The Sumatran orangutan (Pongo abelii) is one of the three species of orangutans. Critically endangered, and found only in the north of the Indonesian island of Sumatra, it is rarer than the Bornean orangutan but more common than the recently identified Tapanuli orangutan, also found in Sumatra.
Description
Male Sumatran orangutans grow to about 1.7 m (5.6 ft) tall and 90 kg (200 lb), while 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 and has a paler red color.
Evolution Fossil orangutans in Sumatra from the Pleistocene had similar diets to present day Sumatran orangutans, consisting mainly of soft fruit as evidenced by dental microwear.
Behaviour and ecology
Compared with the Bornean orangutan, the Sumatran orangutan tends to be more frugivorous and especially insectivorous. Preferred fruits include figs and jackfruits. It will also eat bird eggs and small vertebrates. Sumatran orangutans spend far less time feeding on the inner bark of trees. 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 with its teeth. The orangutan 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. Tools are created differently for different uses. Sticks are often made longer or shorter depending on whether they will be used for insects or fruit. If a particular tool proves useful, the orangutan will often save it. Over time, they will collect entire "toolboxes". 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.
NHNZ filmed the Sumatran orangutan for its show Wild Asia: In the Realm of the Red Ape; it showed one of them using a simple tool, a twig, to pry food from difficult places. There is also a sequence of an animal using a large leaf as an umbrella in a tropical rainstorm. As well as being used as tools, tree branches are a means of transportation for the Sumatran orangutan. The orangutans are the heaviest mammals to travel by tree, which makes them particularly susceptible to the changes in arboreal compliance. To deal with this, their locomotion is characterized by slow movement, long contact times, and an impressively large array of locomotors postures. Orangutans have even been shown to utilize the compliance in vertical supports to lower the cost of locomotion by swaying trees back and forth and they possess unique strategies of locomotion, moving slowly and using multiple supports to limit oscillations in compliant branches, particularly at their tips. The Sumatran orangutan is also more arboreal than its Bornean cousin; this could be because of the presence of large predators, like the Sumatran tiger. It moves through the trees by quadrumanous locomotion and semibrachiation. As of 2017, the Sumatran orangutan species only has approximately 13,846 remaining members in its population. The World Wide Fund for Nature is thus carrying out attempts to protect the species by allowing them to reproduce in the safe environment of captivity. However, this comes at a risk to the Sumatran orangutan's native behaviors in the wild. While in captivity, the orangutans are at risk to the "Captivity Effect": animals held in captivity for a prolonged period will no longer know how to behave naturally in the wild. Being provided with water, food, and shelter while in captivity and lacking all the challenges of living in the wild, captive behaviour becomes more exploratory in nature. A repertoire of 64 different gestures in use by orangutans has been identified, 29 of which are thought to have a specific meaning that can be interpreted by other orangutans the majority of the time. Six intentional meanings were identified: Affiliate/Play, Stop action, Look at/Take object, Share food/object, Co-locomote and Move away. Sumatran orangutans do not use sounds as part of their communication, which includes a lack of audible danger signals, but rather base their communication on gestures alone. In 2024, a wild Sumatran orangutan, called Rakus, was observed applying a paste made from chewed Fibraurea tinctoria leaves to a facial wound, a treatment which appeared to heal the wound weeks later.
Life cycle
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