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.
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.
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.
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.
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.
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.
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