This story appears in the December 2016 issue of National Geographic magazine.

“Sometimes I feel like I’ve chosen the most difficult thing in the world to study,” Cheryl Knott tells me as we sit beneath the rain forest canopy at her orangutan research station in western Borneo. The high-pitched, dental-drill sound of cicadas fills the air, at times forcing us to pause our conversation. As we talk, Knott’s associates are at work in the surrounding forest of Indonesia’s Gunung Palung National Park with GPS units and iPads, following orangutans in their daily wanderings, recording what they’re doing, what they’re eating, and how they’re interacting with others of their species.

Unlike gorillas and chimpanzees—fellow great apes that live in groups and can be followed and observed relatively easily—orangutans live mostly solitary lives. They spend nearly all their time in the treetops, they wander widely, and for the most part they inhabit rugged forest or swampy lowland that’s hard for humans to traverse. As a result, orangutans long remained among the least known of Earth’s large land animals. Only during the past 20 years or so has scientific evidence begun to outweigh speculation as a new generation of researchers has tracked the elusive apes across the islands of Borneo and Sumatra, the only places orangutans live.

For more than two decades Knott has supervised the research at Gunung Palung, looking at many aspects of orangutan life history but focusing especially on the way the availability of food affects female hormones and reproduction. “At the time we started here, no one had really worked on hormones in wild apes,” she says. “People said I was crazy.”

View Images Commanding high prices as pets on the black market, baby orangutans can be captured only by killing their protective mothers. These orphans are being raised at International Animal Rescue.

Knott’s studies have special significance because female orangutans give birth only every six to nine years. No other mammal has a longer interval between births. And there’s no telling what her research might mean for our knowledge of human fertility; we and orangutans are so similar that Knott can use standard drugstore test kits on urine from female orangutans to determine whether they’re pregnant.

Typical of many forests in southeastern Asia, the trees at Gunung Palung produce little or no fruit in most seasons. Then, every four years or so, trees of various species simultaneously bring forth massive amounts of fruit in a process called masting. The phenomenon led Knott to wonder about the connection between food abundance and orangutan reproduction.

Knott discovered that researchers could collect and preserve urine from female orangutans on filter paper so that the samples could be tested for hormones later. Her work has shown that reproductive hormones in female orangutans peak when fruit is most abundant in the forest—an adaptation to the boom-and-bust environment.

“It makes a lot of sense,” Knott says. “They’re putting on weight during these high-fruit periods, and then they live off that during the low-fruit periods. During these high-fruit periods, females are more likely to conceive.”

Tempted by the fruit of a strangler fig, a Bornean orangutan climbs 100 feet into the canopy. With males weighing as much as 200 pounds, orangutans are the world’s largest tree-dwelling animals.

It's an exciting time for Knott and other orangutan researchers, as advances in technology (including the possibility of using drones to find and follow orangutans in rugged terrain) mean that the pace of discovery, already far more rapid than it was just two decades ago, will almost certainly increase. This assumes, of course, that there will still be orangutans left to study in the forests of Borneo and Sumatra.

In the 1980s and ’90s, some conservationists predicted that orangutans would go extinct in the wild within 20 or 30 years. Fortunately that didn’t happen. Many thousands more orangutans are now known to exist than were recognized at the turn of the millennium.

This doesn’t mean that all is well in the orangutans’ world. The higher figures come thanks to improved survey methods and the discovery of previously unknown populations, not because the actual numbers have increased. In fact, the overall population of orangutans has fallen by at least 80 percent in the past 75 years. It’s indicative of the difficulty of orangutan research that scientist Erik Meijaard, who has long studied the species’ population trends, is willing to say only that between 40,000 and 100,000 live on Borneo. Conservationists on Sumatra estimate that only 14,000 survive there. Much of this loss has been driven by habitat destruction from logging and the rapid spread of vast plantations of oil palm, the fruit of which is sold to make oil used in cooking and in many food products.

View Images Rows of oil palms replace rain forest near Borneo’s Gunung Palung National Park. Vast expanses of orangutan habitat have been lost to palm oil, used for cooking, food products, and cosmetics. Composite Photograph by Tim Laman

There’s another factor at work as well. A 2013 report by several top researchers said that as many as 65,000 of the apes may have been killed on Borneo alone in recent decades. Some were killed for bush meat by people struggling to survive. Others were shot because they were raiding crops—or protecting their young. The expressive, heart-melting faces of baby orangutans make them highly valuable in the black-market pet trade, within Indonesia as well as smuggled out of Borneo or Sumatra to foreign destinations. The ferocious protectiveness of female orangutans means that the easiest way to obtain a baby is to kill the mother—a compounded tragedy that not only removes two animals from the wild but also eliminates the additional offspring the female would produce during her lifetime.

At rehabilitation centers such as International Animal Rescue near Gunung Palung, the steady influx of orphaned orangutans shows that this killing remains a serious problem. More than a thousand orangutans now live at rehab sites, and though the goal is to release as many as possible back into the forest, attempting to teach survival skills to young orangutans is challenging and unproven.

Threats to orangutans come as the recent boom in research is revealing a surprising range in their genetic makeup, physical structure, and behavior—including the beginnings of cultural development that could help us understand how we transitioned from ape to human.

For centuries, scientists considered all orangutans to belong to one species, but in the past two decades new insights have led researchers to see Bornean and Sumatran orangutans as distinct species, both of which are critically endangered. Surprisingly, researchers have found that a recently discovered population at a site called Batang Toru in western Sumatra is actually closer genetically to Bornean orangutans than to other Sumatran populations—possibly the result of differing waves of migration to the islands from mainland Asia.

View Images An 11-month-old baby copies mom at mealtime. The juvenile will stay with its mother for up to 10 years as she passes on essential survival skills. Among them: how to find the most nutritious rain forest fruits.

The Batang Toru orangutans are believed by some researchers to diverge from others enough to constitute a third species. Numbering as few as 400 individuals, they’re threatened by a proposed hydropower project that would fragment their habitat and open the area to more human intrusion, including illegal hunting.

What’s more, several populations on Borneo are now deemed to be separate subspecies, based on factors such as differing body types, vocalizations, and adaptations to the environment. The diversity of orangutans extends even further—into differences whose origins continue to resist scientific understanding.

From his perch high in the rain forest canopy of Sumatra, a big male orangutan known as Sitogos jumps to the trunk of a dead tree and, using all his 200 pounds, rocks it back and forth until it snaps at the base. At the last moment Sitogos leaps to a nearby limb, while the tree falls toward me with an enormous crash.

Orangutans do this a lot when they’re mad, and they’re very good at it. The tree couldn’t have been aimed any more accurately if it had been laser guided.

Sitogos means “the strong one” in the Batak language of northwestern Sumatra. True to his name, the big male stares down at me, shakes the branch he’s holding, and gives a guttural, bubbling call. There may be Sumatran tigers and sun bears roaming the forest floor, he seems to say, but up here in the treetops, I’m the king.

Stretching his arms to their full span of seven feet, Sitogos moves through the canopy by using his long-fingered hands and dexterous feet to clamber from branch to branch. A young female, Tiur (“optimistic”), follows his every move, approaching closely whenever he pauses. Much smaller and more delicately built, she persists in her pursuit even though he seems indifferent. They sprawl on a branch together, eating flowers and breaking off cuplike fern fronds to drink the water inside. When he leans forward against a limb, Tiur grooms his back.

Sometime in the recent past, Sitogos had undergone an astounding transformation. He’d spent years hardly larger than Tiur. Then, with testosterone flooding his body, he’d grown powerful muscles, longer hair, fleshy pads called flanges on the sides of his face, and a massive throat sac to amplify his calls.

The sybaritic scene in the forest canopy—the devoted attention of Tiur and access to her and other females for mating—is Sitogos’s reward, but his physical change has a price too. From somewhere in the distance comes the call of another male orangutan. Sitogos stands up, transfixed, and begins moving toward his challenger.

View Images A male Bornean orangutan uses a leafy branch as a makeshift umbrella. This learned behavior is an example of orangutan “culture” passed down from one generation to the next.

Take to the Trees and See What It’s Like to Be an Orangutan



The males of many species of animals undergo major physical changes as they mature, but for orangutans the process is especially intriguing. Not all males develop the massive bodies, facial flanges, and throat sacs shown by Sitogos. Many retain smaller bodies long after they reach sexual maturity, transforming years later than other individuals. Some remain undeveloped their entire lives. The mechanism behind this divergence, called bimaturism, ranks among the greatest mysteries of zoology.

In the forests of northern Sumatra, only one dominant flanged male maintains control over a local group of females. Many males in the area retain smaller bodies and don’t develop flanges, thereby avoiding the confrontations that inevitably occur when several males try to assert dominance (until they themselves can try to move into the dominant role). For the smaller males, the only chance to pass on their genes is to watch from the sidelines, out of reach of the boss, sneaking in for mating whenever possible.

In Borneo, by contrast, nearly all males develop flanges. They wander across large areas, with no one male maintaining an associated group of females. A male’s best chance at mating is to grow strong and join the competition, leading to more confrontations and injuries.

Among the Orangutans Orangutans learn most of the behaviors illustrated here from their mothers before reaching adolescence around age 12. Some of the behaviors are ubiquitous. Others occur only in certain areas, suggesting there could be regional cultures. FERNANDO G. BAPTISTA, Matthew W. CHwastyk, and RYAN WILLIAMS, NGM STAFF; MANYUN ZOU SOURCES: TIM LAMAN; CHERYL D. KNOTT, BOSTON UNIVERSITY; carel p. van sChaik, university of zurich; Serge wich, liverpool john moores university; Meredith Batian, Smithsonian National Zoo Among the Orangutans Orangutans learn most of the behaviors illustrated here from their mothers before reaching adolescence around age 12. Some of the behav- iors are ubiquitous. Others occur only in certain areas, suggesting there could be regional cultures. FERNANDO G. BAPTISTA, Matthew W. CHwastyk, and RYAN WILLIAMS, NGM STAFF; MANYUN ZOU SOURCES: TIM LAMAN; CHERYL D. KNOTT, BOSTON UNIVERSITY; carel p. van sChaik, university of zurich; Serge wich, liverpool john moores university; Meredith Batian, Smithsonian National Zoo

On a trail not far from Knott’s research station, I see evidence of these conflicts. A male orangutan named Prabu sits high in the branches of a strangler fig, occasionally peering down to reveal a fresh puncture wound on his forehead and a lower lip missing a chunk of flesh. Obviously Prabu had been in a fight, but was he the winner or the loser?

As I watch, he rises up and gives the loud series of sounds known as a long call: a complicated and thrilling medley of deep rumblings and bubbling hoots that can carry a mile through the forest. Usually males’ long calls last less than a minute, but Prabu’s continues for more than five minutes. Bloody but defiant, Prabu still proclaims his power to rival males and potential female mates alike.

Some scientists believe the dichotomy between male orangutans arose in part because of the differing geologic histories of Sumatra and Borneo. Sumatra is more fertile than Borneo, where ancient, weathered soil lacks plant nutrients, and many forests see the boom-and-bust cycles of masting fruit trees, leading to periods of low food availability. Orangutans on Sumatra don’t have to travel far to find enough food, and female density is higher. This gives males the ability to remain in a single place and develop associations. The relatively poorer environment of Borneo has created a free-for-all in which individuals roam over large areas, finding food and mating opportunities where they can.

View Images Acting on a tip, a wildlife officer confiscates a baby orangutan kept illegally as a pet. Workers at a rescue center will attempt to teach the orphan how to survive in the wild, but the training is controversial, and finding suitable forest in which to release the apes is growing increasingly difficult.

View Images Unequipped to survive in the wild after years as a pet, this male orangutan will spend his life at a rescue center. More than a thousand orangutans now live at rehab facilities.

This may explain why the development of dominant male characteristics differs between the islands. But it brings up a far more difficult question.

“How does a Sumatran male know that if he grows flanges and he’s not the boss, he’s not going to be successful at mating?” Carel van Schaik asks as we talk in his office in Switzerland, at the University of Zurich, where he and his colleagues have published dozens of scientific papers on orangutan research from both Sumatra and Borneo.

The answer to van Schaik’s question, of course, is that the male doesn’t “know,” in the human sense. “It’s not something they can learn,” van Schaik says. “There has to be a switch, the sensitivity of the switch has to be different for different populations, and it has to be somehow genetic.”

This question of how male development is triggered remains unanswered, in part because of the same challenge that faces orangutan researchers on so many fronts: Their subjects are just so difficult to study.

In addition to their physiological diversity, orangutans exhibit differences in behavior that are passed from individual to individual and generation to generation in ways that can legitimately be called cultural.

“At one of our sites we’ve heard a call used by mothers when they reassure their kid,” Maria van Noordwijk, a member of the Zurich team who studies primate maternal care, told me. “We call it the throat scrape. We had a female that we knew pretty well before she gave birth for the first time. The day after giving birth she already gave that call. It had never been heard before from her. It’s clearly something she learned from her mom.”

“Primates aren’t supposed to do vocal learning,” says Carel van Schaik. “And yet, unless you believe this is genetic, which we think we can reject, then it’s very likely that it’s cultural. What orangutans do isn’t like the human voice, but the comprehension and learning and imitating of sounds is there.”

View Images Wearing masks to shield their charges from human pathogens, rescue workers conduct daily “forest school,” teaching orphan orangutans to practice skills and natural behaviors for surviving on their own.

Researchers see more than just animals’ behavior when they watch orangutans. After all, these scientists (and you and I) took only a slightly divergent route on the great-ape evolutionary highway than did their arboreal subjects. Behind the field notes and data points is the question of what orangutans can tell us about humans.

Unlocking all the secrets contained in the brains and bodies of these great-ape relatives means preserving the entire spectrum of adaptations. “If every group is unique, it’s not good enough to say we’ll protect them at just a few spots,” Knott says. The loss of any single population brings an end to any chance to learn from its unique environmental and cultural adaptations.

I spent time in the field with Marc Ancrenaz, who since 1996 has directed an orangutan research and conservation project on the Kinabatangan River, in the Sabah region of northeastern Borneo. Here several hundred orangutans live in a narrow corridor of degraded habitat along the river, among villages that themselves are surrounded by a sea of oil palm. The patchy woodland is nothing like the “virgin rain forest” usually associated with orangutans.

“Of course we would prefer primary forest, but this is what we have,” Ancrenaz says, as we take shelter from a storm in a hut at his study site. Outside, the muddy ground is dotted with the circular footprints of Borneo pygmy elephants. “Twenty years ago science thought orangutans couldn’t survive outside primary forest. We were very surprised here. How come orangutans are in a place where they are not supposed to be?”

Ancrenaz is among several researchers who see the human-altered landscape as vital to orangutans’ survival. “I think this is the future of biodiversity,” he says.

In western Borneo, Knott has set up an organization to work with local communities to develop sustainable alternative livelihoods, reduce illegal logging and poaching, and provide conservation education in areas surrounding Gunung Palung National Park. In the same spirit, Ancrenaz has established conservation education programs in Sabah schools and communities, trying to find ways that people and nature can coexist. He partners with people living along the Kinabatangan, helping them make money from orangutans and other wildlife through ecotourism and related enterprises. His hope is that residents will become invested in the survival of animals. “Remote villages are the front line for wildlife conservation,” he says. “If we don’t incorporate local people into our plans, I think we’re going to fail.”

For orangutans to survive in their present diversity, governments and conservationists must make smart choices about where to establish preserves, how to manage them, and how to use limited resources. They must find ways for the species to coexist with humans on two islands where habitat is constantly shrinking.

“I see a lot of people trying to do conservation with their heart, with their feelings, which is fine,” Ancrenaz says. “But conservation has to be backed up with strong science. The goal of people doing research is to produce better knowledge, better understanding of orangutan ecology and genetics. The rest is actually using this knowledge to impact land use and communities. This is where conservation takes place.”

In the forests of Borneo and Sumatra, orangutan behavior determined by millions of years of evolution endures: Males challenge each other with their calls, young males wait for their chances to assert dominance, and females teach their young how to survive in the treetops. Some of the mysteries of their lives have been revealed. What else we learn will depend on the success of this teaming of science and conservation, seeking answers about the links between humans and these apes that seem so like us when we look into their eyes.

“As a scientist you’re supposed to be objective,” Knott says, as we talk at her camp deep in the Borneo rain forest. “But you’re also human, and that connection is why I’m here.”