Sumatran orangutan

Extremely sensitive to habitat disturbance

In addition to having longer facial hair, the Sumatran orangutan has a few behaviorial and biological differences from the Bornean orangutan as well.

This species particularly depends on high-quality primary forests, and is less able to tolerate habitat disturbance than Bornean orangutans. Sumatran orangutan densities reportedly plummet by up to 60% with even selective logging.

Models based on logging rates and the effect on orangutans predict that continued habitat loss, together with hunting, will cause population declines of 97% over the next 50 years – and eventually push the species to extinction.

Social structure
Sumatran orangutans are reported to have closer bonds than their Bornean cousins. This has been attributed to mass fruiting of the fig trees, where large groups come together to feed.

Adult males are usually solitary while females are accompanied by offspring. Group size averages only 1.47 and both sexes have been observed to live in home ranges of 2-10 km², with considerable overlap. Both male and female adults form temporary associations with immature individuals.

The Sumatran orangutan is almost exclusively arboreal. Females virtually never travel on the ground and adult males do so only rarely. This is in contrast to Bornean orangutans (especially adult males) which more often descend to the ground. 

Life cycle
The youngest females reproduce at 10-11 years of age, but the average age of first reproduction is around 15 years of age. The interbirth interval for Sumatran orangutans is estimated to be as short as 6 years and as long as 10. 

Diet
The diet of Sumatran orangutans includes more pulpy fruit and figs compared to that of Bornean orangutans.

Population & distribution

Previous population & distribution
Historically, the Sumatran orangutan was distributed over the entire island of Sumatra and further south into Java.

Current population & distribution
The species' range is now restricted to the north of Sumatra, and it has been confirmed that orangutans do not inhabit the majority of the areas south of Lake Toba where they were previously thought to exist. 

The total population has significantly declined over the past few decades. For example, from 1992-2000, the population is considered to have declined by more than 50%.

The total population is currently estimated at about 7,300 orangutans, distributed in 13 populations over 21 forest blocks.

Only 7 of these populations have prospects of long-term viability, with an estimated 250 or more individuals, and only 3 contain over 1,000 orangutans. These 3 populations are found in the Leuser Ecosystem – one of the largest tracts of forest in the northern part of the island, and facing numerous human pressures (see below).

Six of the 7 viable populations are believed to be losing 10-15% of their habitat each year to logging, and it is expected that these populations will rapidly decline (see below).

Where populations are smaller, such as in West Batang Toru, and the estimated rate of habitat loss is relatively low (2% annually), numbers may persist longer than other populations if current conditions do not change.

What are the main threats?

Habitat loss and degradation
Orangutan habitat in north Sumatra is being lost at an extremely high rate, mainly due to logging and forest conversion to oil palm plantations and other agriculture.

Between the beginning of the last centruy and 1997, forest cover is estimated to have fallen from around 82,000km² to 26,000km². There are no signs that the trend has abated in the years since then.

Much of the remaining forest may not be suitable for orangutans or is degraded to some extent.

An ill-conceived plan to build a major road through the Leuser Ecosystem particularly threatens one of the largest-remaining expanses of Sumatran orangutan habitat. Not only will the road fragment the forest, but will open access to illegal logging activities and human settlements. The project is progressing despite proof that conserving the region creates the conditions for long-term sustainable development. Furthermore, because Environmental Impact Assessments (EIAs) were not carried out in some cases, the sections of the road that cross through protected areas are illegal.

• More on habitat loss and degradation

Hunting
In the Batang Toru and east Sipirok areas, it has been reported that orangutans are hunted for food.

Heading towards extinction?
If logging and hunting are stopped, only populations of 250 or more orangutans will have prospects of long-term viability. However, if habitat loss is not stopped, then Sumatran orangutan populations may decline by 50% in about a decade, by 97% in 50 years, and will eventually go extinct.

You can help

• Adopt an orangutan through WWF-UK or WWF-US and support our work to protect orangutans (international adoptions possible)

• Buy sustainable wood, paper and palm oil. By purchasing certified sustainable palm oil and FSC-certified forest products, consumers, retailers, traders, and manufacturers help protect orangutan habitat by limiting illegal logging and forest conversion to oil palm plantations. 
   
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