The largest of the Asian primates, the orangutan, belongs to the Hominidae (or Great Apes) family whose members also include humans, chimpanzees, and gorillas. While fossil records indicate that this primate once inhabited much of Southeast Asia, its current populations are restricted only to the islands of Borneo and Sumatra. Moreover, recent genetic evidence has revealed that the Bornean orangutan (Pongo pygmaeus) is actually a different species from its Sumatran relative (Pongo abelii). Additionally, three subspecies of Pongo pygmaeus (P. p. morio, P. p. pygmaeus, and the most common P. p. wurmbii) are recognized based on the mammal’s home range distributions
Kingdom: Animalia (Animals)
Common name: Orangutan
In terms of physical description, the Bornean and Sumatran orangutans exhibit a characteristic ape-like shape, shaggy reddish fur, and long powerful arms with strong, grasping hands and feet that enable individuals to gracefully traverse the forest canopy by brachiating and grabbing hold of nearby tree branches. Both species also demonstrate similar sexual dimorphism. Adult males are distinguished by their larger size, well-developed throat pouch, and flanges, known as cheek pads, on either side of the face. Yet, there are subtle dissimilarities in body build, coat characteristics, and facial features that serve to differentiate one species from the other in addition to their geographic separation.
Although uniquely adapted for survival as the largest tree-dwelling mammal in the world, orangutan populations have been steadily declining in both range and numbers for many years. They are currently in danger of becoming extinct within the next few decades. Humans have a long history of hunting this particular primate for various reasons including food, exhibition in zoos, or illegal pet trade, but the main threat to the orangutan’s continued existence derives from loss of habitat. In the last half-century, close to 80% of orangutan territory has been exploited by illegal logging operations, gold mining, and conversion of pristine forests to permanent agricultural sites, in particular palm oil plantations. Between 1997 and 1998, it is believed that around one-third of the island’s orangutan species subsequently perished following forest fires caused by slash and burn farming techniques in Borneo.
Because this primate inhabits densely forested regions, it is difficult to determine precise population sizes; however, estimates place the current number of Bornean orangutans at approximately 55,000. Sumatran populations have fared much worse, declining as much as 50% during the 1990s, such that current censuses indicate only about 7,500 individuals remain. Taking into account that both species have extremely slow reproductive cycles and require extensive home ranges to support their dietary needs, their populations have become highly vulnerable to excessive mortality. The population will require a long time to recover even if government-sponsored protective measures can be routinely instituted and sustained.
In their native environments, Bornean and Sumatran orangutans often display a semi-solitary social organization, which is quite uncommon among great apes. Individuals usually travel alone or in small groups consisting of two females, their dependent young, and occasionally an adult male, although temporary travel bands and feeding aggregates have been observed when quantities of food are plentiful. Generally, males and females come together only to mate, at which time dominant males become polygamous. Although there is no distinct breeding season, females typically give birth only once every five to eight years due to the lengthy gestation period (of about nine months) and the prolonged maternal investment in offspring development. In contrast, adult males can afford only sporadic parental care since their reproductive success largely depends on the formation and maintenance of dominance hierarchies with other males whose home ranges overlap with their own.
With regard to their daily activities, orangutans are diurnal and almost exclusively arboreal. A typical routine begins with early morning foraging on fruit and other items such as leaves, insects and tree bark. Afterward, there is midday rest, late afternoon travel, and early evening preparation of a nighttime nest. These nests are normally constructed from branches and other vegetation obtained from the forest canopy, and they resemble platform-style beds situated 40 to 60 feet above ground.
In general, the orangutan bears an ape-like shape with high sloping forehead and bulging snout, but it is the long, shaggy reddish-brown fur that distinguishes this primate from any other member of the Hominidae (or Great Apes) family. Additionally, both males and females possess short legs that contrast sharply with an extremely long, powerful set of arms, which may extend, from fingertip to fingertip, up to 2.25 m (7.4 ft) in length. Such exaggerated upper extremities, coupled with highly flexible hips and strong, curled hands and feet enable orangutans to maneuver gracefully and quickly throughout the forest canopy where most of their lives are spent. Only occasionally does an individual descend from the trees and move about quadrapedally (or sometimes bipedally) because it is cumbersome and slow to walk on clenched fists rather than on knuckles like the other great apes do. Moreover, this awkward form of locomotion makes orangutans more susceptible to certain types of predation. However, older males may become too large to move around in trees easily and sometimes resort to walking on the ground.
Some other features common to both males and females include strong teeth and jaws for pulverizing nut shells and wood; throat pouches for vocalizing resonating sounds; and a long, thick hair covering that serves to accentuate actual body size. Additionally, newborns of both sexes have pink faces, but this coloration gradually changes to dark brown or black during the next few years.
Interestingly, both Bornean and Sumatran species exhibit a significant degree of sexual dimorphism. On average, adult females weigh approximately 30 to 50 kg (66-110 lbs) while males tend to weigh 50 to 90 kg (110-198 lbs). Also, the female’s average length of 122 cm (4.0 ft) differs from the 137 cm (4.5 ft) length of a typical male. In addition to their larger size, male orangutans display more developed throat pouches and a distinct pair of cheek pads (or flanges) composed of subcutaneous fat deposits that continue growing for much of their adult life span. Males also have somewhat longer hair than females and they display facial mustaches.
A comparison of species reveals certain subtle dissimilarities between Borean and Sumatran orangutans. Typically, Sumatran orangutans may be distinguished by their longer fur, more slender build, white hairs on the face and groin, and long beards on both males and females.
Behavior and reproduction
Adult orangutans are relatively solitary creatures, yet their dispersal patterns are partially influenced by philopatric practices. Males occupy large areas (about 10 km2 or 3.86 mi2) often incorporating one or more females that have settled near their mother's home range. While there is no strong indication that either sex exhibits territoriality, or that females’ home range proximities promote increased social bonding among family members, this distribution arrangement facilitates the temporary formation of mating consortships as orangutans travel between feeding sites. Since males establish and maintain dominance hierarchies for breeding rights, these small groups typically consist of a single, polygamous male and several sexually receptive females accompanied by their immature offspring. Meanwhile, both subadult and less dominant males exhibit a rather nomadic lifestyle until such time as they can displace a resident adult male from his home range. These younger, smaller males may also mate with females, although such copulations are usually forced and appear to occur randomly, not because the female is sexually receptive or fertile.
Generally, females attain adult size and experience menarche between the ages of six and eleven, but such development may happen sooner among heavier individuals having more body fat. The female estrous cycle is between 22 and 30 days in length with ovulation normally occurring around the 15th day. There is no genital swelling during estrous, and females have a light menstrual flow for three or four days. Young adult females may undergo a period of infertility lasting one to four years, and successful reproduction usually commences no later than age 15. Copulation is often performed with the sexual partners facing one another while hanging by their arms from tree branches. Gestation lasts from 233 to 263 days, and orangutans typically give birth to a single offspring, or occasionally twins, probably not more than once every five years. However, the inter-birth interval may be longer (i.e. ten years) if habitat quality deteriorates significantly.
Physical development of male orangutans involves a rather interesting set of events influenced by social circumstances. Sexual maturity takes place between ages eight and fifteen, such that testicles become fully descended and individuals are capable of engaging in successful intercourse. However, the appearance of the secondary sexual features of a socially mature adult (i.e., enlarged flanges, well-developed throat pouch) is commonly delayed until ages 15 to 20 whereupon development occurs rather quickly once a resident male position becomes available. Along with this physical transition comes a new mating strategy involving use of the long-call that lures receptive females to seek out dominant males. Additionally, females may associate with mature males in order to gain protection from the undesirable advances of unflanged males.
With the arrival of newborns, mothers routinely administer the bulk of parental care. Infants normally weigh about 1.5 kg (3.3 lbs) and nurse every three to four hours until they begin taking soft food from their mothers’ lips at age four months. However, the young are not completely weaned until they are about 3½ years old, and they may continue to nurse periodically during stressful periods until about age seven. Early on, a young orangutan clings to its mother’s abdomen by entwining its fingers and gripping her fur until it is a year old, then it begins to ride on her back until reaching about two or three years of age. Oftentimes, a young orangutan will scream loudly if separated from its mother, but from ages five through eghti this individual demonstrates increased interest in playing and traveling with peers, although frequent proximity to its mother is still maintained. When menarche commences, a young female will establish her own home range and begin displaying sexual behaviors in the presence of the resident male. In Borneo, where lowland and peat swamps offer significant plant diversity, female home ranges average 3.5 to 6.0 km2 (1.35 to 2.32 mi2) while Sumatran females occupy somewhat larger ranges, nearing 8.5 km2 (3.28 mi2), in mountainous regions where faunal diversity is less.
With regards to daily activities, orangutans tend to adopt a fairly predictable schedule that encompasses early morning foraging, a mid-day relaxation period, afternoon travel (i.e., to the subsequent day’s feeding site), and evening preparation of the night nest formed by bending branches into platforms situated 40 to 60 feet above ground.
In their natural habitats, average female life spans range from 45 to 55 years while males tend to live a bit longer. In captivity, this primate may live to be almost 60 years of age.
Orangutans appear to have a vocal repertoire of about 13 different sounds. For example, they may scream when scared or grind their teeth when frustrated. Males may sometimes spontaneously roar and within a small group setting, individuals often communicate with lip smacking, squeaks, and barks. At other times males emit a short-call immediately after contact or conflict with another individual has subsided. The most dramatic vocalization is the 1-2 minute long-call elicited by a flanged male to advertise his proximity to sexually receptive females and to any other males located within a one to two km (0.6-1.2 mi) range. This call also has been shown to induce increased secretion of stress hormones in adolescent and subadult males, thereby retarding the development of their secondary sexual characteristics. While these individuals tend to flee upon hearing the long-call, sexually receptive females can use the vocalization to help locate dominant males for reproductive activities.
In addition to vocal communication, orangutans are thought to employ facial expressions, gestures, body postures, and tactile demonstrations (i.e., mother-offspring grooming) to convey information. Moreover, compared to all other great apes, orangutans are capable of the most facial expressions, due to their very flexible lips. Also, both Bornean and Sumatran orangutans are exceptionally intelligent and have demonstrated the capacity to learn some complex tasks and sign language during experimental trials.
Pleistocene fossil records indicate that the orangutan was once widely distributed throughout Southeast Asia (including southern China, northern Vietnam, Laos, and Java), but today this primate is restricted only to the islands of Borneo and Sumatra where it occupies various types of lowland forests as well as mountainous areas nearing an elevation of 1500 m (4921 ft).
In Borneo, the Pongo pygmaeus species (of about 55,000 individuals) inhabits home ranges encompassing the Malaysian federal states of Sarawak and Sabah as well as the Indonesian province known as Kalimantan. Moreover, it is in this lattermost region where the majority of orangutans can be found since extensive forests still exist, especially along the eastern coast. In terms of subspecies distribution, the Pongo pygmaeus pygmaeus population is centered in the Danau Sentarum region of the island, yet it is estimated that a mere 1,500 individuals remain due to excessive logging and hunting activities occurring in that area. The main stronghold of Pongo pygmaeus morio is the Berau/Gunung Gajah population, although some individuals continue to inhabit sites within the former Kutai National Park. New evidence suggests that P. p. morio may be expressing a stronger presence than previously noted in Sabah. The largest populations in Borneo, represented by Pongo pygmaeus wurmbii, are situated in the swamp areas of Central Kalimantan, where at least 35,000 individuals currently reside. Other major strongholds of P. p. wurmbii include Tanjung Putting, Sebangau, Arut-Belantikan, Mawas, and Gunung Palung.
Historically, the Pongo abelii species was distributed over the entire island of Sumatra, but its home range is now restricted to the northern portion, and it has been confirmed that these primates no longer inhabit the majority of areas south of Lake Toba where they were previously thought to exist. Currently, there are 13 identified orangutan populations (totaling about 7500 individuals) that are dispersed among 21 forest blocks, but only three of these groups contain over 1000 individuals, and they are situated in the Leuser Ecosystem (which coincides with the former Gunung Leuser National Park). Another seven populations are believed to contain 250 or more individuals, but six of these groups are estimated to be losing habitat at the rate of 10-15% each year to logging, and these populations are expected to rapidly decline within the next few decades. Where populations are smaller, such as in West Batang Toru, and the estimated rate of habitat loss is relatively low (two percent annually), orangutan numbers may persist longer than for other groups if current habitat exploitation trends do not change.
Orangutan populations are most dense in those tropical rain forest regions of Borneo and Sumatra characterized by lowland swamps, peat swamps, and secondary (regenerated) broadleaf forests. In these habitat types, both tree diversity and food quantity tend to be greater and more stable throughout the year than in hilly or mountainous terrains. While lowland dipterocarp forests (comprised of species of the Dipterocarp family of trees) also offer a rich variety of food sources for orangutans. Their highly valued timber content makes these areas somewhat less suitable for inhabitation due to recurrent logging operations.
On the island of Borneo, orangutan populations occupy a total area of approximately 150,000 km2 (57,915 mi2) although their home ranges are scattered among various types of forests whose elevations are below 1,000 m (3,281 ft). Average annual rainfall in these regions is about 4.3 m (14.2 ft) with heaviest precipitation occurring during September and between December and May, while June through August is remarkably dry. Additionally, average annual temperatures range from 18°C (64.4°F) to 37.5°C (99.5°F).
In contrast, Sumatran orangutans inhabit only a total area of about 26,000 km2 (10,000 mi2) and their home ranges are known to include mountainous sites nearing elevations of 1500 m (4900 ft), although elevations of 200 to 400 m (600 to 1,300 ft) are preferred due to greater fruit availability. Year-round rainfall averages three m (8.4 ft) with heaviest precipitation occurring during the time periods of March-June and September-December. Average annual temperatures range from 17°C (63°F) to 34.2°C (93.5°F) and humidity approaching 100% is relatively constant.
Food and feeding habits
According to researchers, orangutans may actually consume more than 400 different food items. About 60% of the primate’s diet consists of various types of fruit such as durians, jackfruit, lychees, mangosteens, and mangoes, but figs are a mainstay. The remainder of the diet may include such diverse items as young leaves and shoots, flowers, tree bark, vines, woody lianas, roots, bird eggs, ants, caterpillars, termites, spider webs, fungi, tree sap, honey, and mineral-rich soil. Water is usually obtained from tree holes or forest canopy leaves that catch and retain frequent rainfall.
These primates frequently forage alone or in very small groups (i.e., a mother and her offspring) during early morning hours, thereby maintaining their semi-solitary lifestyle. This is in part due to the extreme seasonal variability of rainforest food supply as well as the orangutan’s high daily caloric needs. However, during times of abundance, such as the mass fruiting of fig trees (every two to ten years), orangutans have been observed to form large travel band and feeding aggregates that also permits opportunities for social interaction and bonding. During such times these primates will consume excessive calories, creating fat deposits to sustain their activity levels during periods of food scarcity.
Studies conducted on females and their young suggest that orangutans learn at a very early age where and when to obtain specific types of food and which items to avoid. These tactics seem essential to conserving valuable energy that would be wasted if individuals randomly foraged over great distances in the forest.
Threats and conservation status
Due to declines in population size, Bornean and Sumatran orangutans are classified as endangered on the World Conservation Union’s Red List of Threatened Species (2007), and they are listed on Appendix I of the Convention on International Trade in Endangered Species (CITES) which prohibits international trade. Although these primates once occupied most of Southeast Asia, their current habitats are restricted only to the islands of Borneo and Sumatra, and their home ranges continue to be exploited in present times. Current estimates of population size indicate that the number of Bornean orangutans is approximately 55,000 individuals, while only about 7500 of their Sumatran relatives remain.
By far, the most devastating influence on both species’ existence is the human destruction of their habitats by deforestation. Both the slash and burn farming technique and the establishment of permanent agricultural sites (i.e., palm oil plantations) have removed more than 80% of the Bornean orangutan’s home ranges, while the Sumatran species has been restricted to fragmented forest regions due to extensive (and oftentimes illegal) logging operations and establishment of a major road system through the Leuser Ecosystem stronghold. Moreover, such agricultural and commercial practices have significantly contributed to the destruction of millions of acres by natural wildfires that are triggered by prolonged drought. In fact, it is estimated that nearly one-third of the Bornean orangutans succumbed to the forest fires that ravaged the island between 1997 and 1998.
Humans have routinely hunted orangutans as a food source as well as for financial gain by illegal pet trade and sale of primate skulls (which can fetch up to US$70 in some villages). Additionally, these mammals are sometimes killed because they are considered to be competitors for limited fruit supplies in forests or unwanted trespassers on plantation sites.
Ecotourism has also become a more prominent factor in the demise of some orangutan groups. Close contact with wildlife reserve employees and tourists has exposed these primates to a number of human diseases (i.e., tuberculosis, pneumonia, poliomyelitis, hepatitis, malaria, cholera and measles) to which they are susceptible given their classification in the Hominidae family.
Faced with loss of habitats, orangutans have been forced to disperse in search of new home ranges, often necessitating ground travel through open areas, making them highly vulnerable to further predation. During these times of emotional and physical stress when food sources may become quite limited, female orangutans often experience lower reproductive success. Given that females normally demonstrate a long inter-birth interval, typically produce only a single offspring during any gestational period, and may not become sexually mature until age 15, such forced migrations likely contribute heavily to the steady decline in this primate’s population numbers.
Research and conservation
Efforts to conserve Bornean and Sumatran orangutan species have necessarily focused on habitat preservation and enforcement of international agreements banning illegal trade and persecution of this primate. Organizations such as the World Wildlife Fund, the Environmental Investigation Agency and Telepak have been instrumental in developing protocols for the effective re-introduction of captive orangutans into the wild in rehabilitation centers located in Kalimantan, Sabah, and Sarawak. Also, a 25,000 km2 refuge has been proposed for northern Sumatra that would overlap with the existing Gunung Leuser National Park where most remaining individuals of the Pongo abelii species currently reside.
Another mechanism for preserving orangutan populations has been the implementation of public awareness programs to educate island residents regarding the critical role that this primate plays in sustaining lowland rainforests. As widely ranging fruit eaters, these mammals are important in dispersing various types of seeds and maintaining diversity of rainforest woody plants. They also help prune and aid in regenerating plant growth because they only consume green leaves and stalks.
Unfortunately, time is running out for the Bornean and Sumatran orangutans and it is feared that the current rate of population decline could lead to species extinction within the next few decades, if not sooner.
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- Orangutan Conservancy. (n.d.). Orangutans and the rainforest. Retrieved July 14, 2009.
- Urban, K. (2008). Pongo pygmaeus: Encyclopedia of life. Retrieved July 14, 2009.
- World Wildlife Fund. (n.d.). Orangutans. Retrieved July 14, 2009.