Physical description 

Orang-utans have a characteristic ape-like shape, shaggy reddish fur and grasping hands and feet. Their powerful arms are stronger and longer than their legs and can reach 2m in length, long enough to touch their ankles when they stand.

There are two different types of adult male orang-utan: flanged and unflanged. Flanged males have prominent cheek pads called flanges and a throat sac used to make loud verbalizations called 'long calls'. They also have a long coat of dark hair on their back.

The unflanged male looks like an adult female. Both reproduce and an unflanged male can change to a flanged male for reasons that are not yet fully understood. Orang-utans are the only primate in which this biological phenomenon occurs.

Male orang-utans can tip the scales at 90 kg, while females weigh between 30-50 kg.

Social structure
Orang-utan means 'person of the forest' in the Malay language. They live in primary and secondary forests. Although they can occur up to 1,500m above sea level, most are found in lowland areas and prefer forests in river valleys or floodplains.

Orang-utans travel by moving from one tree to another, and usually avoid climbing down to the ground. But when they do, they move on 'all fours', placing their clenched fists on the ground.

Orang-utans make a nest of vegetation to sleep in at night, and rest in smaller nets during the day.

Adults are generally solitary, although temporary aggregations are occasionally formed. Males' large home ranges overlap with the ranges of several adult females. Adult males are generally hostile to one another, although they do not display territorially.

Life cycle
Orang-utans can live up to 50 years in the wild. Females first reproduce between 10-15 years of age. They give birth at most once every 5 years, and the interval between babies can be as long as 10 years.

Orang-utans usually give birth to a single young, or occasionally twins. Orang-utans stay with their mothers for the first 7-11 years of their life. An infant rides on its mother's body and sleeps in her nest until it is able to survive on its own.

The long time taken to reach sexual maturity, the long interbirth periods, and the fact that orang-utans normally give birth to just a single young mean that orang-utans have an extremely low reproductive rate.

This makes orang-utan populations highly vulnerable to excessive mortality, and means that populations take a long time to recover from population declines.

About 60% of the orang-utan's diet includes fruit, such as durians, jackfruit, lychees, mangosteens, mangoes and figs. The rest comprises young leaves and shoots, insects, soil, tree bark, woody lianas, and occasionally eggs and small vertebrates. They obtain water not only from fruit, but also from tree holes.
 

Three species

There are three species of orang-utan - the Bornean, Sumatran and Tapanuli - which differ a little in appearance and behavior. The Bornean and Sumatran species have shaggy reddish fur, Sumatran orangutans have longer facial hair.

Sumatran orang-utans are also reported to have closer social bonds than their Bornean cousins.

The Sumatran orang-utan is almost exclusively arboreal. Females virtually never travel on the ground and adult males do so only rarely. This is in contrast to Bornean orang-utans, especially adult males, which more often descend to the ground.

A third species of orangutan was announced in November, 2017. With no more than 800 individuals in existence, the Tapanuli orangutan is the most endangered of all great apes. This new third species lives in North Sumatra, but is genetically and behaviorally distinct from the two other species.

An international team of scientists described the new species in Current Biology. The Tapanuli orangutan (Pongo tapanuliensis) is distinguished from other orangutan populations based on morphological and genomic evidence. The new species is endemic to 475 square miles of upland forest in the Batang Toru Ecosystem of Sumatra and is believed to have been isolated from other orangutan populations for 10,000-20,000 years.

Population & distribution

Orang-utans were once distributed widely across Southeast Asia, roaming as far north as southern China, and as far south as the Indonesian island of Javar. But today Asia's great ape is confined to just two islands, Borneo and Sumatra.

As the orang-utan's range has decreased so have its numbers. A century ago, there were probably 230,000 orang-utans - around four times as many as there are today.

Their dense forest home makes it difficult to determine population sizes, but the Bornean orang-utan is estimated to number around 104,000 individuals, while there are under 14,000 Sumatran orang-utans. 

The Sumatran orang-utan is now restricted to the north of Sumatra. It depends on high-quality primary forests, and is less able to tolerate habitat disturbance than Bornean orang-utans. Sumatran orang-utan densities reportedly fall by up to 60% with even selective logging.

The population is currently fragmented into 13 populations in 21 forest blocks. Only 6 of these populations boast more than 250 animals and are therefore regarded as viable in the long term, but even these groups are under threat due to ongoing habitat loss.

The Bornean orang-utan was once distributed throughout large areas of Kalimantan (Indonesia) and Sarawak and Sabah (Malaysia) on the island of Borneo. But its population has fallen by more than 50% over the past 60 years, while at least 55% of its habitat has disappeared over the last 20 years.

Most Bornean orang-utans are now found in Kalimantan, especially along the east coast. The majority of wild populations are located outside of protected areas, in forests that are exploited for timber production or are in the process of being converted to agriculture.

Three subspecies live in different parts of the island:

• Northwest Bornean orang-utan is the most threatened subspecies with over 3,000 remaining.
• Northeast Bornean orang-utans are the smallest in size. Around 16,000 live in Sabah and parts of eastern Kalimantan.
• Central Bornean orang-utans are the most common subspecies with around 35,000 surviving.
   

What are the main threats?

Habitat loss is by far the greatest threat to orang-utans. Huge tracts of forest have been cleared throughout their range and the land used for agriculture, particularly palm oil - a product that is found in more than half of packaged products in supermarkets around the world.

Road development, illegal timber harvesting and unsustainable logging, mining and human encroachment also contribute to habitat loss, degradation and fragmentation.

Today, more than 50% of orang-utans are found outside of protected areas, in forests under management by timber, palm oil and mining companies.

But even protected areas are not secure since the boundaries of protected areas in Borneo are often not clearly delineated, which makes them difficult to patrol. Furthermore, many parks are understaffed and underfunded. Consequently, oil palm companies and logging firms have encroached into all the parks.

Along with habitat loss, young orang-utans up to the age of seven are sought after for the illegal pet trade. When infants are targeted, usually the mother is killed so this trade represents a real threat to wild orang-utan populations.

In addition, orang-utans are hunted in some areas for food. They are also sometimes killed when they move into agricultural areas and destroy crops.

And fire is also a major threat. In 1997-98, the drainage of peat-swamp forest contributed to uncontrollable fires in Kalimantan, which lasted for 6 months and killed up to 8,000 orang-utans.