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Taxonomy & Distribution

Orangutan literally means “person of the forest” in the Malaysian and Indonesia languages spoken in the countries where orangutans are found. Currently, orangutans are restricted to the islands of Borneo and Sumatra, although during the Pleistocene they were more widespread across Southeast Asia. Orangutans from the two islands are normally divided into two separate subspecies: Pongo pygmaeus pygmaeus, from Borneo, and Pongo pygmaeus abelii, from Sumatra. While it is argued that Bornean and Sumatran orangutans have a high enough genetic difference to justify classifying them into two species, they can easily interbreed in captivity and produce fertile offspring.

Environment

Orangutans live in rain forest habitats ranging from sea-level swamp forests to mountain slopes rarely exceeding 1200 m. These forests are true wet, rain forests wither average rainfall ranging from slightly over 2000 mm per year to 4500 mm per year. One of the principal orangutan habitats is forest dominated by the large trees of the Dipterocarpaceae family. This type of forest is characterized by “mast fruitings,” a phenomenon that occurs approximately every 2-10 years in which up to 88% of rain forest tree species may fruit at the same time. This causes dramatic fluctuations in the type and quantity of fruit available to orangutans. This resource unpredictability may help us explain many of the unique aspects of orangutan physiology and behavior.

General Description

Orangutans are the largest of all canopy animals, with wild adult males weighing 86.3 kg on average and females 38.5 kg. Such large animals move through the canopy by quadrumanual clambering (using all four hands and feet to grasp and pull themselves along) and occasional brachiation (particularly by smaller individuals). They also effectively use their body weight to bend and sway small trees, using the stored momentum in the tree as a spring to propel themselves across a gap until they can grasp an adjacent branch.

Female orangutans are less than half the size (approximately 45%) of developed adult males. This is one of the highest degrees of sexual dimorphism seen in primates. Factors such as male-male competition, female choice, and sexual coercion may have been important in the evolution of large male body size in orangutans. Female orangutans are considered to be the ecological sex, that is, to exhibit a body size that is primarily constrained by nutritional factors rather than competition.

Fully adult males are also striking for their secondary sexual characteristics, such as the production of the long call and their projecting cheek pads. Intriguingly, it has been proposed that there may be two types of fully adult males with one type, developed males, exhibiting the large body size and secondary sexual characteristics described, and the other, undeveloped males, retaining a smaller, subadult morphology. Males have been reported to still be undeveloped at an estimated age of 20 years in the wild and up to 18 years in captivity.

Activity Patterns

Orangutans spend in different activities varies depending on the availability of food, social conditions, and reproductive status. On average, orangutans spend approximately 44% of their time resting, 41% feeding, 13% traveling, 2% nest building, and less than 1% engaging in other activities, such as fighting, mating, and socializing. Because orangutans are primarily solitary, their activity patterns may be very individualistic.

Feeding Ecology

The orangutan diet varies dramatically depending on what foods are available. Fruit, both pulp and seeds, is the preferred food of orangutans. Orangutans prefer to feed in trees with large patches of fruit when it is available. Adult males have longer feeding bouts than do females, and males tend to feed in fewer food patches per day than do females. When fruit is abundant, such as during a mast fruiting, the orangutan diet may consist of 100% fruit. However, during fruit-poor times, orangutans must rely on more abundant, but relatively lower quality, food such as the inner cambium layer of bark, leaves, pithy plants, and insects. Orangutans have been seen to eat meat only on rare occasions. In Sumatra, three adult females have been observed on seven occasions to hunt and eat slow lorises, and one female was observed to eat a gibbon. At Gunung Palung, a juvenile female orangutan once caught and ate a rat.

Social System

The orangutan diet varies dramatically depending on what foods are available. Fruit, both pulp and seeds, is the preferred food of orangutans. Orangutans prefer to feed in trees with large patches of fruit when it is available. Adult males have longer feeding bouts than do females, and males tend to feed in fewer food patches per day than do females. When fruit is abundant, such as during a mast fruiting, the orangutan diet may consist of 100% fruit. However, during fruit-poor times, orangutans must rely on more abundant, but relatively lower quality, food such as the inner cambium layer of bark, leaves, pithy plants, and insects. Orangutans have been seen to eat meat only on rare occasions. In Sumatra, three adult females have been observed on seven occasions to hunt and eat slow lorises, and one female was observed to eat a gibbon. At Gunung Palung, a juvenile female orangutan once caught and ate a rat.

Reproduction and Life History

Females reach sexual maturity at approximately 11-15 years in the wild, although maturation can occur as early as 7-9 years in captivity. Sexual maturation in males ranges broadly. Males have been known to father offspring as early as 6.5 years in captivity. The transition from an adolescent to an undeveloped adult male stage occurs at approximately 7-10 years of age. After that, full development may proceed directly or may be delayed for 10 years or more. Average completed orangutan life span in the wild is not known, but captive individuals have lived into their late fifties.

Orangutans have been reported to have an average completed interbirth interval of 8 years – the longest of any primate and indeed on of the longest of any mammal. Infants and juveniles nurse for approximately 6 years, during which time female hormonal production appears to be suppressed. The length of gestation is approximately 8 months.

Like humans, orangutans have no estrous swellings and no visual indicators of ovulation, and the average orangutan menstrual cycle has a mean length of 28 days. A surprisingly large percentage of orangutan matings have been characterized as forced copulations. In the wild, undeveloped males appear to engage in more forced copulations and, usually, more copulations in general than do fully developed males. However, both forms of male can and do force females to copulate. Current studies are examining the relationship between the menstrual cycle and reproductive activity in the wild, where data are showing that matings are much more common during periods of high fruit availability when estrogen levels are higher in females.

Cognition & Tool Use

Orangutans in captivity have been found to be highly intelligent, habitually making and using tools. Orangutans raised in people’s homes and later brought to rehabilitation centers have been taught sign language and regularly emulate human activities. New evidence of tool use in the wild is now emerging. In the recently establishing field study at Suaq Balimbing in Sumatra, orangutans regularly use tools to extract seeds from Neesia fruits as well as to access insects from tree holes. Stick tools of specified lengths are made for each of these tasks. Other tool-using behaviors are also observed in the wild. Orangutans regularly make leaf umbrellas to cover themselves during heavy downpours and use branches in agonistic displays. Occasional observations have been made of orangutans using leaves for self-cleaning and as protection in food acquisition, dead wood for opening up durian fruits, and sticks for scratching. At Gunung Palung, we have recently witnessed orangutans using leaves as drinking tools.

Conservation Status

Orangutans are internationally recognized as endangered species. Their rain forest habitat has declined by more than 80% over the past 20 years due to timber extraction and conversion of rain forest for plantations and agriculture. This, augmented by hunting orangutans for meat and killing adult females to obtain infants for the illegal pet trade, has resulted in an estimated decline in the orangutan population of 30 to 50% over the past 10 years. Uncontrolled forest fires have also destroyed significant portions of orangutan habitat in both Sumatra and Borneo. The continued survival of the orangutan is in significant peril. Only through concerted efforts to preserve orangutan habitat and prevent continued hunting of individuals can we hope to sustain populations of wild orangutans into the twenty-first century

 

Jari Manis, a dominant flanged orangutan male.
Note the wide cheek flanges, throat poach,
long hair, and impressive size.

This is Jari Manis several years later, displaying the loss of
condition in past-prime males.

Rob is an unflanged male, considerably smaller and less
magnificent than the flanged males. Rob seems to be
sexual mature, but may be "arrested" in the smaller
male morph.

Beth, a very pregnant female. Female reproduction is
very costly and very slow, with about 1 infant borth
every 8 years.

Marissa has just given birth to her daughter, Martina. Martina will
remain dependent on her mom for 6 years or more.

 

 

For more information and references, consult: Knott, Cheryl. 1999. Orangutan behavior and ecology, pp. 50-57 in The Nonhuman Primates, P. Dolhinow & A. Fuentes (eds), Mayfield Publishing Co., Mountain View, CA.

All Photos © Tim Laman

 

Contact: Dr. Cheryl Knott, Boston University Department of Anthropology, 232 Bay State Rd, Boston, MA 02215
knott@bu.edu


This material is based upon work supported by the National Science Foundation under Grant No. 0936199. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.


 

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