Distribution and Habitat
Everything we know about the Tapanuli orangutan (Pongo tapanuliensis) is limited to a single population remaining on the island of Sumatra, in a region called Batang Toru, south of Lake Toba. Less than 800 Tapanuli orangutans live here, restricted to the fragmented rainforests of this region’s uplands.
Before this small population was discovered in 1997, it was believed that no orangutans lived south of Lake Toba. Researchers also assumed that these newly discovered individuals must be more Sumatran orangutans, like the ones found to the north. But in 2017, following genetic and morphological research, it became suddenly apparent that the members of this isolated population represented a new species of orangutan altogether!
The discovery of a new great ape was exciting but not all good news. With less than 800 Tapanuli orangutans remaining in the wild, researchers had discovered a new species on the very brink of extinction.
A more recent study has shown that the area Tapanuli orangutans once roamed was likely much larger than it is today. Another has shown that the remnant population’s current whereabouts were probably not part of its original range or habitat. Although they are better adapted for life at a lower altitude, the orangutans seem to have gradually retreated into the hills for protection from human activities.
Size, Weight, and Lifespan
Tapanuli orangutans are approximately the same size as other orangutan species. The average female measures a little under four feet (1.21 m) and weighs 82 pounds (37 kg).
Males have two stages of maturity. A fully developed male is significantly larger, measuring just under five feet (1.53 m) and weighing twice the average female at 165 pounds (75 kg). Before reaching this second stage, an adult male maintains a size and weight more similar to females.
Orangutans have been known to live up to 60 years in the wild.
The three species of orangutans can be hard to tell apart. They all have striking orange fur covering their torso and limbs. Their faces and bodies look uncannily like our own except for their unique proportions. Exceptionally long arms and stout legs make them look top-heavy and unbalanced. In their arboreal environment, however, they are flawless acrobats. Their powerful hands have phenomenal gripping powers, and their hooked shape makes them master brachiators. Additionally, their feet look and work a lot like hands and are used regularly like an extra set.
Orangutans are sexually dimorphic, meaning that females and males look different. Females are noticeably smaller than males. Males actually have two stages of maturity, however. It is only during a secondary stage of maturity that males develop features that very obviously distinguish them from females. Adult males who have not yet entered this secondary stage closely resemble the females in both size and appearance.
Researchers refer to developed males as flanged. Flanged males are significantly bigger than unflanged males. They also develop prominent cheek pads and large, pendulous sacks hanging under their chins. They use both of these features to produce their long and booming calls that resonate for miles through the forest. It is still not clear how or why this extreme form of bimaturism occurs.
To the trained eye, Tapanuli orangutans do exhibit some features that make them distinguishable from other species. For instance, their smaller and differently shaped skulls are originally what prompted researchers to take a closer look at their genes and led to their distinction as a unique species.
With the naked eye, Tapanuli orangutans can be distinguished by their thicker and frizzier fur. Flanged males have other characteristics that set them apart as well. They grow a mustache and beard that Sumatran and Bornean orangutans do not, and their cheek pads lay flatter and are covered with a thin layer of blonde fuzz.
Orangutans’ dietary habits vary not only from species to species but from individual to individual. They eat a large variety of things including leaves, saplings, buds, and other plant parts like tree bark. Insects are also on their menus. Fruits make up the largest percentage of any orangutan’s meals, however. Some of the fruits they eat include lychees, figs, mangos, and wild durian.
Throughout their evolution, orangutans have become master foragers. They have developed many specialized techniques to harvest, process, and make palatable plants that other creatures—even other primates that share their ranges—might avoid or ignore altogether.
Tapanuli orangutans display many unique dietary behaviors never observed in other species. For instance, they are the only species known to eat certain types of caterpillars and pinecones.
Some orangutans have been known to occasionally eat slow lorises. These small, nocturnal primates sleep during the day—easy pickings for a hungry orangutan that might come across their sleeping spots. So far, this behavior has only been observed among Sumatran orangutans. Will future research reveal that their Tapanuli neighbors eat slow lorises as well?
Behavior and Lifestyle
Tapanuli orangutans are arboreal. Long and powerful arms are perfect for pulling their large great-ape bodies through the trees. Both their thumbs and big toes are opposable and just the right size to give them a firm grasp without impeding their potential to swing under branches as needed. Their legs are significantly shorter than their arms, and their feet actually act like another set of hands, holding them steady as they reach out to pluck a fruit. This ability makes them great multitaskers.
While their bodies are adapted for life among the branches, orangutans are not nimble. They may jump and leap if need be but, because of their size, most of their acrobatics are limited to slow, thoughtful movements that show off their exceptional dexterity and poise. Indeed, it’s not uncommon to catch orangutans in all sorts of peculiar poses as they make their way through the canopy or gather food. Additionally, they have fully rotating hips, which give them an unprecedented range of motion. As if that wasn’t enough, their shallow hip joints allow them to bend their legs a full 90 degrees. With this ability, orangutans can hang by a hand and foot combination while they gather fruit using their two free limbs.
Orangutans hardly ever venture to the ground. Males do so more often than females but typically only to overcome a gap in the trees. Only Bornean orangutans, both males and females, are known to spend a significant amount of time traveling on the ground—a behavior perhaps explained by the lack of ground predators in Borneo or the notion that it could help them conserve energy. Tapanuli orangutans prefer to keep to the canopy.
Orangutans are resourceful and fantastic problem-solvers. They not only construct nests to sleep in, but use tools. A small branch becomes the perfect toothpick or ear swab. A slightly larger one acts as a back scratcher. When a fruit-laden branch is far away, a long stick can help bring it within reach.
Sometimes, tools help orangutans solve more complex problems. A leafy branch held in the air at just the right angle gives a hot orangutan some shade from the sun or shelter from the rain. One with less foliage might be a means of fishing tasty ants from their nests. An orangutan might even bend or break a branch in a certain way in order to make it better fit the task.
Orangutans are great at coming up with unique solutions to problems as they arise. Presented with a brand new situation, an orangutan uses his or her prior experiences to come up with a suitable solution. This means two orangutans living in the same general area can exhibit completely different behaviors.
For instance, some Bornean orangutans are known to use leaves as napkins. But not all Bornean orangutans do that. Meanwhile, Sumatran orangutans use leaves to safely handle spiny fruits or thorny branches like they are gloves. Some lay leaves down on spiny tree branches to create more comfortable places to sit. Others probably use leaves in completely unique ways that researchers have yet to observe!
While individual orangutans find their own solutions to life’s problems, the most useful behaviors tend to spread through a population. This ability to transfer knowledge from one orangutan to another forms the basis of culture. It means that two groups living relatively near each other can have different ways of dealing with the same problem. For instance, all orangutans build nests—but details like preferred construction materials, their building processes, and favorable position in a tree vary widely from region to region. Sumatran orangutans are even known to build a second nest around midday where they take an afternoon nap—a behavior Bornean orangutans do not do. Is this behavior also found in Sumatran orangutans’ nearby Tapanuli cousins?
Orangutans’ knack for culture is encouraged by their uncanny abilities to mimic. Learning a new behavior for an orangutan is as simple as watching what others do and copying them. Behaviors that prove useful, or that make a task more efficient become more likely to spread throughout the population over time and without the need for direct communication or instruction.
They are such phenomenal copycats that orangutans have even been known to copy the behavior of other animals, particularly humans. In the 1970s—when such experiments still seemed ethical—some orangutans were even taught to use human sign language. More recently, near a village where people regularly bathe in rivers, one group of orangutans has learned to copy the villager’s hygiene practices. Sometimes, they even steal bars of soap to use for a good scrub down! In another area, one orangutan was observed attempting to spear fish—though not with any success—as he had seen humans doing just the day before.
Orangutans are uniquely solitary primates. But they are in no way anti-social. Though they only rarely come together in groups, they develop important emotional ties with other orangutans that share their ranges. Bonds are merely limited and depend on an individual’s age and sex. The mother-child bond, for instance, is the strongest by far. Outside of this relationship, relationships are more casual. Only flanged males are territorial and potentially aggressive towards other flanged males.
Tapanuli orangutans are not well-studied at this time. What unique behaviors might they display that the other two varieties do not? Sadly, with only 800 left in the wild and confined to a few small patches of forest, we have already missed out on answering this question to an unfortunate degree.
What Does It Mean?
Of, relating to, or being aggressive, or defensive social interaction (such as fighting, fleeing, or submitting) between individuals usually of the same species.
Physically adapted to living primarily or exclusively in trees.
Also called arm swinging, it’s a form of arboreal locomotion in which primates swing from tree limb to tree limb using only their arms.
Beneath the emergent layer, the canopy layer is the primary layer of the forest and forms a roof over the two remaining layers (the understory and forest floor). Many animals live in this maze of leaves and branches, where food is abundant.
To have sexual intercourse.
A recurring period of sexual receptivity and fertility in many female mammals.
Thick pads of flesh that frame the face and develop more prominently in dominant male orangutans. Some less dominant males never develop cheek flanges.
When a species’ population is reduced in size (i.e. by a cataclysmic event, habitat fragmentation, etc.) limiting the genetic diversity of the species.
The process by which a large, continuous stretch of habitat gets divided into smaller, disjointed patches of habitat.
A system of organization in members of a group who are ranked one above the other according to status or authority.
The breeding of closely related individuals, especially over many generations.
Refers to a living organism’s distinct physical (biological) characteristics.
A small surviving group of individuals from a once larger population.
Describes a difference in developmental timing between males and females or developmental differences within a sex related to secondary sexual characteristics.
Distinct difference in size or appearance between the sexes of an animal in addition to difference between the reproductive organs themselves.
A forest that is situated at the foothills or lower elevation slopes of a mountainous region.
Visit the Glossary for more definitions
Daily Life and Group Dynamics
As tree-dwelling primates, orangutans are quite unique among great apes. Life in the canopy influences many of the aspects of their daily lives and group dynamics. Chimpanzees, bonobos, and gorillas spend the majority of their time on the ground. Here, there is plenty of space to congregate so it is quite easy to gather regularly and in large groups.
Orangutans do not have this luxury. Not only would there not be enough fruit in one area to sustain a large group of big great apes, but several orangutans would be too much weight for one tree to hold. Since it would be dangerous and unsustainable for orangutans to gather in large groups, they lead quite solitary lives compared to their cousins on the African continent.
In spite of these environmental limitations, orangutans lead rich social lives. Females and subadult males occasionally gather with other females and subadult males for short periods of time and in small groups. These groups never form long enough for any hierarchy to emerge, however. So, the political games other great ape species play are nonexistent for orangutans.
The bimaturism of adult orangutan males affects their social standings. Only fully matured, flanged males are truly solitary, traveling far and wide to find sexually mature females who are ready to mate. Unflanged males are more likely to gather with other unflanged males and occasionally even females.
The most universal relationship for orangutans is that between mother and child. This relationship forms the foundation for all that it means to be an orangutan. Deprived of it, a young orangutan is not able to survive in the wild. Offspring remain with their mother for seven to eleven years, dependent on her until they have learned all the skills they will need in order to survive as adult orangutans.
A mother and her young wake when the sunlight first hits the trees. They crawl out of their nest and begin the day. They move slowly through the trees together. The mother composes her movements carefully, keeping a close eye on her young whose own movements are not nearly so graceful. Eventually, he crawls onto her back—tired—and she whisks them off to a nearby foraging spot.
They haven’t visited this spot in a long time, but the mother knows this is the time of year that the fruit of that particular tree ripen. Another female and her young are already there. The four act perfectly pleasant towards each other and share the bounty happily. The young ones watch their mothers as they eat, silently studying how to open the shell of the fruit to get at the meat inside. When the fruit is open, the young orangutans eagerly beg their mothers for a piece.
Recent studies have shown that mother orangutans change their reactions to their offspring begging depending on the food up for grabs and the abilities of their young. Foods more difficult to process, like fruits with hard shells, are more readily shared with young ones than those, like leaves, that are abundant and easy to harvest. As their young grow and gain the skills to process the more difficult items, mothers share less and less of their own food with them. In this way, their young are passively encouraged to take initiative in order to practice the skills they will one day need when they leave her side.
Orangutans spend the majority of the day either feeding or resting. Sometime between meals and naps, they might travel to a new foraging spot. Before dark, they either build a new sleeping nest or return to an old one to settle down for the night.
Though the last to be described, the Tapanuli orangutan is likely the most ancient orangutan lineage.
Orangutans have a greater ability to store fat than other great apes.
Orangutans use their feet and legs to gesture more often than any other great ape species.
Legend has it that orangutans are secretly able to talk but choose not to, knowing that if humans ever found out we might enslave them. While this is only a myth, it is one that speaks to orangutans’ amazing communication skills. Orangutans are so adept at communicating that many of their methods look uncannily like our own.
In the 1970s—before the scientific community realized such research was unethical—a handful of orangutans were taught to use sign language. One, called Chantek, could even combine signs he already knew to talk about things he didn’t yet have a sign for. This research seldom ended very happily for their subjects, but did reveal for the first time just how competent orangutans are as communicators.
Of course, if scientists had initially studied orangutans in their natural environment, they would have also found this to be the case. After all, orangutans already use hand signals to communicate in the wild. In 2019, the first study to look closely at the use of gestures in wild Bornean orangutans found that mothers and their offspring use more than 400 unique gestures to communicate with each other. The same study described over 800 orangutan vocalizations. Not only do orangutans combine gestures to mean different things—like Chantek did—they even combine gestures with vocalizations to the same end!
Do all orangutans learn and use similar gestures? Or do they vary between species? What are the cultural differences between regions? Hopefully future studies will give us clearer answers to these questions.
Researchers theorize that orangutans’ complex and layered communication methods are driven by their long development periods. In order to learn what food is safe to eat, mother and child naturally develop ways of communicating with each other. Thus, while offspring are studying and practicing how to harvest and process various foods, they are also honing their communication skills.
Another recent study has shown how orangutans are even able to communicate about past events. Other primates, particularly monkeys, often use specialized alarm calls to warn others immediately of approaching predators. In the presence of predators, however, a mother orangutan becomes visibly distressed but does not vocally react until the coast is clear. Only then does she sound the alarm to her oblivious offspring. In this way, she avoids drawing the predator’s attention but still teaches her young that the creature they recently saw should be considered dangerous.
Flanged male orangutans make a special vocalization, called a “long call.” This loud, booming call attracts sexually receptive females at the same time that it warns other males to keep their distance. Only fully mature orangutans have the developed throat sacks necessary to make these calls, and their cheek pads help to amplify and direct the call, ensuring it is broadcast for miles around. The long calls of Tapanuli orangutans are quite different from those made by Bornean and Sumatran orangutans. Not only do they last much longer, the calls contain more pulses emitted at a faster rate.
There is still much to be learned about the communication abilities of orangutans, especially the Tapanuli variety.
Reproduction and Family
Any adult male, flanged or unflanged, is capable of mating. But females overwhelmingly prefer flanged males, suggesting that their special status gives fully developed males a leg up when it comes to mating.
A flanged orangutan has a larger range than females and unflanged males. He travels throughout this range in search of potential mates, regularly issuing long calls to attract females and intimidate his rivals. When he finds a receptive female, he may still have to compete with another flanged male to win her favor. The two opponents make agonistic displays at each other. If neither one backs down, things can become violent. Flanged males frequently show the scars and wounds of their past fights. The winner mates with the female and, after 8.5 months, she gives birth.
The mother-child relationship forms the basic familial unit for orangutans, and fathers play no role in bringing up offspring. Besides humans, orangutans have the longest development period of any mammal. Offspring remain under the close watch and care of their mothers for approximately at least a decade. As such, mothers generally give birth to one infant at a time and do not become sexually receptive again until their child has gained independence.
For the first two years or so, a mother’s infant is completely dependent on her for food, protection, and transportation. He clings to her stomach, side, or back, clutching her fur tightly. Eventually, her infant ventures from the safe warmth of her fur. As he grows, he practices more and more how to use his specially adapted body to move about the canopy on his own, trying out all kinds of novel and risky maneuvers.
He studies his mother’s behavior closely, watching how she harvests and processes different fruit. At first, his mother shares her food willingly with him. But as he gets older, he is more and more expected to procure it for himself. In this way, she encourages him to take initiative to practice the skills he will need to survive as a solitary adult one day. He imitates what he has seen her do to the best of his ability. It might take several tries before he successfully procures even a single morsel. Luckily, his mother knows at what point he deserves a helping hand so that he won’t grow disheartened and give up.
For at least two more years, he remains close to his mother. The two eat, sleep, and travel together. Sometime between eight and fifteen years of age, he reaches sexual maturity. While he can mate, he won’t develop cheek pads or a throat sack for several more years. Armed with the skills and knowledge learned from his mother, he finally sets off on his own. Their separation is in no way dramatic, however. In the years to come, he even occasionally pays her a friendly visit.
Until he develops his cheek pads, he either keeps to himself or socializes with other unflanged males. As an unflanged male, however, finding a mate is likely to prove difficult for him. In fact, unflanged males are known to force copulation with females more often than other primates as a way to compete with the female-preferred flanged males.
Researchers are still quite baffled by the phenomenon of bimaturism in orangutans. There are many theories about why, when, and how the change occurs but little is certain. Do all males eventually develop cheek pads or only a select few? What factors keep unflanged males in arrested development?
Some researchers hypothesize that the long calls of a flanged male could release a special hormone in the unflanged males who hear it, which might inhibit their development. While somewhat similar phenomena have been confirmed in some species of monkeys, it has yet to be found in orangutans.
A female orangutan reaches sexual maturity as early as twelve years of age, but few females give birth before they are fifteen years old.
Though similar to Bornean and Sumatran orangutans, the development of Tapanuli orangutans has not been studied in detail at this time. There is some evidence that Sumatran orangutans grow up faster than Bornean orangutans. How does the Tapanuli variety compare? Hopefully these sorts of questions will be answered in due time.
Orangutans live in some of the most biologically diverse places on the planet, and eat hundreds of species of fruit throughout the year. Thus, they play vital roles in their ecosystems as seed dispersers. As they travel, seeds from the fruit they have eaten work their way through their digestive systems until they come out in their feces far from where they were ingested.
While many primates help to disperse seeds in this way, two features make orangutans extremely effective in this role. First, they are such large primates that they can ingest large seeds that smaller gibbons and monkeys cannot. Second, thanks to their resourcefulness, orangutans are able to process fruits that other species simply leave alone, ensuring their seeds are also dispersed.
Orangutans are keystone species, meaning that their existence is critical to the health of their ecosystems. Landscapes would look very different without them. Researchers have found that forests with a higher density of orangutans tend to have richer and more biologically diverse species than those with fewer orangutans. Thus, the better a habitat is for orangutans the better it is for other species, both plants and animals.
An orangutan’s large size and arboreal lifestyle mean they have essentially zero predators. Mothers do need to keep their eyes peeled to protect their smaller dependents, however—especially from tigers.
For now, the ecological roles of Tapanuli orangutans are not well-studied. However, this species is known to eat a number of plants that the other two do not. How do these specialized dietary choices affect their particular ecosystems? The recent evidence suggesting that the remaining 800 wild Tapanuli orangutans may no longer live in their natural habitat complicates matters. If their ecosystems are already extinct, are these orangutans our only hope to return them to their natural states?
Conservation Status and Threats
The Tapanuli orangutan is classified as Critically Endangered by the International Union for Conservation of Nature (IUCN, 2017), appearing on the IUCN Red List of Threatened Species. With less than 800 remaining in the wild, this species is considered one of the world’s most endangered primates and the most endangered great ape.
The less than 800 Tapanuli orangutans in the wild today are limited to a small area of fragmented forest in the uplands of Batang Toru. Researchers estimate that Tapanuli orangutans have held onto less than 3% of their historical range since 1890. Recent evidence even suggests they already live outside their historical range, having retreated to higher elevations due to the rapid destruction of their natural lowland habitats by humans during the last several decades. Therefore, the hilly submontane forests where they live today are not only miniscule and severely fragmented but suboptimal conditions for their conservation.
Sadly, this remnant population is surrounded by threats on all sides. For decades, the forests of Sumatra have been steadily cleared to make way for human settlements and infrastructural projects. Palm oil plantations are especially prominent nowadays, claiming large swaths of the Tapanuli orangutan’s estimated historical range.
Settlements require roads. For arboreal species like orangutans, roads can easily fragment whole forests. Orangutans have no way to bridge the gaps roads make in the trees and become isolated. In order to cross, they must descend to the ground where they risk getting hit by oncoming traffic. For a species that moves so slowly on the ground, this is especially hazardous.
When forests become fragmented, it has adverse, long-term effects on primates. Traveling to new regions to find food or mates becomes difficult, especially for arboreal primates. As a species with such long development periods, such limitations make it unlikely for orangutans to find sexually receptive partners. In this situation, genetic bottlenecks can develop quite quickly. As gene flow slows to a halt, inbreeding is likely to increase. In just a few generations, this causes offspring to be born with debilitating defects. Inbred primates are also more vulnerable to diseases and parasites, making them less capable of surviving to adulthood.
Agriculture causes more problems than just habitat destruction. The physical closeness of locals to wild orangutans ensures conflict. Desperate orangutans raid crops, angering farmers who then plot how to harm or kill them. Recently, migrants from nearby Nias Island, just west of Sumatra, have settled on land along the forest edge of Batang Toru protected forests, leading to an uptick in the hunting of Tapanuli orangutans in particular.
As if that wasn’t enough, young orangutans are a prized commodity in the illegal pet trade. Poachers almost always kill the mother in order to get their hands on her otherwise helpless infant. Orphaned orangutans who end up as pets are likely to die of malnutrition as their captors make no effort to feed them diets similar to what they would eat in the wild. If they manage to survive, their captors frequently abandon them once they become too large and difficult to control. Captors believe they are doing their victim a favor by releasing them back into the wild but don’t realize that an orangutan who has grown up in captivity knows nothing about how to survive in the wild and quickly dies.
Since 2017, the forests where Tapanuli orangutans live have become protected—sort of. A select number of companies still retain limited rights to develop or alter the land in some way to meet their own financially motivated ends. One logging company, for instance, is still permitted to log 300 km2 of primary forest inside the Tapanuli orangutans’ current range. Additionally, gold and silver mining operations persist in the southwest corner of the Batang Toru region. Mining companies not only continue to destroy and fragment forests—disturbing entire ecosystems—but continue to speculate on land as they seek to extend their operations indefinitely.
Currently, the biggest existential threat for Tapanuli orangutans is the planned construction of a large hydro-electric dam. The proposed dam would stand in the area where their population happens to be densest and—if completed—will fragment their range beyond repair. The project has suffered several set-backs since its initial proposal, but the debate surrounding it unfortunately continues.
With less than 800 left in the wild, Tapanuli orangutans are already disturbingly vulnerable, and our opportunity to study them is running out. If we want to have any chance at conserving these charismatic creatures so vital to their ecosystems, then we must ensure that we can gather the necessary data to do so. Therefore, any action that could possibly speed up their extinction to any degree would be irresponsible.
Some hope endures for the critically endangered Tapanuli orangutan.
The myriad organizations already long at work for orangutan conservation have quickly come to the aid of this novel species. Furthermore, grassroots movements world-wide have been protesting the Batang Toru hydro-electric dam project. Together, they have provided enough public pressure to cause several major sponsors of the project to back out completely, including the World Bank and the Asian Development Bank.
In 2019, the Bank of China—the dam’s biggest backer—agreed to conduct a thorough review of the project’s potential environmental impacts. Since that time, several studies have been conducted by a number of parties, beginning a long debate about whose review was the most trustworthy. In 2020, the Bank of China officially pulled out of the project.
The project was set to proceed in spite of these trends but, in 2020, the world-wide pandemic forced the project to come to an indefinite halt. This has given conservationists an unexpected opportunity to conduct more research in the meantime and hopefully stop the project altogether.
Researchers estimate that if a mere 1% of adult Tapanuli orangutans are killed, translocated, or captured per year their extinction is not only inevitable but imminent. Therefore, foregoing construction of the dam completely is the only option if we want to conserve them.
But that alone won’t be enough. In order to curb poaching, protections already in place must be more strictly enforced and methods to calm human-orangutan conflicts implemented. The entire Batang Toru ecosystem needs to be ruthlessly protected, and clearer boundaries drawn so that there is no more confusion about what is allowed and where. Research, conducted using noninvasive methods, is equally paramount since a well-rounded understanding of Tapanuli orangutans is the only way to write an effective plan to conserve them.
Written by Zachary Lussier, January 2022