The Sumatran orangutan (Pongo abelii) is one of the 2 species of orangutans. Found only on the island of Sumatra, in Indonesia, it is rarer than the Bornean orangutan. They grow to about 1.4 m (4.6 ft) tall and 90 kg (200 lb) in males. Females are smaller, averaging 90 cm (3.0 ft) and 45 kg (99 lb). Compared to the Bornean species Sumatran orangutans are thinner and have longer faces; their hair is longer with a paler red color.
Wild Sumatran orangutans in the Suaq Balimbing swamp have been observed using tools. An orangutan will break off a tree branch that is about a foot long, snap off the twigs and fray one end. It then will use the stick to dig in tree holes for termites. They will also use the stick to poke a bee’s nest wall, move it around and catch the honey. In addition, orangutans use tools to eat fruit. When the fruit of the Neesia tree ripens, its hard, ridged husk softens until it falls open. Inside are seeds that the orangutans enjoy eating, but they are surrounded by fiberglass-like hairs that are painful if eaten. A Neesia-eating orangutan will select a five-inch stick, strip off its bark, and then carefully collect the hairs with it. Once the fruit is safe, the ape will eat the seeds using the stick or its fingers. Although similar swamps can be found in Borneo, wild Bornean orangutans have not been seen using these types of tools.
The Sumatran orangutan is more social than its Bornean counterpart; groups gather to feed on the mass amounts of fruit on fig trees. However, adult males generally avoid contact with other adult males. Subadult males will try to mate with any female, though they probably mostly fail, since mature females are easily capable of fending them off. Mature females prefer to mate with mature males. Male Sumatran orangutans sometimes have a delay of many years in the development of secondary sexual characteristics, such as cheek flanges and muscle mass.
Sumatran orangutans are endemic to Sumatra island and particularly restricted to the north of the island. In the wild, Sumatran orangutans survive in the province of Nanggroe Aceh Darussalam (NAD), the northernmost tip of Sumatra. The primate was once more widespread, as they were found more to the south in the 19th century such as in Jambi and Padang. There are small populations in the North Sumatra province along the border with NAD, particularly in the Lake Toba forests. A survey in the Lake Toba region found only two inhabited areas, Bukit Lawang (defined as the animal sanctuary) and Gunung Leuser National Park. The species has been assessed as critically endangered on the IUCN Red List since 2000. It is considered one of “The World’s 25 Most Endangered Primates.”
A survey in 2004 estimated that around 7,300 Sumatran orangutans still live in the wild. Some of them are being protected in five areas in Gunung Leuser National Park; others live in unprotected areas: northwest and northeast Aceh block, West Batang Toru river, East Sarulla and Sidiangkat. A successful breeding program has been established in Bukit Tiga Puluh National Park in Jambi and Riau provinces.
Orangutans have 48 chromosomes. The Sumatran orangutan genome was sequenced in January 2011, based on a captive female named Susie. The researchers published less complete copies from 10 wild orangutans, five from Borneo and five from Sumatra.
The genetic diversity was found to be lower in Bornean orangutans (Pongo pygmaeus) than in Sumatran ones (Pongo abelii), despite the fact that Borneo is home to six or seven times as many orangutans as Sumatra. The comparison has shown these two species diverged around 400,000 years ago, more recently than was previously thought. The orangutan genome also has fewer rearrangements than the chimpanzee/human lineage.