Native to tropical rainforests and mossy mountain areas, orangutan is the largest arboreal mammals on Earth and spend approximately 90% of their time in trees. Coincidentally, the word “orangutan” is said to translate to “person of the forest” in Malay, which, despite their tree-dwelling disposition, is rather appropriate since orangutans and humans share 96.4% of their DNA.
Species: Pongo pygmaeus (Bornean orangutan), Pongo abelii (Sumatran orangutan), Pongo tapanuliensis (Tapanuli orangutan)
Subspecies: Pongo pygmaeus pygmaeus (Northwest Bornean orangutan), Pongo pygmaeus wurmbii (Southwest Bornean orangutan), Pongo pygmaeus morio (Northeast Bornean orangutan)
IUCN Status: Critically Endangered
Population: Approximately 104,000 (Bornean), 13,000 (Sumatran), 800 (Tapanuli)
The genera Pongo consists of three species of orangutan, each classified under the great ape family: the Bornean orangutan; the Sumatran orangutan; and the Tapanuli orangutan. Of the three species, only the Bornean orangutan is further divided into three separate subspecies, in accordance with their geographical location within Borneo. The Tapanuli orangutan, which was identified as a distinct species in 2017, is found exclusively in the Northern Sumatran region of Batang Toru, a forested area of incredible biodiversity. Although all three species share certain physiological and behavioural similarities, geographical distribution has inevitably influenced their evolutionary development in distinct ways.
By virtue of their classification as great apes, all three species of the orangutan have certain physiological features in common: hair as opposed to fur; nails instead of claws; opposable thumbs and thus prehensility; padded digits with unique fingerprints; comparatively large skulls, brains, and bodies; a high brain-to-body size ratio; and no tail. Additionally, arboreal primates, such as the agile orangutan, have incredibly long arms (almost one and a half times longer than the rest of their body), as well as lengthy, narrow, curled hands and feet. Such morphological adaptations allow for their unique method of movement called ‘quadrumanous scrambling’: due to the sheer weight of orangutans, brachiation (swinging from tree to tree) is no simple feat, so they must use their feet as well as their arms to reach for nearby branches and carefully swing across.
Despite these commonalities, distinguishing orangutans from their fellow great apes is a relatively straightforward task, on account of their unique reddish-orange tinted hair and wide-set faces. Yet, between each orangutan species there exist slight aesthetic differences, which are only amplified by the pronounced sexual dimorphism present in all three species.
The hair colour of the Bornean orangutan tends to lean towards a darker shade of red, with a coarser texture. Males are substantially larger than females, with males at an average height of 97cm and a weight of 60 to 90kg, and females at a height of 78cm and a weight of 40 to 50kg. Females typically have long, narrow faces with medium or short facial hair, as do adolescent males. Fully matured males, on the other hand, have much wider faces as they develop ‘flanges’ (flaps of fatty tissue on the cheeks) and large laryngeal sacs (throat pouches). These appendages assist sexually-developed males in producing specialised vocalisations, which are long in duration and can carry over half a kilometre, to attract potential mating partners. Males will usually develop these alluring accessories after approximately 20 years, as testosterone levels gradually increase.
Flanged male orangutan. Photo by Joshua J. Cotten/Unsplash
The hair colour of the Tapanuli orangutan is somewhere in between its two counterparts, with a cinnamon-like complexion, and a thick, frizzy texture. Female and male body sizes are similar to those of the tall and lanky Sumatran orangutan, yet two significant morphological differences found in the Tapanuli orangutan are what led to its classification as a distinct species: a smaller head, with narrower eye sockets; and wider canines. Females have a lengthy face with short facial hair, whilst unflanged males sport a long beard and a distinctive moustache. Flanged males have a slightly wider face than that of the Bornean and Sumatran flanged male, and have a noticeably greater quantity of hair around the mouth and head.
2. Foraging and Feeding Habits
The foraging and feeding habits of each orangutan species vary due to the differences in vegetation, prey and predators found within each habitat. However, a common trait amongst all three is the predominantly frugivorous nature of their diet, with fruit constituting 60% of their daily calorific intake. Figs, mangoes, litchis, jackfruits, and durians are popular choices among orangutans, yet they are also known to consume leaves, shoots, termites, ants, insects, birds eggs, caterpillars, and flowers. Tree bark and soil, which are rich in nutrients that are otherwise absent from their predominantly fruit-based diet, are also a resource exploited by orangutans. Due to the fact that the Tapanuli orangutan inhabits forests at a much higher altitude than its two counterparts, it has been known to consume a number of tree species which were not previously documented as orangutan food. Regardless of the disparity in size between the two genders, both females and males eat approximately 8,500 calories per day.
Like many other forest-dwelling primates, orangutans exhibit an extensive understanding of the endemic species of plants and vegetation within their habitat, thereby avoiding poisonous types and utilising those with medicinal properties. Anti-parasitic and anti-inflammatory flora provide a multitude of benefits, such as preventing diarrhoea and expelling toxins, which contribute to the relatively long life-span of orangutans (40 to 60 years in the wild). Their ability to calculate the seasonality of fruit and produce, as well as their understanding on how to consume fruits with hard shells or spikes, also aids in their survival. This knowledge is acquired through social learning, since young orangutans stay with their mothers for seven to 15 years after birth in order to learn how to survive solitarily.
Tool-use has also been widely observed amongst orangutans when foraging and feeding, as well when building nests for sleeping. Common techniques include: the use of a stick to lure termites and ants from tree holes; using leaves as make-shift gloves when handling spiked fruits; fishing high-hanging fruit out of trees using branches; and using sponges fashioned from chewed-up leaves to extract water from tree cavities. However, due to their propensity for lingering in the canopy, some argue that orangutans lack the proficiency in tool-use that they perhaps possessed millions of years ago when they spent more time on the forest floor. As Bornean orangutans venture more regularly onto ground level, they are thought to utilise a greater variety of resources for foraging when compared to the Sumatran orangutan, who must remain watchful of Sumatran tigers.
Female orangutan with her infant. Photo by Justin Bland/Unsplash
3. Social Behaviour
In stark contrast to the majority of primate species, orangutans are amongst the most solitary members of the great ape family, presumably due to three concomitant factors: the frugivorous feeding preferences of the orangutan; the relatively wide, seasonal, and often scarce distribution of food; and the lack of arboreal predators. When ripe fruit is abundant, groups of orangutans have been observed feeding together and socialising without incident. However, once food is scarce, individuals venture out solitarily in search of sustenance and resources. As a result, males orangutans tend to disperse and inhabit large territories, spanning up to 30 square kilometres, once sexual maturity is reached. Females, on the other hand, remain in a modest area of approximately one square kilometre with their offspring.
Within territorial ranges, flanged males often exhibit aggressive behaviour towards fellow flanged males, producing consistent grumbling calls throughout the day to avoid encounters, whilst unflanged males are typically regarded as docile and non-threatening. Adult females are thought to maintain social structures of greater complexity, with groups of females and their respective offspring being frequently observed in the wild. Social interactions between fully-matured males and both adolescent and adult females are normally limited to sexual intercourse, either reciprocal or forced, as males playing no part in the rearing of offspring.
One of the most fascinating characteristics of orangutans, and amongst the most renowned features of great apes, is their incredible intellect. From their advanced use of tools, remarkable memory retention skills, and ability to adopt sign language, to their capacity to learn through observation, numerous primate species have proven to be far more intelligent than ever imagined. However, behavioural studies conducted in recent years on wild orangutans have begun to shed light on the true extent of the orangutan’s intellectual capacity, arguably setting the orange-tinted titans a cut above the rest.
In 2018, a postdoctoral student at the University of St. Andrews, studying orangutan alarm calls in the Ketambe forest of Sumatra, noticed two distinct warning strategies adopted by female orangutans when spotting predators. The first, which approximately half of the test subjects opted for, was to emit a loud “kiss-squeak” sound in order to warn fellow orangutans of potential danger. The second, which greatly intrigued researchers, was to remain quiet so as to avoid drawing attention; after approximately seven minutes, with the predator no longer in sight, the originally silent orangutans suddenly emitted a warning call. Scientists interpreted the action as serving two purposes: firstly, to warn fellow orangutans that the predator may still lurk beneath the canopy; and secondly, to teach infants and adolescents about the danger that just passed. This ability, displaced referencing, is a skill previously believed to be unique to humans. It is the capacity to discuss events or objects that are not occurring or physically present in time. This feat constitutes a significant step in the evolutionary development of the orangutan language, as it appears to be a controlled, calculated behaviour as opposed to a reflex or conditioned response.
5. Environmental Services
In addition to their unique appearance, expertise in botany, peculiar social habits, and remarkable intelligence, orangutans also play an important role in maintaining the vitality and robustness of their surrounding ecosystem. Often described as “gardeners of the forest”, orangutans inadvertently aid in daily seed dispersal by virtue of their predominantly fruit and plant-based diet. As a matter of fact, many of the seeds they consume would not germinate successfully without a trip through an orangutan’s digestive system. The health and regeneration of rainforests in Borneo and Sumatra therefore rely relatively heavily on stable orangutan populations. This is especially true of Tapanuli orangutans since their native habitat, the Batang Toru forest, has already suffered from extensive environmental degradation and loss of biodiversity. If the current declining population trend of orangutans continues, numerous species of flora and fauna will perish alongside their humble, frizzy-haired gardeners.
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Palm oil plantation in Malaysia. Photo by Nazarizal Mohammad/Unsplash
Despite the indispensable services they provide for their surrounding environment, all three species of orangutan are currently listed as ‘Critically Endangered’ under the International Union for Conservation of Nature (IUCN) Red List.
Sumatran orangutans are reported to have lost approximately 60% of their key habitat between 1985 and 2007, largely due to logging, mining, agriculture, and infrastructure. As one of the major producers of palm oil, Indonesia is home to over 60,000 square kilometres of palm plantations, with Kalimantan and Sumatra accounting for 96% of Indonesia’s oil yield. Rampant deforestation within areas of supposed conservation and environmental protection, as well as the construction of 2,370 square kilometres of illegal plantations, has forced more than 50% of the orangutan population into unprotected forests owned and managed by timber, oil and mining companies. Expelling orangutans from their familiar, native habitat not only exposes populations to malnutrition and starvation, but also leaves them vulnerable to poaching, as well as conflicts with humans over crops. Forests not yet fated for complete demolition are still at risk of logging, which causes an estimated population decline of approximately 60% regardless of selectivity. Additionally, extensive and continuous political conflict in the Aceh Province of Sumatra has caused an increase in instances of illegal land use, encroachment and illegal settlement in ecologically protected areas, such as the Leuser Ecosystem. Land-use estimates indicate a potential loss of 4,000 Sumatran orangutans by 2030, and a population decrease of 81% by the year 2060.
Tapanuli orangutans have suffered a similar loss of 60% of their natural habitat since 1985, with mining and agriculture accounting for the majority of deforestation and fragmentation. Despite the government-mandated protection of prime orangutan forest habitat in 2014, an unidentified company currently owns a logging permit for 300 square kilometres of said land, and gold mines occupy a large part of the remaining habitat range. In 2015, a novel threat arose in the form of a USD$ 1.6 billion hydroelectric power plant project, located in the area of highest orangutan density within Batang Toru. It is estimated that approximately 10% of the Tapanuli orangutan population will be directly affected by the project upon its completion, with fragmentation of forest corridors potentially affecting much more. Quantitative population estimates indicate a loss of around 700 Tapanuli orangutans since 1985, from 1,489 individuals to the current 800. By 2060, researchers estimate a population decline of 83%, with a mere 257 individuals remaining.
The Bornean orangutan population lost over 100,000 individuals between 1999 and 2015, mainly due to two key threats: the destruction and fragmentation of their habitat through logging, agriculture and forest fires; and the illegal hunting of orangutans for bushmeat, the pet trade, or from human conflicts. As with their Sumatran counterparts, Bornean orangutans suffer the consequences of pervasive deforestation, largely caused by the prolific emergence of palm oil plantations; 39% of forests in Borneo were destroyed between 1973 and 2010, with a projected loss of 220,000 square kilometres of forest between 2010 and 2030. Approximately 61.5% of original orangutan habitat will be destroyed by 2025.
Although Bornean orangutans are comparatively better equipped to tolerate habitat disturbance, haphazard logging has degraded 59% of orangutan habitat since 1979. A similarly calamitous matter, one which lacks sufficient media attention, is the draining and burning of peat-swamp forests in Borneo and across Southeast Asia. In addition to contributing as much as 3% of global carbon dioxide emissions, this process often gives rise to uncontrollable fires which quickly spread to surrounding forests, endangering the lives of numerous species. In 1997, a forest fire in Kalimantan, caused by the drainage of a peat-swamp, lasted six months and took the lives of 8000 orangutans. Lastly, the illegal hunting of orangutans in Borneo has contributed to approximately 12% of the population decline between 1973 and 2025. In Kalimantan alone, 2,256 orangutans are poached each year for the pet trade in Indonesia and Taiwan, as well as for meat consumption. Increasing instances of orangutans entering agricultural plantations in search for food has also caused a spike in conflicts with humans, which often end in unreported orangutan fatalities.
Exacerbating the vulnerability of all three orangutan species is their extremely slow reproductive rate. Sumatran orangutans have the longest birth intervals of any mammalian species at 8.2 to 9.3 years, whilst Bornean orangutans give birth every 6.1 to 7.7 years. Gestation lasts approximately 254 days, and infants are weaned for seven years. Female orangutans thus only bear three to four offspring throughout their entire lifetime. With mounting anthropogenic pressures on orangutan populations, and a lengthy population recovery time, orangutans edge closer to extinction with each passing day.
7. Conservation Efforts
In recent years, the devastating effects of unrestricted palm oil production on Southeast Asia’s various environments and ecosystems has garnered extensive media coverage. Despite the unfavourable reputation it often receives, studies have demonstrated that palm oil is in fact a very efficient crop, as oil palm trees produce more oil per measure of land than any other vegetable oil crop. Although palm oil accounts for 40% of the global vegetable oil demand, plantations occupy just 6% of the total landmass currently utilised to produce vegetable oils. Therefore, if palm oil production were to cease, alternative crops would require four to 10 times the amount of land to produce the same amount of oil. The solution to palm oil-induced deforestation thus lies in establishing sustainable practices for plantations to follow and local governments to enforce.
Widely recognised as an independent standard for the sustainable production of palm oil, the Roundtable on Sustainable Palm Oil (RSPO) outlines a set of criteria that manufacturers must abide by in order to achieve their certification. These include: conservation of natural resources and biodiversity; transparency; compliance with local laws; and the responsible development of new plantations. Major corporations such as Kellogg’s, The Body Shop, Nestlé and Ferrero have begun implementing policies of zero-deforestation and full traceability, attempting to source product ingredients from reputable manufacturers.
Nevertheless, whilst this sustainability initiative is certainly a step in the right direction, rigorous scrutiny of RSPO-certified organisations is required to retain external accountability. A study carried out in 2018 by the University of Queensland and the ARC Centre of Excellence for Environmental Decisions (CEED) indicated that “no significant difference was found between certified and non-certified plantations for any of the sustainability metrics investigated”. Numerous plantations certified by the RSPO were found to have been built in primary forest areas, with little regard for preserving biodiversity or orangutan habitat. The ensuing public backlash effectively forced the RSPO to review their certification process, leading to the implementation of a more stringent assessment regime. It is therefore clear that continued public demand for transparency is incredibly important in ensuring that environmental organisations such as the RSPO remain responsible for their actions.
Aside from palm oil plantations, the primary forests of Sumatra and Borneo, as well as their numerous inhabitants, are constantly threatened by logging, local agriculture, unsustainable spatial planning, infrastructure, and poaching. Although areas such as the Leuser Ecosystem and the Batang Toru Ecosystem are officially protected under national legislation, such laws are rendered useless by a lack of surveillance and enforcement. The continued existence of orangutans therefore depends largely on the designation and protection of key forest areas, as well as forest corridors, where deforestation, logging, infrastructure development, and hunting being effectively prohibited. By selecting areas of high orangutan population density, and where resources necessary to orangutans are plentiful, instances of human-orangutan conflict can be minimised. Furthermore, due to the relatively high tolerance of Bornean orangutans to habitat disturbance, local governments can work with timber companies to perform well-planned, selective logging, whereby trees frequently utilised by orangutans are kept intact.
Community nursery training as part of Borneo Nature Foundation’s one million trees program. Photo by the Borneo Nature Foundation
Whilst public pressure on governments and international bodies occasionally leads to tremendous change, the mountain of bureaucratic hurdles involved can often depreciate the urgency of the situation. Thus, at the forefront of conservation efforts are local NGOs such as the Borneo Nature Foundation, who implement conservation and biodiversity research projects across the island of Borneo in order to protect its native species. Some of their work includes: replanting one million trees in rehabilitating forests to prevent soil erosion, restore habitats, and reduce the risk of wildfires; introducing environmental education campaigns in local villages to encourage the conservation of resources and wildlife; conducting scientific research into the ecological traits of numerous species, the impacts of habitat disturbance, and the effects of conservation efforts; and performing regular forest patrols in Sebangau National Park to prevent illegal logging, poaching, and forest fires.
What Can You Do?
- Support a local NGO. Organisations such as the Borneo Nature Foundation, Re:wild, and Saving Nature are committed to the protection and conservation of wildlife and the environment. You may donate to fund tree replanting projects, wildlife patrols, or awareness raising campaigns.
- Purchase sustainably. On your next grocery trip, try and make sure that the products you buy contain sustainable, RSPO certified palm oil. Similarly with paper or wood, have a look to see whether they come from a Forest Stewardship Council certified forestry.
- Reach out to your favourite brands. Write a letter letting them know that you’d prefer to purchase products with sustainably sourced ingredients, and get your friends to write one too!
- Cut down on your paper and plastic consumption. Bring your own thermos to your favourite coffee shop, take a Tupperware to your go-to takeaway restaurant, think before you print, recycle your notes, and use a hand towel instead of paper towels.
- Raise awareness. Acts of shameless environmental degradation, such as the draining of peat-swamp forests and the export of orangutans to Taiwan for the illegal pet market, lack the international media attention they deserve. If you read an article on the topic, share it with your friends and loved ones and find out how you can all help to make a difference.
Featured image by: Dan Dennis/Unsplash
If you want to learn more about endangered species, make sure to check out other articles from our Endangered Species Spotlight Series