Article body copy
An update from December 2022 is included at the end of the story.
Choosing a good place to sleep should be simple. But along the Kinabatangan River in Borneo, the monkeys swinging through the trees lining the water’s edge are picky and rambunctious as they search for a spot that’s just right. Their lives can depend on it.
Monkeys, in general, prefer trees without a lot of cover so they can spy predators, such as clouded leopards. Some spots are safer than others—like the slender ends of long branches where a monkey is out of reach of most predators and could be shaken awake by a stealthy cat approaching. But there’s a downside—a monkey that snags a perch on a tapering limb hanging over the river could tumble into the water if the branch snaps.
“We did a GPS survey on the crocodiles; they hunt below trees,” says Benoît Goossens. We’re in a boat with several students, learning how to conduct a primate survey. Goossens, an ecologist and the boat’s pilot, points to a macaque settling onto the branch of a dead tree overhanging the river. “That one’s ripe for falling in and getting eaten by a croc.” A monkey squabble, though, is usually the reason for an unintended dive.
To spot monkeys, Goossens’s first instruction is: look for moving branches. Counting them is fairly straightforward too—at least from a boat on the river—since the monkeys are nicely silhouetted by the setting sun. Identifying individual species, however, takes practice.
Silvered langurs, for example, have triangular heads with tufts of hair sticking out the sides and growing up in a spiky tuft. They have long tails and very orange offspring. They’re the placid onlookers in this monkey world. They avoid the shenanigans in the nearby trees, where gray-brown long-tailed macaques bounce about like young children putting off the moment they wink out at night—jumping on beds, scurrying about the room, monkeying around until their little bodies give out.
Just now, some macaques are leaping into a tree occupied by the much bigger proboscis monkeys. Proboscis females weigh up to 12 kilograms (about the same as an average-sized four-year-old child) and males up to 24 kilograms (about the same as an average-sized seven-year-old child). The macaques—around three to seven or so kilograms—are sassy. Their jumps are like a pffft in the face of any being within their vicinity. As they advance, the proboscises leave, one by one. The macaques follow; the proboscises spring back to the original tree. A real-life page from Where the Wild Things Are unfolds above me as the monkeys cavort in the canopy. Back and forth, back and forth, back and forth they go in a wild monkey rumpus.
Even from a distance, the proboscis monkeys exhibit an air of forbearance for their naughty little primate cousins. The proboscises appear sangfroid. Equanimous. Unflappable. Considering today’s zeitgeist of endless outrage, such tolerance in our primate cousins is conspicuous.
Composure aside, the proboscis monkey is most famous for its looks. The female has a sweet face with an upturned nose; bright, wide-open eyes; a pudgy belly. The male is more … striking. A pronounced brow cloaks his eyes and his nose can reach an impressive 17.5 centimeters long—a smidge longer than an iPhone X—a protuberance straight out of a Roald Dahl book. Bulbous. Fleshy. Floppy. His stomach is so Dahlishly pronounced, it’s perhaps the best example ever of the word potbelly. The paunch, the hooded eyes, and the fur piled on his shoulders like loose skin add to the furry–old man look. In a treetop perch, gazing over the river, resting on his haunches, a big male wears an impassive expression but is thoroughly attentive to his surroundings, though he forgets his manners and scratches his tummy and nether regions now and again.
Proboscis monkeys are a favorite among tourists in Borneo, and to see them is special. So too the silvered langurs, listed as near threatened by conservationists, are another highlight for visitors. No one worries about losing long-tailed macaques. In the wider world, they outnumber the other two monkeys; they are a species of least concern, being their cheeky selves in habitat that stretches across Southeast Asia. Silvered langurs are found beyond Borneo, too. The proboscises, however, live only on this island in the southwest corner of the Pacific Ocean, and they are endangered.
Out of the more than 600 primate species on the planet, the proboscis monkey is one of the more puzzling. No known fossilized remains exist, and historical data is scant. Their social structure is not fully understood. Yet despite some persistent knowledge gaps, they have become far less mysterious than they used to be: over the past 15 or 20 years, research has turned a fuzzy image of this Old World monkey, drawn in the past century by old-school naturalist-explorers, into a more sophisticated, if still pixelated, portrait.
Today, around 1,960 proboscis monkeys range along the Kinabatangan, all the way to the ocean. They probably once inhabited the entire Bornean coast, but as humans encroach on the beaches and turn mangroves into shrimp farms, the monkey’s riverine lowland forest habitat has become more vital. The latest efforts by scientists to learn about the proboscis monkey matter in a place where, between 1980 and 2000, more wood was harvested than in Africa and the Amazon combined. To save them, scientists must probe into even the minutest details of the monkeys’ existence.
For much of the 20th century, proboscis monkeys flunked as subjects for scientific study. Aside from their shoreline night-night ritual, they’re elusive animals and kept their secrets safe from science for a long time. Early European explorers and naturalists had little to say about this odd-nosed primate, one of 10 odd-nosed monkey species scattered across Southeast Asia. In Borneo’s jungle, the proboscis was hard to see, hard to follow, hard to shoot, and was (and still is) likely to die in captivity. Around a century ago, British zoologist Charles Hose sent two live specimens back to England, but they died before reaching Sri Lanka.
Just when the monkey was down on its luck—mostly due to the slaughter of its habitat in the late 20th century—and could have used some serious scientific attention, wildlife research underwent something of a revolution. New tools—GPS tracking, and less-invasive DNA analysis, for example—allowed for a depth of understanding never imagined by early naturalists. It was as if a door to a secret society opened, and in walked all sorts of curious people, such as Goossens. He began his career learning how to study wildlife less invasively. His PhD adviser, Pierre Taberlet, pioneered noninvasive genetic sampling in the mid-1990s when studying brown bears in France’s Pyrenees mountains by sampling DNA from scat and from fur caught on strategically placed barbed wire.
Today, Goossens is the director of the Danau Girang Field Centre (DGFC), a scientific research station in the Malaysian Bornean state of Sabah (Borneo rests partly in Malaysia, partly in Indonesia, and partly in the tiny sultanate of Brunei). Nestled within the rainforest on the Kinabatangan River, the center is a joint venture of Cardiff University in Wales and the Sabah Wildlife Department. Students from near and far come to study monkeys, birds, crocodiles, pangolins, slow lorises, snakes, and plants.
Goossens launched the center in 2008, eight years after he first arrived in Borneo to study orangutans on a grant from the Darwin Initiative. “I fell in love with Sabah,” he says. Goossens has no desire to return to Belgium, his native country. From his expressive eyebrows and sturdy build, Goossens calls to mind a character from the French comic book series Asterix, minus the droopy mustache. He has Asterix’s can-do spirit: when he stumbled on a collection of empty concrete buildings in the middle of the Sabah rainforest while doing fieldwork, he saw his opportunity.
The buildings belonged to the Sabah government but were no longer in use. Goossens convinced officials to lend him the space and Cardiff University (where he was working as a postdoctoral fellow) to fund a scientific field station. Not long after opening its doors, in walked a Canadian graduate student smitten with the proboscis monkey: Danica Stark.
Over 10 years and one PhD dissertation later, Stark entered the ranks of experts who really know the proboscis monkey. I call them Team Proboscis.
Stark, hair pulled back into a ponytail, folds her tall frame into a chair in one of the center’s buildings—dining room, makeshift classroom, library, offices, and lab. We sit in the dining area next to a wall open to the elements but lined with a screen to deter animals, from nosy macaques to mosquitoes. The building, orange with burgundy trim, sits in a clearing a few meters off the ground on concrete blocks, its underside full of napping mammals: at dawn, bats scoot from the jungle back to their roosts beneath the building.
Stark’s deep-set eyes are friendly, though she exudes a Western Canadian reserve. She’d rather not talk about herself, but, on prodding, she shares her inspiration to become a primatologist: around her second year at the University of Alberta, she watched a spot aired on the Canadian Broadcasting Corporation about famous Canadians. “There was one about Biruté Galdikas, an orangutan researcher,” Stark says, and grins. Stark wasn’t interested in orangutans so much as primatology, so she dropped her plan to become a veterinarian and transferred to the University of Calgary. Stark briefly studied howler monkeys in Panama, then mona and ursine colobus monkeys in Ghana, but it was the proboscis monkey that tugged at her curiosity, and she committed to this odd-nosed monkey for her master’s degree.
“From the beginning, it was their appearance,” Stark says. The monkeys’ mystery was also a factor: they were a puzzle with a whole lot more pieces missing than other primates. They melt out of sight and into the forest every day. They gallivant among the trees in swamps, around lakes and mangroves. “You just can’t follow them in. It’s been really restricting that way,” she says.
The environment is so restricting that estimating the population is guesswork. Based on the most recent estimate over 10 years ago, around 7,500 exist in Malaysian Borneo. The bulk of the population lies in the bigger portion of Borneo—in Kalimantan, the Indonesian part of the island—and the guesstimate there is even fuzzier, numbering 20,000 to 25,000 from a survey also done over a decade ago.
Confirmation bias has undermined our understanding of the monkey: researchers assumed the proboscises prefer water because boats were the main method of human travel for most of Borneo’s history. Conveniently, they saw the monkeys in trees on ocean and river shorelines at dawn and dusk, and pushing further into the forest seemed unnecessary.
But where did the monkeys spend their days?
Trees. They block your path, catch your clothes, thrust twigs at your face, and drop leaves and seeds on your head. In a rainforest, they’re in charge, weaving and soaring, almost sentient. If you’re not paying attention, it feels as if you’ll walk forever among the trees, forever lost in a forested purgatory. Ask the researchers tramping the forest along the Kinabatangan River to name these tall beings and they give a one-word answer: dipterocarp.
The genesis of dipterocarp forests can be traced to the southern supercontinent of Gondwana that broke apart around 180 million years ago. Despite their inability to dodge chainsaws, like all trees, dipterocarps are so evolutionarily successful—there are so damn many of them—that they almost defy scientific classification. Some 240 species of dipterocarp can coexist in an area the size of a sports field; one genus people from outside the ecosystem will recognize is Ficus, the fig tree, a popular houseplant.
Visitors to the field center are not allowed to head into the forest alone. It’s easy to lose your way and bump into danger in the form of snakes or orangutans; we stick to the boardwalks and trails linking the various buildings and the dock. One morning, I finally walk into the rainforest when I tag along with Elisa Panjang, one of Goossens’s graduate students and one of Borneo’s few pangolin (scaly anteater) researchers, to check on some wildlife cameras. Walking through the jungle must be done single file. The foliage is so thick, the sun pokes through the canopy only in small patches. Dead leaves form a brown deep-pile carpet and the air is heavy with moisture. Panjang and Roslee, a research assistant, carry parangs (machetes). The tread of gumboots and swish of quick-dry clothing accompany the occasional riff of a parang clearing the way.
As we march along, I consider how much easier and more wondrous it would be to be a monkey here. To trudge is earthly. To vault from one tree to another, scamper along the canopy, and spy on the world from above would be transcendent.
Stark’s goal was to follow the proboscises’ movements, so she began placing GPS collars on monkeys in 2010, eventually tracking 10 and their extended families—about 15 individuals per collared animal. All she had to do was wait for the data to beam from satellites to cellular towers and onto her computer. Stark’s research revealed that an average troop roves about 940 meters over the course of a day within an area smaller than a square kilometer, a relatively modest spatial need, and the monkeys are not especially territorial. Troop territories overlap, even those of so-called bachelor troops, which, oddly enough, are not always solely male; they’re known to accept a female or two into their ranks.
They’re easygoing creatures. Except when it comes to where they live. Stark’s research showed that proboscis monkeys need a forest canopy; nothing else will do.
A mere 15-minute hike from the riverbank, we emerge from the trees and enter a world the proboscis never will: a domesticated and regimented place, with row after row of stout, uniform trees devoid of understory. A palm oil plantation.
The DGFC lies within the Lower Kinabatangan Wildlife Sanctuary, a 270-square-kilometer patch of wild, a third of the size of New York City, New York. The sanctuary, however, is hemmed in by palm oil estates. In some parts, you only have to walk 300 meters from the river to enter an orderly plantation laid out in a grid like a city. Borneo has thousands of such plantations. The huge international market for palm oil is one reason the island has lost so many trees.
Panjang checks on wildlife cameras she placed in the estate to record evidence of pangolins, or, as she calls them, “walking money.” Laborers may find it difficult to resist the temptation of poaching a pangolin trundling among palm oil trees or sleeping on a branch. A pangolin is worth around US $600 on the black market. Its meat may end up on a dinner plate at a private dinner in Kota Kinabalu, Sabah’s capital, and its scales in traditional Chinese medicine concoctions. Poachers also target proboscis monkeys to sell as pets or for their meat and bezoars—concretions formed in their digestive tracts, which are incorrectly considered medicinal.
An armed poacher with great aim can take down a proboscis, yet up in the sheltered canopy they’re beyond the reach of traps or snares, and they are consummate tree huggers. Proboscis monkeys, Stark learned, spend most of their daytime hours in the trees, presumably filling their potbellies. The dipterocarp canopy is particularly generous when it comes to nourishment.
Exactly what are they eating though?
The European explorers who set down in Borneo in the 19th century were, in keeping with the times, myopic researchers, more collectors than scientists, content to describe and count. So they dissected the monkeys’ brains or weighed their testes and at times declared truths based on few observations. Over a century ago, the Field Columbian Museum in Chicago, Illinois, mounted a taxidermic proboscis in a diorama where it was shown ravaging a woodpecker’s nest and tearing the bird to pieces.
The keen early observers knew that display was nonsense. They kept notes and reported proboscises eating shoots and the fruit of mangrove trees. Yet no one knew enough to keep a proboscis monkey captive—even the basics, like the right combination of foods, eluded early captors. The monkeys’ needs are complex, like their stomachs.
A proboscis potbelly is a multichambered stomach, sometimes accounting for 25 percent of the animal’s body weight. A chewed leaf lands in the proboscis’s foregut, a chamber full of cellulose-busting bacteria. From there it goes through a tube into another chamber, then empties into the small intestine. The process takes around 40 to 50 hours.
The Kinabatangan proboscis monkeys are also, so far, the only primate known to chew their cud—they regurgitate a meal to chew it again, like cows. This finding was a bit of a science sensation in 2011 when reported by Ikki Matsuda, a Japanese researcher who, like Stark, was drawn to the mystery of the monkey.
Matsuda has studied proboscis monkeys living along the Kinabatangan River—in Sukau, a village about two hours by boat downriver from the field center—since 2005, beginning as a young graduate student at Hokkaido University in Japan. Like Stark, he found his way to the proboscis through his country’s public broadcaster, Nippon Hoso Kyokai (NHK), when he watched a nature documentary.
“The TV program is saying this monkey is living in very swampy areas and their ecology is unknown,” Matsuda says over Skype, his short hair a spiky cap on his head, his eyes framed by black glasses. Now a primatologist at Chubu University in Japan, Matsuda was in his early 20s when he first watched the monkeys on the little screen and he too was intrigued: where do they go and what do they do in the forest all day?
In 2005, Matsuda arrived in Sukau and went into the bush with a couple of research assistants and searched for a proboscis monkey troop. Matsuda and his colleagues identified and focused on a male and his harem, trailing the monkeys every day from morning until night for a month.
Finally, they habituated the group to their presence and could get close enough to regularly observe the monkeys’ daily routine in the forest. This troop had a big, dominant male, six females, and several other immature monkeys. The Japanese scientists named the monkeys after characters in the famous manga series Dragon Ball: Bejita, the dominant male, and Bulma and Chi-Chi, two of the six adult females, for example. Chi-Chi was one of Matsuda’s favorite monkeys.
She was something special, Matsuda says. Binoculars are great for studying behavior, but are not always helpful in the dense forest when trying to see exactly what the monkeys are eating. Chi-Chi was the first to allow the researchers to move closer. Eventually, the rest of the troop opened up, and Matsuda and his team could identify specific foods and which parts of the foods the monkeys preferred.
Feeding behavior, physiology, and spatial use of habitat tell researchers a lot about an animal’s needs. Given the data from the cooperative proboscis troop, Matsuda surmised that the monkeys needed 800 meters of forest on either side of the Kinabatangan River to comfortably survive. Another useful takeaway was that they like to chow down between 3:00 p.m. and 5:00 p.m., just before they hunker down for the night, giving them time to digest what turned out to be a hugely diverse diet: Chi-Chi and her family dined on 188 different species of plant. As for their fruit eating, the monkeys are primarily after the seeds, Matsuda and his team determined. The monkeys will eat both the flesh and the seeds of unripe fruit, but if they have to settle for ripe fruit, they leave the flesh and strictly go for the seeds: the sugars from ripe fruit can cause bloating and occasionally even death.
Questions answered invariably lead to more questions, and a peek at Chi-Chi and company’s eating habits led Matsuda and his colleagues to search for more detailed data—from inside the monkeys’ potbellies.
Those magical bellies are essential to dining so broadly in a dipterocarp forest. But there is a lingering suspicion that the monkeys first evolved to live in a mangrove forest.
Sonneratia trees dominate Borneo’s coastal mangroves. Their leaves are hard to digest and rich in tannin. “Proboscis monkeys are one of the only ones that can eat the stuff that’s in the mangroves,” Stark says. Their webbed fingers and toes are also handy in mangrove habitats, lending an ease to swimming and traipsing across mucky ground, where they’re less likely to be caught by a predator than in a dipterocarp forest.
But researchers also assume that as long as there’s a water source—a river, lake, or swamp—proboscis monkeys will colonize an area. Nutritionally, a dipterocarp forest, even a disturbed one, is superior. A monkey in a mangrove forest has basic sustenance: the dutiful diet of a hospital cafeteria. In contrast, a monkey in riverine forest enjoys the kind of salubrious nutrition found in a hip vegan restaurant. We know this from the monkey’s microbiome.
The microbiome is a hot topic among proboscis monkey researchers. Matsuda, Stark, and others from Team Proboscis wanted to figure out the diets that promoted the most biotic diversity in the monkeys. So they collected the yet-to-be-digested contents of proboscis monkeys’ stomachs.
Six adult proboscises from different habitats provided the pre-gastric gunk. Two monkeys spent most of their time in a riverine forest, two dwelled in mangroves, one semi-free-range monkey lived in a mangrove surrounded by palm oil plantations, and one was captive.
The monkeys with the most species-rich microbiomes, more microbial diversity, and presumably healthier foreguts? Those that lived in the forest lining the Kinabatangan River. Their microbiomes had double the species of the other four microbiomes. The free-ranging mangrove dwellers had slightly more species richness than the other two monkeys, but mostly ate only Sonneratia caseolaris leaves and unripe fruit so had less overall microbial diversity. In contrast with the free-ranging mangrove monkeys, the captive monkey, held at a wildlife park near Kota Kinabalu, and the semi-free-range mangrove monkey ate foods with a greater variety of nutrients: sweet potatoes, ipil-ipil leaves, cucumbers, long beans, carrots, and sunflower seeds. And yet, though nutritionally sound, those two monkeys also shared microbiome species with humans, which includes potential human pathogens.
As with how Stark’s research revealed the parameters of a monkey’s home, the diet study drew a finer portrait of the proboscis and the depth of the impact of biodiverse ecosystems. Much like how converting the Amazon rainforest to agricultural lands limits the soil’s bacterial community, degrading the primate’s landscape limits the microbiota in its gut.
Expanding protected areas to include more biodiverse hotspots would be good for the proboscis monkeys. And recent research has shown the relationship is two way.
Matsuda’s time with Chi-Chi’s family raised questions about how the monkeys might be spreading seeds. Were they seed predators or seed dispersers? Did a journey through the monkeys’ complicated guts kill the seeds or were they viable when pooped out?
So, a couple of grad students, Valentine Thiry and Oriana Bhasin, collected 201 fecal samples along 21 kilometers of the Kinabatangan River. They collected in the mornings from under the sleeping sites of various proboscis troops. They brought the samples back to the field center—from 77 percent of the proboscis poop, they collected 28,452 intact seeds, mostly of the species Nauclea orientalis (or bangkal, a tree that thrives in the soil around rivers). The students germinated those seeds and control seeds that never traveled through a monkey’s gut, from both ripe and unripe fruit.
They found that seeds traveling through a proboscis’s gut were much more likely to germinate than the other seeds. Also, seeds from unripe fruit were more likely to germinate than seeds from ripe fruit. So, given their use of habitat, proboscis monkeys were providing a service: they were dispersing intact seeds far from the parent plant to suitable riverine habitats.
Instead of a one-way relationship—the dipterocarp jungle giving life to the monkeys—there is reciprocity—or mutualism, in ecological terms. The trees and the monkeys benefit each other. If you’re restoring a forest, this is useful information. Plant bangkal, for example, and the monkeys will come and spread seeds for you.
Stark published her dissertation in 2018. When I ask her what surprised her most from her decade of studying proboscis monkeys, her answer is counterintuitive, the opposite of what most of conventional wisdom says about how reliant animals that live in old-growth forests are on the old growth itself.
“They actually do quite well in a disturbed forest, if they have enough forest,” Stark says. Old growth is not essential to the monkey’s well-being. “[A disturbed forest] has a lot of really young growth, which provides a lot of food. It’s really cool how they’re supposed to be super-specialized, but they seem to be able to make it as long as there’s forest.”
By 2010, Sabah had lost almost 40 percent of its old-growth forest, including trees up to 1,000 years old. Much of the forest along the Kinabatangan River is highly disturbed, with young trees resplendent with shiny new leaves that folivores—leaf eaters—like the proboscis covet. Without monkeys dispersing seeds, forest regrowth could go in a different direction. A restored ecosystem isn’t the same as it was before, but if you have the original species around, they can help a lot. For conservationists, this is powerful knowledge. For instance, the reintroduction of wolves and beavers in Yellowstone National Park helped re-establish a diverse ecosystem. The life span of palm oil trees is about 30 years. Peak crop yield is nine to 18 years. If a plantation company hands the land back to the state, or even abandons it, the forest regrows quickly, especially with a little help, as the seed research showed.
The chances of a palm oil company willingly abandoning or handing over a plantation seem slim. Sometimes a firm nudge helps.
In 2014, Goossens noticed an estate alongside the wildlife sanctuary creeping beyond its borders. He contacted Sabah’s assistant minister of tourism at the time, Datuk Pang Yuk Ming. Pang had the boundary of the riparian reserve surveyed and found that about eight hectares—about eight sport fields—of sanctuary had been converted to palm oil. The government had an excavator rip out about 400 illegally planted palm oil trees, and a nearby community ecotourism co-op, Kopel, replanted the area, which already boasts a canopy.
There’s another bit of good news. Drone footage taken by a researcher at DGFC in 2015 led to an important discovery—and to another government action. The imagery showed that almost 50 hectares of forest had been cleared from the sanctuary shortly before the drone flights. The logging had wiped out 30 percent of the home range of one of the troops Stark had followed via GPS collar. Drone images also showed that the felling extended down to the river’s edge, despite a law requiring riparian reserves of a minimum of 20 meters alongside rivers over three meters wide. A local landowner had logged reserved land with permission from a national government program that supported the rubber industry. The federal agency hadn’t consulted with the Sabah Wildlife Department.
Following this discovery, the field center sent out a press release with the aerial footage combined with Stark’s GPS data to show and explain this habitat destruction. Locals were outraged. The next day, the Sabah State Government ordered an immediate stop to further land clearing on sensitive areas along the river. Sabah’s decision was helped along by an emotional social media appeal, aided by the fact that there was already a regulation and proboscis monkeys had a proven economic value to ecotourism.
So what about that 800 meters on either side of a river that Matsuda identified as necessary habitat? Team Proboscis has not had luck convincing Sabah to change its riparian reserve law. “That’s obviously not enough,” Goossens says. To protect and expand the habitat, to give a voice to the voiceless flora and fauna, Goossens and his colleagues produced a 10-year action plan that the Sabah State Government approved last year. A buffer of 800 meters may be unrealistic, but Team Proboscis would be happy to begin with a minimum of 100 meters. Ultimately, the scientific data provided a starting point.
Once you’ve measured, you can manage more effectively. And conservation management is rarely about managing animals or plants, it’s mostly about managing people.
The air smells like black pepper. Four-beat calls—ohwoo oo oo oo—from crow pheasants lend a rhythmic line to our chatter in the boat. We’re back on the water for another primate survey.
A bachelor group of proboscises moves quickly from tree to tree, the macaques on their tails. Big, dominant proboscis males sit where they can watch over their female charges in the lower boughs. One male sits high up in the tree, looking down, it seems, at the primates looking up from the boat. We idle and listen. To non-jungle dwellers, proboscis sounds are a mash-up of the sounds of different animals: donkey brays, cat meows, bird whistles, pig snuffles, and human baby squawks.
How wonderful to see this magnificent creature in the flesh. The first published illustration of the proboscis was created in 1775 by a German naturalist, J. C. D. von Schreber, who probably never saw a proboscis, even a dead one. He likely read a French naturalist’s description of the animal. He drew a tall, muscular, hairy, confident, manlike bipedal creature sporting a furry muffler and a long, pointed nose.
By the time a French naturalist bestowed the Latin name Nasalis larvatus, nasal mask, the locals had dubbed the monkey orang belanda, Malay for Dutchman: the potbelly and pointed nose reminding them of their white intruders. Figuring out the function of that paunch has been easy compared with understanding why the male needs so huge a honker.
Charles Hose, the English zoologist, wrote in 1893 that only proboscis monkeys of advanced age, males and females, have a big nose. Wrong. Over a century ago, naturalists argued over whether the nose was so big the male had to physically move it to eat. They figured it did. Again, wrong. “Their nose movement can be a bit hypnotizing when they’re eating,” Stark says. “It’s just flopping around.”
Of course, many a historical male naturalist explained the nose by intoning the old adage: big (insert nose, hands, or feet) = big (insert penis, dick, junk) = reproductive success. And, well, they were kind of right.
In a group of studies that took over 15 years, Matsuda and others on Team Proboscis found the best evidence yet to explain the big nose: the ladies like it. The bigger the male, the bigger the nose and the bigger the testes. And bigger testes correlates with higher sperm counts in primates. Smaller-nosed males take one look at a mega-schnozzle and they’ll stay away—they cannot compete.
The nose also calls a troop to attention. Big-nosed males can turn up the volume on their calls, making it easier to hear them in the dense rainforest. The ability to create a hullabaloo, apparently, makes a quality male.
Goossens waves to get everyone’s attention and points up toward a tree: a proboscis alarm call cuts through the chatter. The big male at the top of the tree sees danger: it’s a sound much like my friend’s cat made after it was hit by a car and couldn’t meow properly anymore, its cry catching midway in its throat. Although when Goossens imitates the cry it comes off sounding more like kahau—the local name for the monkey.
When a kahau pierces the air, female proboscises and their offspring fling themselves—their arms extend out, legs back, bellies fully stretched—out. We slide past. A glance back reveals the male eyeing us. Goossens salutes him.
Update December 2022:
Since we published this story in the spring of 2020, we’ve been checking up on our favorite monkeys and monkey researchers sharing space along the Kinabatangan River in the Malaysian Bornean state of Sabah.
New scientific findings—even if worrisome—are good. How can we manage ourselves and make the best conservation decisions unless we know how humans can upend an ecosystem? Take boats, for example. Please.
In the marine biology world, everyone knows the brum, brum, brum of boat engines disturbs a host of animals, from whales to fish. Benoît Goossens and his merry band at the Danau Girang Field Centre (DGFC) wondered about proboscis monkeys: are boats and their engines a bother? Their odd looks, endangered status, and exotic locality make the monkeys a huge ecotourism draw, and they’re easy to see at dawn and dusk, when they’re either waking up or settling down in trees along the riverbank. So, typically, tour boats motor along the shoreline. In probably the first study of how primates feel about boat noise and proximity, published this year, the DGFC researchers concluded boats can stress out proboscis monkeys. The monkeys are fine with slow boats—cruising at no more than 3.6 kilometers per hour—that stop 60 meters away. They do not like fast boats that get too close, nor do they like slow boats that get too close.
Now, the trick is translating that into ecotourism rules.
Another couple of new papers underscore just how chill these primates are, the Dalai Lamas of the rainforest. In one study, researchers recorded just under 2,000 calls of proboscis monkeys along the lower Kinabatangan River and recorded video and audio of monkeys in a nearby sanctuary. They wanted to know if proboscis monkeys vocally interject themselves into a vocal situation between other monkeys. They do.
Mostly it’s the males with those big honkers braying to soothe the worried, like when an obnoxious neighboring monkey species or a lurking predator rattles the babies, mothers, and juveniles so much they’re a chattering mess. Or braying to diffuse a confrontation—like when a couple of females in their harem shriek at each other—and avoid physical violence. It’s thoroughly human behavior, not much studied in nonhumans.
Proboscis monkeys revealed more to the DGFC researchers about their preference for peace. By correlating canine size with nose size, they found that the bigger the nose, the smaller the canines, unusual in the primate harem world. Howler monkey and hanuman langur males will fight to the death with their canines to win a harem. Proboscis monkeys honk to let other males know they’re big and scary, and if that doesn’t work, they shake a few branches. What an evolutionary win, especially since big canines make eating more of an ordeal.
And what is an ideal life other than hanging out with friends and family, sharing a good meal?