The Sumatran orangutan (Pongo abelii) is one of three types of orangutans. It is very rare and lives only in the northern part of the Indonesian island of Sumatra. It is less common than the Bornean orangutan but more common than the recently discovered Tapanuli orangutan, which also lives in Sumatra. The name "orangutan" comes from two Malay words: "orang," meaning "person," and "hutan," meaning "forest." Together, they mean "person of the forest."

Description

Male Sumatran orangutans can grow up to about 1.7 meters (5.6 feet) tall and weigh around 90 kilograms (200 pounds). Female Sumatran orangutans are smaller, averaging about 0.9 meters (3.0 feet) tall and weighing approximately 45 kilograms (99 pounds). Compared with Bornean orangutans, Sumatran orangutans have a slimmer body shape, longer faces, and longer hair that is a lighter red color.

Evolution

Fossil orangutans in Sumatra from the Pleistocene had diets that were similar to those of present-day Sumatran orangutans. These diets mainly included soft fruits, as shown by tiny marks on their teeth.

Behaviour and ecology

The Sumatran orangutan eats more fruit and insects than the Bornean orangutan. It prefers figs and jackfruits but also eats bird eggs, small animals, and tree bark. Unlike the Bornean orangutan, the Sumatran species spends less time eating the inner bark of trees.

Wild Sumatran orangutans in the Suaq Balimbing swamp use tools to find food. They break off a branch about a foot long, remove the twigs, and chew one end to make it frayed. They use the stick to dig for termites in tree holes or poke bee nests to collect honey. For eating Neesia tree fruit, they choose a five-inch stick, strip its bark, and use it to remove the painful hairs around the seeds. They then eat the seeds safely. These tools are stored and reused, forming a collection of tools over time. Bornean orangutans do not use these tools in the same way.

In a documentary called Wild Asia: In the Realm of the Red Ape, a Sumatran orangutan was shown using a twig to pull food from hard-to-reach places. Another scene showed an orangutan using a large leaf as shelter during a rainstorm.

Tree branches also help Sumatran orangutans move through the forest. They are the heaviest tree-dwelling mammals, so they move slowly and use many supports to balance on flexible branches. They sway trees to reduce the effort needed to move.

Sumatran orangutans spend more time in trees than Bornean orangutans, possibly because of predators like tigers. They move using all four limbs and a method called semibrachiation, which involves swinging with their arms.

As of 2017, about 13,846 Sumatran orangutans remain. Conservation efforts include breeding them in captivity, but this may affect their natural behaviors. In captivity, orangutans may lose skills needed for survival in the wild, such as finding food or avoiding danger.

Orangutans use 64 different gestures to communicate. Twenty-nine of these have clear meanings, such as signaling to play, sharing food, or moving away. Unlike Bornean orangutans, Sumatran orangutans do not use sounds for communication.

In 2024, a wild Sumatran orangutan named Rakus used chewed leaves from a Fibraurea tinctoria plant to treat a facial wound. The wound healed over weeks.

Sumatran orangutans have five life stages:
1. Infancy (birth to 2.5 years): Weighs 2–6 kg. Has light facial markings and is carried by the mother.
2. Juvenilehood (2.5–5 years): Weighs 6–15 kg. Still carried by the mother but explores nearby.
3. Adolescence (5–8 years): Weighs 15–30 kg. Face darkens, and social bonds with peers grow.
4. Sub-adulthood (8–13–15 years): Weighs 30–50 kg. Develops cheek features and avoids adult males.
5. Adulthood (13–15+ years): Weighs 50–90 kg. Fully developed with thick beards and cheek pads.

Female Sumatran orangutans live 44–53 years, while males live 47–58 years. Females can give birth until about 53 years old. Both genders remain healthy with thick hair and strong cheek pads until old age.

Sumatran orangutans are more social than Bornean orangutans. They gather to eat fruit from fig trees. Groups usually include females and a dominant male. Adult males avoid other males, while subadult males try to mate with females, though they are often rejected. Mature females prefer to mate with older, dominant males. Some male Sumatran orangutans develop large cheek features and muscle mass later than others, a trait called bimaturism.

Genomics

Orangutans have 48 chromosomes. In January 2011, scientists mapped the genome of the Sumatran orangutan using DNA from a female named Susie who lived in captivity. This made the Sumatran orangutan the third living species related to humans to have its genome fully studied, after humans and chimpanzees.

Scientists also shared less complete genome copies from ten wild orangutans—five from Borneo and five from Sumatra. Genetic studies found that Bornean orangutans (Pongo pygmaeus) have less genetic diversity than Sumatran orangutans (Pongo abelii), even though Borneo has six to seven times more orangutans than Sumatra. Research shows these two species split into separate groups about 400,000 years ago, which is more recent than earlier estimates. The orangutan genome also has fewer changes in the structure of DNA compared to the genomes of chimpanzees and humans.

Conservation

Sumatran orangutans face dangers such as logging (both legal and illegal), the large-scale clearing of forests to create farmland and oil palm plantations, and the splitting of forests by roads. Oil companies use deforestation methods to reuse land for growing palm oil. This land is taken from forests where Sumatran orangutans live. A study from the 1990s found that forests supporting at least 1,000 orangutans were lost each year within the Leuser Ecosystem alone.

By 2017, about 82.5% of the Sumatran orangutan population lived only in the northernmost part of the island, in the Aceh Province. Orangutans are rarely found south of the Simpang Kanan River on Sumatra’s west side or south of the Asahan River on the east side. The Pakpak Barat population is the only group predicted to survive long-term, despite current challenges like habitat loss and human activity.

While poaching is not a major threat to Sumatran orangutans, occasional hunting by local people reduces their numbers. In the past, they were hunted in northern Sumatra for food. Although hunting is rare today, some local groups, like the Batak people, eat many animals in their area. Farmers also see orangutans as pests and may try to kill them if they damage crops. In the past, zoos and institutions in Europe and North America also hunted orangutans for display.

Sumatran orangutans have a highly developed heart and blood system. However, their large air sacs in the lungs have made them more likely to get a condition called air sacculitis, which is similar to a strep throat infection in humans. This bacterial infection is common in captive orangutans because they are exposed to the human strain of Streptococcus in zoos. At first, antibiotics and rest can treat the infection. However, in 2014, a Sumatran orangutan that had lived in captivity for ten years became the first of its species to die from Streptococcus anginosus. This was the only known case, but it raises questions about why human treatments did not work for this orangutan.

The Sumatran orangutan is found only in the northern part of Sumatra. In the wild, they live only in the province of Nanggroe Aceh Darussalam (NAD), the northernmost tip of the island. In the past, they were found farther south, such as in Jambi and Padang. Small populations also live in the North Sumatra province near the border with NAD, especially in the Lake Toba forests. A survey in the Lake Toba region found only two areas where orangutans live: Bukit Lawang (a sanctuary for the animals) and Gunung Leuser National Park. Bukit Lawang is a village in the jungle, 90 kilometers northwest of Medan, near Gunung Leuser National Park. A Swiss organization set up a sanctuary there in the 1970s to help orangutans rescued from the logging industry. Rangers taught the orangutans survival skills and provided extra food. Recently, the program ended because the orangutans were successfully reintegrated into the wild, and the forest could no longer support more animals.

The Sumatran orangutan has been listed as critically endangered by the IUCN Red List since 2000. From 2000 to 2008, it was among "The World’s 25 Most Endangered Primates." A 2016 survey estimated that 14,613 Sumatran orangutans live in the wild, doubling earlier estimates. A 2004 study found about 7,300 orangutans in the wild. That same study estimated the orangutans occupy about 20,552 square kilometers of land, but only about 8,992 square kilometers have permanent populations. Some live in Gunung Leuser National Park, while others live in unprotected areas like northwest and northeast Aceh, West Batang Toru River, East Sarulla, and Sidiangkat. A successful breeding program has been started in Bukit Tiga Puluh National Park in Jambi and Riau provinces.

Two strategies are being considered to help save the species: 1) rehabilitating and reintroducing orangutans that were previously in captivity or displaced, and 2) protecting their forest homes by stopping threats like deforestation and hunting. The first method is cheaper but takes 10–20 years to show results. The second method offers better long-term protection for populations. The World Wide Fund for Nature has worked with other groups to stop clearing forests near Bukit Tigapuluh National Park.

A new population is being created in Bukit Tigapuluh National Park by reintroducing orangutans that were taken as illegal pets. This group has about 70 individuals and is reproducing. However, protecting forests costs twelve times less than reintroducing orangutans and helps preserve more wildlife.

Orangutans need large areas to live and have low population numbers, which makes conservation difficult. Their numbers depend on the availability of soft-pulp fruits. Sumatran orangutans move seasonally between lowland, midland, and highland areas to follow fruit supplies. Forests with a wide range of altitudes can support more orangutans, while deforestation and forest loss disrupt their seasonal movements. Sumatra has one of the highest deforestation rates in the world.

Predation

Possible predators of Sumatran orangutans in the Sikundur Monitoring Post are Sumatran tigers, Sunda clouded leopards, python species, and grey-headed fish eagles.