guide books, she matched the monkey to the saki genus, but not to a species. Marsh dug through everything she could find describing sakis, only to be frustrated by vague, confusing, and repetitive publications.

She sent an informal email to a few primatologists asking for help. They suggested she straighten out the taxonomy herself. “That’s what started this journey,” she told me over coffee in Santa Fe, New Mexico, her home base.

It took Marsh ten years to sort out and update the saki monkey taxonomy—essentially a record of how many species there are, how they’re related, and where they live. She visited every specimen in museums around the world and took several trips to South America to see live animals in captivity and the wild. She described and delineated the ranges of 16 species, including five new ones, in two volumes of the journal Neotropical Primates published in 2014. She was able to find evidence of live individuals for every species except the Vanzolini. She corresponded with scientists and field researchers in the Amazon, but no one could send her photographs or evidence of the Vazolini. So Marsh decided she should try to find it.

A review of the scientific literature on the Upper Jurua watershed, the proposed home range of the Vanzolini sakis, provided very little useful information about the area. Maps showed that Cruzeiro do Sul and Eirunepé, relatively small cities at either end of the study area, were surrounded by widening swaths of deforested land and agricultural fields. Aside from that, the view via Google Earth looked promising: The forest appeared intact, with very little in the way of human settlements.

Along the banks of the Eiru River, a few small dots indicated riberinho communities, or subsistence villages, along the river banks, tucked within the solid green of vast untouched forest. Marsh and her colleagues expected the area would be much the same as what Olalla, the Ecuadorian collector, encountered in the early 1930s when he set up camp. All signs suggested the Vanzolini saki would still be thriving there. “It looked like a monkey paradise,” Marsh said. But scientists never know what they’ll find until they arrive.
On the evening of day nine, the sunset turns puffy clouds into neon cotton candy while the field team gathers on the houseboat’s lower deck for dinner. The cook, an ex-Brazilian jungle army soldier, serves us all generous portions of chicken stew from a closet-sized kitchen too small to stand up in.

The conversation turns to the day’s observations. One measurement primatologists use to determine species abundance is the amount of effort required to locate animals. Eight pairs of eyes searched the forest for 11 hours that day. They came up with a capuchin, a squirrel monkey, and a possible saki sighting, too far away to be sure. “That’s a lot of effort for not a lot of payoff,” Marsh says.

The Rio Eiru is turning out to be more disturbed by settlements and hunting than any of the scientists expected. While the forest remains relatively intact in most places, the animals are sparse. “It’s not the monkey paradise we imagined we were headed to,” says Marsh.

Over the course of a month on the Rio Eiru, the expedition team’s evening meetings mirror a much larger debate that’s taking place within conservation biology circles: whether the Amazon ecosystem is so threatened by hunting practices that it’s becoming an “empty forest.” In 1992, biologist Kent Redford, then working for the Wildlife Conservation Society, coined the term to refer to forests that otherwise had excellent habitat but were devoid of large mammals. In the absence of these animals, the forest loses critical ecological functions.

On an average day of surveying along the Eiru, four survey teams turn up a total of two to three species of monkeys. Over the course of a week, that adds up to around ten species, but in low numbers. For comparison, Marsh says, in Manu National Park in Peru, and other forests in Ecuador where hunting pressure is minimal to none, a single observer could record five species before noon.

Lemos, who works for Brazil’s Mamirauá Institute, has been sorting through 14 years of hunting data from two forest reserves in the northern state of Amazonas. There, local communities work with scientists to track hunting, fishing, and logging activities and to manage wildlife populations. Monkeys have low reproductive rates, which means hunting can have an outsized impact on them. So far, however, Lemos’s research shows that, at least for howler monkeys, the population is reproducing, or recruiting from surrounding forest, at a rate that’s keeping pace with what hunters kill.

For now, those reserves are connected to vast stretches of intact forest. In the neighboring states of Rodônia and Pará, though, where the so-called “arc of deforestation” has clear-cut the ecosystem into isolated patches, monkey populations are in steep decline. As the arc of deforestation creeps northwest, the same thing could happen here, too, says Lemos. “Howler populations are stable for now, but we can’t say anything definitive about the future,” she says. “If one variable changes, the whole ecosystem could shift.”
What Marsh couldn’t tell from looking at maps before the expedition was that hunting pressure along the Rio Eiru and in the larger Juruá watershed is far more intense than what’s allowed legally. Underneath the intact tree canopy, hidden from satellite view, the region’s animals face threats from more than local villagers looking to put food on the table—and they have for a long time.

The hunting trails that crisscross the Juruá watershed were originally blazed by rubber tappers, or seringuerios, during the first rubber boom in the late 1800s and then again when World War II created a second boom. When the market for natural rubber bottomed out, an international trade in Amazonian furs opened up, and continued for 80 years. Spotted cats were the main target, but giant otters, capybaras, caiman, deer, and peccaries were also on the list for their skins. From data collected and analyzed from commercial trade logs, researchers found that populations of animals and fish along waterways of the Amazon basin fell throughout the 20th century. Although monkeys and other mammals, such as tapir, weren’t taken for their hides, they were regularly hunted for food.

With rubber collection mostly a thing of the past, seringal communities in the Juruá watershed are dying out. The children of many rubber tappers have either left or become manioc farmers, fishermen, and loggers who hunt for subsistence and sell their goods in the city. Fish, açai, pets, fruit, and bushmeat—staples of the Amazonian diet—all are being transported from the villages to urban centers in a near constant stream of dugout canoe traffic.

Officially, hunting is illegal throughout Brazil except in traditional or indigenous communities. Since many seringal families have moved to the city but maintain ties to riberinho communities through extended family, some are operating in a gray area. “A lot of our hunting laws depend on the interpretation of the person enforcing them,” says Lemos. “What’s written in the laws is not very clear.”

In one community, we learn that illegal hunters, those with no family ties to the area, arrived from the city a few days earlier and chopped down a large tree to kill a family of howler monkeys. “That’s a lot of effort for a hunter,” says Marsh, who helps analyze hunting pressure for primate species on the IUCN Red List. “It’s indicative of the intense hunting pressure this region faces from outside the riberinho communities.”

Riberinhos’ ongoing subsistence hunting, villagers hunting to sell in city markets, city dwellers illegally hunting along the waterways—all have combined to severely reduce the density of animals in the region.

Still, as the scientists found out, saki monkeys persist in the forests. At least for now.

At 5:30 a.m. on day ten, we awake in the village of Santa Lucia, the last community we’ll survey on the Rio Eiru. It’s close to where the Olalla brothers collected Vanzolini sakis in the 1930s. The morning light fills the overcast sky as four of us lower ourselves from the houseboat into a red striped canoe. Three other survey teams set out on foot or in other canoes, with GPSs, notebooks, and cameras at the ready. We motor away from the community along a channel through the flooded forest. The now-familiar screech of squirrel monkeys is audible above the roar of our motor.

We alternate between motoring and paddling through a forest fractured and broken by human pursuits. Chainsaws roar in the distance, chickens squawk, and accordion notes of forró, Brazilian country music, dance around us from unseen settlements. We pass large farms of manioc and bananas, newly slashed and burned from the forest to feed the nearby city. A loud shot echoes through the jungle. The previous evening, we heard from the villagers that a hunting party recently killed 14 uakari monkeys.

After a couple hours in the canoe, we’ve still seen only squirrel monkeys. We decide to change course and head toward the lakes where the Olalla brothers collected the Vanzolinis. The canoe driver picks up the long rabeta motor, swings it to the back of the canoe, and we speed off.

The closer we get to the Juruá River, the more the trunks of trees fill out and fill up with bromeliads, yellow orchids, conical purple flowers, and flor-de-maracuja, the iconic red flower of the Amazon flooded forest. We hit the Juruá and the landscape opens for a couple of minutes before we duck back into a side channel. The tops of submerged cecropia trees clutter the waterway. Vines laden with purple morning glories weave the trees together in a tight tapestry. Floating meadows and blooming water hyacinth edge the channel. Fragrance molecules heavy with moisture fall from flowers in the forest canopy. Every few feet, I’m bathed in the scents intensified by the wet, sticky air: jasmine, fresh cut pink roses, persimmon.
We pull into a second channel and the driver slows the motor. High on a bare branch, a three-toed sloth scratches itself. We stop to photograph it and mark a GPS point. I shift in my seat, sore from eight-hour days on hard wooden canoe benches.

Further along, the driver cuts the motor and whispers, “Parauacu! Parauacu!” From a distance, we see the outline of a black monkey with a long fluffy tail surveying us from a branch overhanging the open water. As we approach, it stands up, walks cat-like on all fours, its limbs covered in unmistakable golden fur. Then it leaps from the channel’s edge and disappears into the dark forest. “E bem diferente, o parauacu,” says the driver, noting how distinct the Vanzolini saki’s appearance is from other monkeys we’ve seen.

“Let’s follow it,” I suggest. He quickly swings the long stem of the motor back into the boat, picks up a paddle and steers us silently into the thicket of the flooded forest. I jump when we bump a tree trunk and a tarantula falls at my feet. It scurries under my seat, where I lose sight of it. I tuck in my shirt and hope for the best as we continue up the channel.

From behind the boat, a loud huff catches my attention. A pink river dolphin (Inia geoffrensis) has followed the canoe into the forest, using its flexible neck to negotiate the maze of submerged tree trunks and branches. Two blue-and-yellow macaws (Ara ararauna) fly overhead, their long tails and loud voices unmistakable. The rainforest seems to be revealing its secrets to us all at once. I’m not sure which way to look or point my camera. Turning toward the sound of rustling branches, I see them: three Vanzolini sakis now overhead. One slinks slowly along a thick branch above the canoe and props itself in the crook of the tree to stare down at us. Another vaults from tree to tree making a wide circle around us. A smaller juvenile follows behind, working hard to keep pace.

We duck under hanging vines and push our way in between tree trunks as the paddlers oar the water to pull us forward. Following the sound of their soft clicks and chatter, we pull up under a second group of two sakis. Again the female finds a perch while the male runs off, attempting to lead us away. My heart is pounding and I hold my breath to steady my camera. I get a few good shots before the sakis bound away into forest too dense for our long canoe to follow. We high-five each other, ecstatic about our close encounter, then mark the spot on the GPS. With the sun low in the sky, the driver plops the motor in the water and we head off for the houseboat at top speed.

The next morning, the team gathers around the boat’s galley for breakfast. “In case you haven’t heard the news,” Marsh says, “we got word over the satellite last night that a graduate student from the University of São Paulo published a paper with the title, ‘Rediscovery of Vanzolini’s Bald-Faced Saki.’” The air, already thick with humidity, sags with the weight of disappointment.

A graduate student doing unrelated field work near the southern end of the Juruá watershed collected the skins and skull of a Vanzolini saki a hunter killed while he was there. Having learned of Marsh’s expedition to find the Vanzolini, the grad student rushed to publish the discovery, scooping the expedition team’s planned publication of a paper with the same title. Three weeks into a three month expedition, Marsh’s team has endured showers of ants falling from trees, stings from hornets hiding in pants, waist-deep treks through mucky marshes; they’ve hauled canoes over low-water sand bars, tumbled off logs while crossing streams, endured weeks in close quarters on a small ship with 18 people, and been covered in sandfly and mosquito bites—only to have their discovery scooped by a guy who was handed a Vanzolini by a hunter. They’re crestfallen.

After a couple of days blowing off steam and more than a few references to the infamous jostle between Charles Darwin and Alfred Wallace, they decide the expedition won’t be sabotaged by the rediscovery of a single Vanzolini. They have come to find as many sakis and mammals as possible across the monkey’s entire range. Their glory will come in ending the Vanzolini sakis’ reign as a species that scientists know almost nothing about.

Over 21 days along the Rio Eiru, Marsh and her team ultimately record 20 saki group sightings—compared to only eight groups combined for howler and spider monkeys. As the largest primate left in the Eiru forests, the Vanzolini’s saving grace may be that sakis are hard to hunt and the locals don’t love the taste of its meat.

There’s a good argument for continued boots-on-the-ground research in the Vanzolini’s home range. “Working here is like piecing together a puzzle,” Ennes tells me. “We are still figuring out what species of primates live here, and if they are different from their counterparts in other parts of the Amazon.” Marsh and Ennes are now working to confirm at least two potential new discoveries made along their route.

During three months in the Jurua watershed, the team uncovers stark differences between the influence of hunting on the Eiru and Liberdade rivers, which are closer to larger cities, and the Gregorio, which flows through a protected reserve. Duarte finds evidence of illegal wildlife trafficking and hunting on all three rivers. But along the Gregorio, they record significantly higher numbers of primates and other mammals than on the unprotected rivers. While not perfect, the reserve system seems to be working.

Now that they’ve found the Vanzolini saki, the scientists hope it can serve as a model species for conservation planning. In the Juruá watershed, they want to start a citizen science program, where local people would work alongside scientists to study wildlife and manage resources sustainably.

Marsh also hopes to someday help establish a research field station in the interior forests of the Brazilian Amazon. “We scientists go where the rivers are because that is where we can get to. Our version of reality is tainted by locations that are easy to reach. We need to find a way to get beyond rivers, past the people, and find what’s out there—before it’s lost.”