Geographic Distribution and Habitat
The Sumatran orangutan (Pongo abelii) is endemic to the northern portion of the island of Sumatra, in Indonesia. A species is endemic to an area when it is found naturally nowhere else in the world. Most Sumatran orangutans, about 82% of them, live in Aceh Province in Indonesia, with the remaining populations in North Sumatra Province. However, of these more southern populations, only one is believed to be viable in the long term. About 90% of Sumatran orangutans live in what is known as the Leuser Ecosystem, a diverse landscape of rainforests, rivers, lakes, and peatlands on the north end of Sumatra covering 6 million acres (2.6 million hectares), an area about the size of New Hampshire in the US. Sumatran orangutans inhabit moist lowland forests, montane forests, and peat swamps. They do not tend to live at high elevation, preferring elevations around 650 to 1300 feet (200 to 400 m) above sea level, where their favorite fruiting trees grow, though they can be found as high as 5,000 feet (1,500 m) above sea level. They are believed to have a total range of about 6,500 square miles (16,800 sq km). Sumatran orangutans are less tolerant of habitat disturbance than their cousins, Bornean orangutans (P. pygmaeus), and thus are highly dependent on high-quality primary forests. Primary forests are those that have remained relatively undisturbed by human activity for at least several hundred years—and often significantly longer.
Historically, there was believed to be only one species of orangutan: Pongo pygmaeus, of which Sumatran and Bornean orangutans were considered subspecies. Though they look very similar upon first glance, the two orangutans are genetically distinct. After taxonomic reviews in the early 2000s, they were elevated to full species status, with Bornean orangutans retaining the P. pygmaeus scientific name and Sumatran orangutans adopting the scientific name P. abelii. More recently, in the late 2010s, a new species has been proposed: the Tapanuli orangutan (P. tapanuliensis), making for a total of three orangutan species in the most widely accepted classification.
Size, Weight, and Lifespan
Females and “unflanged” males (see below) weigh between 66 to 110 pounds (30 to 50 kg) and have an average height of about 4.3 feet (1.3 m), while flanged males are significantly larger at 110 to 200 pounds (50 to 90 kg) and reach an average height of 5.9 feet (1.8 m)—about the same as the global average height for human males. Sumatran orangutans can live to an age of about 50 years.
As fully arboreal primates, Sumatran orangutans have a very distinct body type. Their long, muscular arms dwarf their legs, which, while certainly important, are quite visibly not the limbs that the apes rely on the most for getting around. An adult male’s arm span can reach 7.5 feet (2.25 m), and are about 1.5 times the length of their legs. Their fingers are very strong, and their thumbs are opposable, allowing them a rock-solid grip on branches and to manipulate tools. They are “quadrumanous”—this means that their feet are shaped similarly to their hands, complete with opposable big toes that allow them to grip. They have a bit of a potbelly, and their entire body is draped in long, orange hair—fringe has never gone out of style for Sumatran orangutans.
Unusual for mammals, sexually mature orangutan males have two “morphs”—unflanged males, which look very similar to females, and flanged males. This phenomenon is known as “bimorphism”. Unflanged males are smaller, faster, and more agile than flanged males. Flanged males are those that have formed wide cheek pads, called flanges, that give their face a very distinct, round appearance. They also grow significantly larger, have longer hair, and develop throat sacs for amplified vocalization. Flange development has been known to happen anywhere between the ages of 11 years up to 30 years, although developing the flange at the age of about 20 seems to be the most common. Rarely, a male will never develop a flange. Unlike some other primates with bimorphism, such as mandrills who can gain and lose their bright face and rump depending on their dominance rank, these physical changes are not reversible. Research is ongoing to better understand why male orangutans have two morphs and what factors may influence the age at which they develop their flange.
Though they look very similar to Bornean orangutans, there are a few visible differences between the species. Sumatran orangutans have longer hair, including longer beards on both males and females, a more slender build, and scatterings of white hairs on their faces and pubic region that their Bornean counterparts lack.
Though labeled as frugivores, or fruit eaters, Sumatran orangutans aren’t choosy and, in addition to fruit, consume a wide variety of foods, including leaves, seeds, flowers, insects, and bark. Interestingly, Sumatran orangutans have even been documented hunting and eating slow lorises, though researchers believe this is rare and opportunistic, having only been observed a total of nine times from six individuals. While data is limited, some researchers believe this may represent an interesting difference between Bornean and Sumatran orangutans, as Bornean orangutans have not been observed eating slow lorises, and indeed have been observed letting them peacefully pass by or even attempting to engage with them in a playful manner.
Behavior and Lifestyle
Sumatran orangutans are arboreal, meaning they spend their lives in trees, and in fact, they are the largest of the arboreal primates. Sumatran orangutans are even more arboreal than Bornean orangutans. Unlike their cousins who occasionally travel terrestrially, females virtually never set foot on the ground, and males do so only rarely, and if they do, it’s often because they are forced to when they grow so large that trees can’t support their weight. They move about via brachiation—swinging from branches using their long, muscular arms. Very large males sometimes lose the ability to brachiate due to their size, and instead climb around on branches using both their hands and feet.
They are also diurnal, meaning that they are awake during the day and sleep at night. They build new nests out of branches and leaves each night to sleep in, and often make a new one during the day to nap in. Sometimes, if they find an old nest, they repair it instead of making a new one. If it’s raining, they’ve even been known to make a crude roof to the nest to keep the rain off their backs.
A typical day for a Sumatran orangutan involves waking up at dawn, feeding for a few hours, then taking a midday rest. They then travel and feed for a few more hours before settling down to sleep in the late afternoon.
Sumatran orangutans are known to be highly intelligent. They have the third most extensive tool usage repertoire among the primates, after humans and chimpanzees. They can use sticks to pull bugs out from crevices and, as seen in some charming photographs, even using large leaves as “umbrellas” to protect themselves from rain. They also use rocks as hammers. In one study of captive orangutans, they were supplied with a piece of sharpened flint and were able to use it as a knife. In a separate experiment, they were supplied with a rock and an unsharpened piece of flint and were able to hit the flint such that sharp flakes came off. However, they didn’t seem to understand that they could then use the flint flakes as a knife, suggesting that they were not capable of forming a complex plan.
In 2011, the Sumatran orangutan became only the third great ape to have its genome fully sequenced, after humans and chimpanzees.
Though primarily fruit-eaters, Sumatran orangutans have been known to hunt and consume slow lorises, a small, nocturnal primate.
Orangutans are one of the least social primates, though Sumatran orangutans are a bit more social than Bornean orangutans and are often considered “semi-solitary”. One hypothesis for the difference is that fruit tends to be more abundant in Sumatra, so there is less food-related competition among the orangutans. Mating pairs will stay together for a time, and females stay with their offspring for years. Females may also associate with other females or adolescents in loose groups. Adult males, on the other hand, are usually aggressive at worst or avoidant at best with each other. Typical home range sizes for females range from 200 to 370 acres (0.8–1.5 km²). Average male home range sizes are not known, but are believed to be significantly larger, over 11.5 square miles (30 km²). Male ranges overlap with female ranges, but not with other males. Flanged males are the most solitary of the Sumatran orangutans. In a given area, there will often be a dominant flanged male that has a significant share of the mating opportunities in an area.
Sumatran orangutans rely heavily on vocal communication. Flanged males have a throat sac that can amplify their calls over long distances. Their “long calls” are used to advertise themselves to the local population of females for mating opportunities and to keep out intruding males. These long calls are often accompanied by pulling and cracking branches to add to the noise. Other sounds made by both sexes for communication include grunts and grumbles, lip-sucking, burping, and teeth grinding. Young orangutans squeak, bark, and scream.
Because of their mostly solitary nature, grooming is not a key form of social bonding as it is for many primates. However, play is an important form of bonding and learning among young orangutans, and juveniles are known to engage in behaviors like non-aggressive biting and playful posturing. Visual communication is also very important among Sumatran orangutans. In fact, because of their highly flexible lips, they are able to form a particularly wide variety of facial expressions with which to communicate.
Sumatran orangutans have an extremely slow life history, with females averaging 15 years of age at their first birth and having the longest interbirth interval of any mammal species—females average 8.2 to 9.3 years between births, compared with an average of 6.1 to 7.7 years for Bornean orangutans. Females have an estrus cycle and are only able to become pregnant for a few days each year. Mating tends to occur in conjunction with heavy fruiting seasons. This not only means the annual rainy season, from December to May but also mast fruiting years, in which trees of one species all fruit at the same time, which can occur every two to ten years. During these events, mating is especially common.
Unflanged males are capable of reproducing but employ different mating tactics compared to flanged males. Unflanged males seek out females and tend to harass them into mating, and can even be aggressive and force them to mate. Unflanged males are more likely to mate when the chance of conception is lower. Sometimes the unflanged males even take a female’s young in an attempt to get her to mate. Females, on the other hand, prefer to mate with the local dominant flanged male and will seek him out to initiate mating when in estrus. Females sometimes form bonds with other females or even non-mating pairs with adult males, which protect them from harassment. Unfortunately, harassment is becoming more common among Sumatran orangutans as habitat loss has forced more individuals into a smaller space, leading to increased tensions as these largely solitary animals can’t as easily move away from an antagonistic individual.
Unfortunately for the non-dominant flanged males, they are at a disadvantage. If they employ “long calls” to attract females, they will also attract the attention of the local dominant male, which could result in an aggressive encounter. At the same time, they are not as mobile as unflanged males, so they cannot actively seek females out very effectively. It’s believed that roughly half of all Sumatran orangutan babies are fathered by an unflanged male and the other half by a dominant flanged male. Unfortunately for them, non-dominate flanged males need to fight for dominance in order to have a real shot at fatherhood.
Once a female is pregnant, she has a gestation period of about 8.5 months and gives birth to a single baby, or rarely twins. The baby becomes her focus for the next eight to nine years of her life. The baby is carried by its mother for the first two or three years of life and young nurse from their mother for about seven years on average. Males play no role in parenting. Females will never go into menopause and a typical female can expect to birth four or five offspring over the course of her life. Female offspring become sexually mature at about 12 years of age. Males become sexually mature at around 14 years of age but don’t develop a flange until they are about 20 years old.
Sumatran orangutans are preyed upon by large, arboreal predators, including Sumatran tigers and clouded leopards. They also play an important role as seed dispersers due to the fruit that they eat.
The International Union for Conservation of Nature lists the Sumatran orangutan as Critically Endangered (IUCN, 2017) appearing on the IUCN’s Red List of Threatened Species. The total population as of 2016 was believed to be about 13,800 individuals, with a population of about 13,600 if non-viable populations are removed. While the previous population estimate points to a total population of about 6,600 individuals, this supposed increase is due to better surveying techniques and coverage, not a true population increase. In fact, the population of Sumatran orangutans is in sharp decline. Models show a potential 81% decline in Sumatran orangutan population from 1985 levels by 2060. This dramatic decline is largely due to habitat loss and fragmentation and poaching. Sumatran orangutans are particularly at risk because of their extremely slow generation time of about 25 years—about the same as humans’. This means that it takes the species a very, very long time to recover from population loss.
Between 1985 and 2007, 60% of Sumatran orangutan habitat was lost. The main causes of habitat loss include logging, mining, agriculture, settlement, and road building. Rampant development and settlement has resulted in a significant amount of land lost to settlements. Oil palm plantations have been responsible for a significant loss of habitat as well—a single plantation can be responsible for the loss of hundreds of square miles of habitat. When a new plantation is created, it forces the resident orangutans to any remaining forest patches. The sudden influx of many large primates in a small space inevitably leads to a lack of food, and as a result, the orangutans often suffer from malnutrition and starvation. Sumatran orangutans are also deliberately killed by humans, sometimes to collect infants for the pet trade, as a by-product of clearing forests for oil palm plantations, or in retaliation for crop raiding.
One of the single largest threats to Sumatran orangutans is a land use plan that was ratified by the province of Aceh in 2013. The majority of Sumatran orangutans, about 82% of them, live in Aceh. The land use plan allows for the building of roads, hydropower infrastructure, oil palm plantations, and settlements in sensitive rainforest habitats. Despite the Leuser Ecosystem being recognized by the Indonesian government as a “National Strategic Area” for environmental function, the land use plan ignores this designation. Both the province and the national government have publicly acknowledged that the land use plan is illegal for this reason, but it still went on to be ratified and implemented. Though a legal battle was fought to force the land use plan to recognize the ecosystem and its designation as a National Strategic Area, the lawsuit was dismissed by the Central Jakarta District Court in 2016. Environmental activists, such as Farwiza Farhan, who has won the TIME100 Impact Award for her work, continue to challenge the land use plan in the hope that it can be revoked before more irreplaceable habitat is destroyed.
Sumatran orangutans are listed in Appendix I of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival. They are also strictly protected under Indonesian national law, which makes it illegal to capture, injure, kill, own, transport, or trade a Sumatran orangutan.
About a quarter of Sumatran orangutans reside in Gunung Leuser National Park, which has also been designated a UNESCO Man and Biosphere Reserve and is part of the UNESCO “Tropical Rainforest Heritage of Sumatra” World Heritage Cluster. A smaller protected area, the Singkil Swamps Wildlife Reserve, is also home to some Sumatran orangutans. While the orangutans in the park and reserve are fairly well protected, most live outside those boundaries. Nearly all—about 95%—of Sumatran orangutans live in the Leuser Ecosystem. While it has been nationally recognized as a National Strategic Area and the importance of the ecosystem’s sustainable management and conservation is stressed in the law, this has not come to pass in practice. The province of Aceh has disregarded this designation and the ecosystem is effectively not protected outside of the national park and wildlife reserve.
A reintroduction program is in the works to release confiscated Sumatran orangutans, many of whom were illegal pets, and establish two new genetically viable and self-sustaining wild populations in and around Bukit Tigapuluh National Park and Jantho Pine Forest Nature Reserve. Confiscated baby orangutans are sent to the Sumatran Orangutan Reintroduction Center, where they are taught crucial survival skills such as nest building, food identification, and moving through the canopy—skills that would be taught over nearly a decade by their mother in the wild. Once the young orangutans are deemed ready, they are released into the wild but are closely monitored to ensure a successful release. If needed, they are supported until they can thrive on their own, such as by being provided supplemental food. Multiple released females have gone on to give birth and raise their own babies in the wild, a momentous achievement for a critically endangered species with a very slow life history.
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Written by K. Clare Quinlan, February 2024