Bornean Orangutan

Quick Take

• Orangutans share so much DNA with humans that their Malay name literally calls them something else entirely, a fact that changes how you see them. Explore their name's origin →
• The world's largest tree-dwelling animal is incapable of jumping, so how does it move through the canopy at all? See their unique limb mobility →
• Bornean orangutan mothers keep their offspring dependent for nearly a decade, and the reason goes far beyond protection. Discover the long dependence period →
• Their population has collapsed by a staggering percentage in just 100 years, and the leading cause isn't what most people assume. See the population collapse →

Since their arrival in south-east Asia, modern humans have been fascinated by this slow-moving and majestic mammal, with their name in the native Malay language meaning “man of the forest” or “forest people”. Bornean orangutans are remarkably similar to us both in their appearance and behavior, with these highly intelligent creatures having evolved perfectly to their tropical, tree-dwelling lives.

However, the hunting of the Bornean orangutan and the deforestation of its once vast natural habitat have led to drastic declines in their population numbers, particularly over the past century. Adults are often killed if they are seen in or close to plantations when looking for food, and their young are captured to be sold into the exotic pet trade, despite their heavy legal protection.

Classification and Evolution

The Bornean orangutan is a large primate species found in the humid tropical jungles of Borneo. Also known as the Red Ape, the Bornean orangutan is the third-largest ape species in the world (after the western gorilla and eastern gorilla) and is the largest tree-dwelling animal on the planet. Although the Bornean orangutan is closely related to the great apes found in Africa, they differ greatly in their behavior with the biggest difference being that the Bornean orangutan is largely solitary where other ape species tend to adopt complex social hierarchies.

Until recently, there was thought to be only one orangutan species, but recent genetic research has shown that there are, in fact, three species of orangutan, which are the Bornean orangutan (Pongo pygmaeus), the Sumatran orangutan (Pongo abelii), and the Tapanuli orangutan (Pongo tapanuliensis), which is slightly smaller in size and tends to be more sociable than its larger cousin. There are considered to be three subspecies of the Bornean orangutan, which, although similar in appearance, are distinguished by the areas of the island they occupy. They are the Northwest Bornean orangutan (Pongo pygmaeus pygmaeus), the Southwest Bornean orangutan (Pongo pygmaeus wurmbii), and the Northeast Bornean orangutan (Pongo pygmaeus morio).

Anatomy and Appearance

The Bornean orangutan is covered in patchy red or orange hair, with its coarse grey skin being visible in some places. As they are largely arboreal mammals, the Bornean orangutan has a number of adaptations that help it in the canopy, including having feet that can grip as effectively as its hands, and arms that can grow to a span of more than two meters and are actually one and a half times longer than their legs.

The face of the Bornean orangutan is bare and displays the gray or black color of its skin. Mature males also develop fleshy cheek pouches, which are made up of fat deposits under the skin, along with throat pouches that are able to produce a deep, resonant sound through the forest. Both male and female Bornean orangutans are specially adapted for opening and eating fruits, with dexterous hands and feet for peeling and large flat teeth that help to grind down harder seeds and tree bark.

Distribution and Habitat

The Bornean orangutan is natively found on the island of Borneo, where it inhabits areas of dense primary forest, mainly in the lowlands and valleys. Although they can be found at elevations of up to 4,921 feet (1,500 meters), they tend to prefer the lush slopes further down due to the higher abundance and variety of food.

Although the Bornean orangutan (and indeed the Sumatran orangutan) were once widespread throughout south-east Asia, they are today confined to the two islands after which they are named, but are seriously vulnerable in their natural habitat due to logging and forest fires which have decimated much of their once vast native regions. Because the Bornean orangutan spends nearly all of its life high in the tree canopy, they rely on these dense and rich jungles to survive, and with ever less of their natural habitat remaining, they are being forced into smaller and more isolated regions away from the increasing levels of human activity.

Behavior and Lifestyle

Unlike the other great ape species, the Bornean orangutan is largely solitary with the exception of mother and infant pairs and the gathering of a number of individuals around heavily laden fruit trees. Bornean orangutans spend much of their days sitting and eating in the canopy before constructing a nest by folding leafy branches over, where they sleep at night. Bornean orangutans move slowly through the steamy forests, and, as they are too heavy to jump, they rely on swinging tree branches back and forth until they can get close enough to grab onto the next one.

Despite only traveling just over half a mile a day, Bornean orangutans can occupy vast home ranges that are loosely shared with other individuals, with males having breeding rights with any female that enters their patch, marking their presence with deep, loud calls which echo through the trees.

Reproduction and Life Cycle

The Bornean orangutan is known to breed year-round, but these slow-developing and maturing animals are not able to reproduce until they are often nearly 15 years old. After a gestation period of approximately 8 to 8.5 months, the female Bornean orangutan gives birth to a single infant, which clings to its mother’s fur and is completely dependent on her for around 7 to 9 years.

Although young Bornean orangutans begin to transition to solid foods at around three to four years old, offspring continue to shadow their mother to learn what to eat and where to find it, and for safety. Eventually leaving her to establish their own territories when they are at least eight years old, young females tend to remain close to their mother, whereas young males can roam the forest for a long time before they finally find a patch of their own.

Bornean orangutans tend to live for up to 35–45 years in the wild but can reach around 60 years of age in captivity.

Diet and Prey

The Bornean orangutan is technically an omnivorous animal, which means that they find and eat a variety of both plant and animal matter. Despite this, Bornean orangutans love fruit, with around 60% of their diet comprising a wide variety of ripe and unripe fruits, including mangoes, durian, figs, and lychees. In the tropics, though, different trees bear their fruits at different times of the year, so Bornean orangutans must remember where the fruit trees are and when they ripen.

In order to supplement their diet, though, Bornean orangutans also eat a range of plant matter including young shoots and leaves along with insects, eggs, and small vertebrates such as lizards on occasion. Although Bornean orangutans are known to congregate together in areas with a rich and abundant food supply, one reason they are so solitary is that living together in one area would leave simply not enough food to go around.

Predators and Threats

Historically, Bornean orangutans would rarely come down to the ground for fear of being preyed upon by large carnivores like tigers, bears, and clouded leopards, but with the extinction of the tiger, particularly throughout much of the island, males are known to spend around 5% of their time on the forest floor.

However, 40,000 years ago, a new threat emerged in the form of modern humans, who hunted the Bornean orangutan to local extinction across much of their once vast natural range. Today they are protected by law, but infants are still captured for the exotic pet trade, and the mothers are often killed in the process.

The biggest threat, though, to Borneo’s remaining orangutan populations is habitat loss in the form of deforestation for logging or to clear land for agriculture, primarily to make way for palm oil plantations.

Interesting Facts and Features

The sound that male Bornean orangutans make to mark their territories comes from their developing throat pouch and is so booming that it can travel for more than a mile throughout the forest, to not only attract females but also to intimidate rival males.

Bornean orangutans are unique amongst great apes as their arboreal lifestyle has led to their limbs having more mobility than other species, allowing them to negotiate around tricky branches more easily.

Similarly, like other great ape species, Bornean orangutans have been observed using tools to help them in their daily lives. Depending on where the individual lives, skills are passed on to infants by their mothers, with sticks being used to extract termites and honey, and even large leaves are used as a form of umbrella to keep the Bornean orangutan dry.

Conservation Status and Life Today

Today, the Bornean orangutan is listed by the IUCN as a species that is Critically Endangered in its natural environment and is therefore severely threatened by extinction in the near future. The loss of their historic, unique, and incredibly biodiverse habitats has led to a 92% drop in their population over the past 100 years, and if things continue as they are, it will not be long before they have disappeared from the wild forever. Rehabilitation programs exist throughout the island and so far seem to be having relative success, but with deforestation (now even in protected areas) for logging and clearing land to make way for palm oil plantations, population numbers can only be expected to continue to fall.