SUMATRAN ORANGUTAN

Pongo abelii

Geographic Distribution and Habitat
Orangutans are the only great apes found in Asia, specifically on the islands of Sumatra and Borneo. Fossils found in Java, China, and Vietnam indicate that they once roamed larger territories than they do today. Originally thought to be one species, two distinct species were recognized in 1996: Pongo abelii in Sumatra and Pongo pygmaeus in Borneo. A third species, the Tapanuli orangutan, Pongo tapanuliensiswas discovered in November 2017, also in Sumatra.

Sumatran orangutans live in rainforests, swampy forests, and mountain forests, at altitudes lower than 5,000 feet (1,500 m). Most of the current population is found in the Aceh Province at the northernmost tip of Sumatra. This area of contiguous rain forest, covering 16,000 square miles (26,000 km2), is known as the Leuser Ecosystem and is the last stronghold for the Critically Endangered Sumatran orangutan. There are some patches of forest with individuals south of the Leuser Ecosystem, but only the populations of west Batang Toru and Pakpak Barat are considered viable.

Sumatran orangutan range, IUCN 2019

Size, Weight, and Lifespan
Male Sumatran orangutans are larger than females. They are on average 54 inches tall (137 cm) and weigh between 155 and 200 lbs (70 to 90 kg). Females are about 43 inches tall (110 cm) and weigh between 90 and 110 lbs (40 to 50 kg). In captivity, they can be much larger and some individuals in zoos are double the weight they would be in the wild at 300 to 400 lbs (136 10 180 kg).

Sumatran orangutans can live up to 40 years in the wild and 50 in captivity.

Appearance
Sumatran orangutans have long orange hair. Their faces are somewhat elongated and both males and females have a beard. Males have a mustache and thin hair covers their cheek pouches. The forehead fairly large and bulges a little. The nose is flat with large nostrils, and the jaws and mouth protrude outward. They have 32 teeth, like humans, but large canines and central incisors are longer and have a heavier enamel surface. The thick layer of enamel is required for gouging on bark and other rough food items. Ears are fairly small compared to the size of the head. Eyes are brown and protected by eyelids and eye lashes. Dominant males develop large throat pouches and large cheek bulges, called flanges, which they retain for as long as they maintain a dominant role in their territory. They lose them if they are dethroned.

Babies are born with a pinkish face that changes to a leathery brown as they age.

Sumatran orangutans have very long arms spanning 7 feet (2 m), as well as long hands and feet. Extremely strong, they can lift 500 pounds (240 kg) effortlessly—which is handy when they want to tear off crowns of grown palm trees to retrieve the hearts for a meal. Their finger bones are bent inwardly and padded by fleshy cushions that improve their grip and allow them to hold up their body weight. Fingernails are black and both feet and hands have ridges on the inside that are unique to each individual. Feet look like hands with a big toe that acts like a thumb. Both hands and feet are perfectly adapted for moving in the trees.

The Sumatran orangutan’s mode of locomotion is called “quadrumanous scrambling.” Because they are so large, they cannot swing from branch to branch. Instead, they use both feet and hands to hold onto branches and climb to treetops. From there, they throw their weight in a specific direction, forcing the branch to bend until they can grab another branch and move on.

What Does It Mean?

Gene flow:
The transfer of genes from one population into another through breeding.

Mast fruiting:

Expression used to refer to the time period when many species of trees flower and bear fruit simultaneously.

Quadrumanous clambering:

Using both feet and hands to hold onto branches and move in the forest.

Visit the Glossary for more definitions

Diet
Orangutans know their territory intimately and, thanks to their highly developed visual and memorization abilities, they recognize which tree will bear ripe fruit and when.

Their diet is composed of mostly fruit, but they also eat leaves, saplings, buds, tree sap, roots, flowers, mushrooms, honey, termites, ants, spider webs, earth rich in minerals, caterpillars, birds’ eggs, and small tree dwelling vertebrates. The bark of tropical trees, which they chew and spit out, is a food source not often exploited by other forest animals. Among their favorite staples are the strong smelling fruit of the durian tree, which reportedly tastes cheesy and garlicky. Despite their differences in size, males and females consume approximately the same amount of calories per day (8,500 kcal).

With 300 endemic species of fruit, food is abundant in Sumatra and orangutans can have their pick. Figs, mangoes, litchis, and jackfruit are some examples of the food items they forage for. It is believed they know at least one thousand plants and pass on that knowledge to their descendants. They systematically avoid poisonous plants and use some for their medicinal properties–anti-parasites or anti-inflammatory—like Fordia splendissima, known to fight headaches. 

Behavior and Lifestyle
Sumatran orangutans are rather solitary, quiet, even-tempered, and deliberate in their actions compared to other great apes. Sitting 70 ft (20-30 m) above the ground, they tend to blend in with the foliage and are not easily observed, hence their nickname of the “elusive ape.” Females remain close to their natal range, whereas males travel a bit further in search of a territory they can claim.

These highly intelligent creatures find creative solutions to everyday life problems. The fruit of the neesia tree, for instance, is very hard to crack, but contains rich kernels of protein that orangutans love. Because the inside of the fruit is lined with crystals of itchy calcium oxalate, orangutans use thin sticks to get the seeds out without affecting their fingers. They also develop specific regional cultural habits like using leaves as a pillow while sleeping or to cover their heads from the rain. Tools selected for use are not the same for all individuals and vary depending upon where they live.

Daily Life and Group Dynamics 
Every day starts with a quest for food at sunrise upon waking. To find their meals, orangutans usually travel on their own, except mothers who travel with their baby and, potentially, an adolescent. Groups of young males and females without a territory of their own have been observed traveling together from one feeding station to another. Because fruit is particularly abundant in Sumatra, it is not unusual to see several orangutans feeding on the same tree at the same time in mast fruiting periods. Socialization is therefore a little more frequent than in Borneo where fruit is not as abundant.

Orangutans feed for a couple of hours then nap. Sumatran orangutans build rudimentary nests for their siestas, whereas Bornean orangutans do not. After an hour of rest, they travel again, picking up food items along the way until they find fruit trees and resume eating. Their day ends around 5 p.m., when they settle for the night and build another nest by bending large branches and weaving smaller branches in to make a sturdy, comfortable, rounded platform. If they sense a storm coming, they may even add a roof made of leaves.

The “pasha” or dominant male’s territory usually spans up to 4 square miles (10 km2). The females’ territories are nested within his territory and span up to 2 square miles (5 km2). The size of the territory is dependent on food abundance. Males defend the feeding areas where females live and thus indirectly defend them from unflanged males that do not have any territory of their own and are transient. These males do not have the physical characteristics flanges of the dominant males (large cheek and throat pouches).

An unflanged male may cross a female’s territory to visit his mother, to look for a territory he might claim, or in search of an opportunity to mate. Females assaulted by an unflanged male scream loudly to call the dominant male and fiercely resist their assailant. However, since orangutans move rather slowly and the territory of each female is quite large, it can be difficult for a dominant male to defend a distressed female and maintain exclusive mating rights.

Fun Facts

Before Captain Beeckman published “A Voyage to the Island of Borneo” in 1718, no westerner had ever heard of the intelligent red ape. He referred to them in his writings as “Oran Ootan,” which is a direct phonetic translation of the Dayak words meaning “person” and “forest.” He wrote that these creatures hid in the forest because they were once humans transformed into beasts. Specimens were brought back to England and other countries to fill up the royal menageries and their sight excited the crowds of the time. However, it was not until the 1960’s that serious studies of the species in the wild began.

Orangutans share 97% of their DNA with humans.

Communication
Infants whine and attract their mother’s attention with calls that let her know they need food or help. Some females make unique sounds aimed at reassuring their offspring. It is believed that the sounds are passed down from mother to daughter.

Dominant males can produce a “long call” thanks to their throat pouches, which they use to attract females and tell other males to keep their distance (a sort of mediation tool to avoid conflict).

Baring of teeth and shaking branches clearly communicates to a rival that he needs to back off. Males also push tree snags (dead or dying trees) over to make noise as part of dominance displays. Both males and females sometimes make a “kiss squeak” sound to threaten another individual.

Female encounters are usually amicable. Adolescent females are the most gregarious. They groom adult males and each other. The behavior of a female changes at the birth of her first offspring when she becomes less tolerant of others.

Reproduction and Family
Female orangutans mate with the dominant male in their territory. They have their first offspring at around 15 years of age and give birth every five to seven years. They are most fertile when fruit is abundant, as their bodies require a lot of calories. After a female is impregnated, the male usually leaves.

A rosy-faced baby is born after a gestation period of 227 to 270 days (7.5 to 9 months) and weighs 3 lbs (1.3 kg). Twins are extremely rare. The youngster will grow to be 33 lbs (15 kg) by the age of three. Baby orangutans are very dependent on their mothers for a long time. They spend at least seven years with her. For the first three years of their lives, they need constant physical contact with their mothers. This is probably why infant mortality is very low in the wild. Young are nursed through the age of five and become independent when they reach the ripe age of eight. 

Males become sexually mature at 15 and leave to be on their own. Females become sexually mature at 11, but remain with their mothers a few more years to learn the art of motherhood.

Ecological Role
Orangutans are sometimes referred to as “the gardeners of the rainforest.” Indeed, the seeds of many of the fruit and plants they eat only start germinating after passing through their digestive system. ​Because orangutans play such an important role as seed dispersers, their disappearance is, in turn, causing the disappearance of numerous plant and tree species.

Conservation Status and Threats
Like all orangutans, Sumatran orangutans are Critically Endangered (IUCN, 2017). The International Union for Conservation of Nature Red List of Threatened Species indicates that their population has endured a considerable loss of individuals since 1985. Per the 2016 estimates, only 13,000 individuals remain in the wild.

Indonesian National Law No.5/1990 on the Conservation of Natural Resources and Ecosystems renders illegal the capture, injury, killing, ownership, and trade of Sumatran orangutans. Unfortunately, despite legal protection, the Leuser Ecosystem is threatened by logging, burning, mining, and road construction. These are all factors that contribute to habitat fragmentation. Logging is particularly attractive to organized crime because high-quality wood is sought after and trade is so lucrative. As crime organizations operate on an international scale with many partners, it is difficult for local authorities to enforce the law. Poaching is another threat. Mothers are killed so their babies can be abducted and sold for the illegal pet trade.

Commercial oil palm (Elaeis guineensis) cultivation started in Sumatra in 1911, but rapidly expanded in the last decade in the north and west of the island to replace rainforest habitats. Due to an increasing international demand for vegetable oil and biofuels, this is one of the fastest growing crops. Plantations often replace degraded forests destroyed by logging and fire, but illegal plantations are also found in protected areas. The rapid destruction of the rainforest and the many direct attacks on its population has brought the Sumatran orangutan to the brink of extinction.

Conservation Efforts
Many organizations are trying to save the rainforest and the Sumatran orangutan. For instance, orangutans rescued from the illegal pet trade are being rehabilitated and reintroduced in the wild—specifically around Bukit Tigapuluh National Park in Jambi and Riau provinces and around Jantho Pine Forest Nature Reserve, north of Aceh. The hope is to set up viable, self-sustaining populations able to reproduce and thrive to counter the dramatic loss already suffered.

Some organizations are looking into the possibility of partnering with oil palm producers to engage them into investing into rainforest protection efforts.

Community-based protection initiatives are in place, where the local population around a protected area is in charge of guarding the forest from illegal logging. 

Funds are being invested into efforts to build corridors between fragmented forested areas and allowing individual orangutans to travel, thereby promoting gene flow–a crucial factor for the survival of the species.

Gunung Leuser National Park, which supports 34.5% of the Sumatra orangutans population, is classified as a Man and Biosphere Reserve and is part of the Sumatra World Heritage Cluster Site by UNESCO.

References:

  • National Geographic website
  • Primate Behavioral Ecology – Karen B. Strier
  • The Effects of Forest Phenology and floristics on populations of Bornean and Sumatran Orangutans.   Andrew J. Marshall, Marc Ancrenaz, Francis Q. Brearley, Gabriella M. Fredriksson, Nilofer Ghaffar, Matt Heydon, Simon J. Husson, Mark Leighton, Kim R. McConkey, Helen C. Morrogh-Bernard, John Proctor, Carel P. van Schaik, Carey P. Yeager and Serge A. Wich
  • Interpol website – Patrolling Indonesia’s natural heritage
  • PNAS on line – Remotely sensed evidence of tropical peatland conversion to oil palm – Lian Pin Koha, Jukka Mietinen, Soo Chin Liew and Jaboury 
  • How will oil palm expansion affect biodiversity? – Emily B. Fitzherbert, Matthew J. Struebig, Alexandra Morel, Finn Danielsen, Carsten A. Bru ̈ hl, Paul F. Donald and Ben Phalan
  • Saving species website
  • Primate Societies – Orangutans: Sexual Dimorphism in a Solitary Species – Peter S. Rodman and John C. Mitani

Written by Sylvie Abrams, November 2017