Orangutan Specifics

map of orangutan distribution Fossil evidence suggests the orangutan once ranged throughout most of the southeast and mainland Asia, encompassing both high and lowland areas in the wet and seasonal tropics of modern-day Vietnam, northern India, and southern China (van Schaik & Delgado, 2000). Today, the orangutan is the only great ape found in Asia. It lives on just two islands: Borneo and Sumatra. Located in the Malay/ Indonesian archipelago and part of the large Sunda continental shelf, Borneo and Sumatra are two of the world’s largest islands. They are isolated from each other by the South China Sea.

Until recently, the orangutan was classified as one species with two subspecies that differ in appearance and behavior: the Bornean orangutan (Pongo pygmaeus pygameus) and the Sumatran orangutan (Pongo pygmaeus abelii). However, recent DNA analysis shows that the genetic difference between them are no greater than the differences that comfortably separate species in other animals like chimpanzees and gorillas. As a result, scientists now say that they are two distinct species. The small population of Sumatran orangutans from the Batang Toru area south of Lake Toba was reclassified as a new species named Pongo tapanuliesis in 2017, based on both morphological and genetic differences (Nater, et al., 2017).

There are a number of sub-species of the Bornean orangutan. Molecular data suggests the Bornean and Sumatran orangutans diverged between 2.7 and 5 million years ago, and that the Bornean orangutan underwent a population expansion between 39,000-64,000 years ago, with the island’s rivers strongly determining migration routes and contributing to the genetic differences in Bornean populations (Goossens et al, 2009).

There are currently two accepted sub-species:

  • The Western Bornean orangutan (Pongo pygmaeus pygmaeus), which lives north of the Kapuas River in West Kalimantan in Indonesia and in parts of west Sarawak in Malaysia
  • The Southern Bornean orangutan (Pongo pygmaeus wurmbii), which lives in southwest Kalimantan between the Kapuas and Barito rivers

A third population of orangutans living south of the Mahakam River in East Kalimantan and in the Malaysian state of Sabah is either a third, separate sub-species (pongo  pygmaeus  morio), or a separate population of the Western Bornean orangutan (Goossens et al, 2009).



The similarities between orangutans and humans and the closeness of our evolutionary history are often cited as important reasons to protect them. But what does this really mean, and just how closely related are we?

genes Genetic studies show that our genome - our hereditary information - differs by a remarkably small amount. Orangutans and humans both belong to the scientific order known as the primates, the group of mammals that contains all the monkeys, prosimians (the pre-monkeys) and apes living today. One of the oldest surviving mammal groups, the primate lineage is thought to go back at least 65 million years ago, when small, arboreal, insect-eating mammals, referred to as the Euarochonta, were beginning the process of speciation that eventually led to the primates we see today, one of the most diverse and varied groups of animals (Dawkins, 2004). Primates inhabit almost every part of the world, with the non-human species found mostly in tropical and sub-tropical regions of Africa, Asia, and Central and South America.

hands While there isn’t one unique characteristic that defines a primate, there are a number of characteristics that are found throughout species of this order, including:

  • An opposable thumb, which aids strong, grasping hands
  • Forward-facing eyes, which enable three-dimensional sight
  • Eye sockets, which protect the eyeball
  • Fingerprints unique to each individual
  • Much larger brains, in comparison to body size

Primates are divided into two scientific sub-orders. The primates considered the most primitive, or those that have retained the most number of features of the ancestral primates, are classified as being Strepsirrhines. They include the lemurs of Madagascar; the galagos, pottos, and angwantibos of Africa; and the lorises of Asia. The tarsiers, monkeys, and apes are classified as Haplorhines due to their larger brains, specialized hands and feet, a wider range of facial features, and the lack of a rhinarium (wet snout).

Around 30 million years ago, the primates of the Haplorhini divided into two further groups, the Platyrrhini, which now consists of the modern day New World monkeys of South and Central America, and the Catharinni, made up of the Old World monkeys of Africa and Asia, and the apes. This latter group split into two superfamilies at approximately 20-25 million years ago, as evolutionary pressures gave rise to drastic differences in the primate species. Modern-day Old World monkeys make up the superfamily Cercopithecoidea, The apes - the gibbons, chimpanzees, bonobo’s, gorillas, orangutans, and humans - are the only surviving members of the superfamily Hominoidea.

proconsul skull Around 20-19 million years ago, a primate had evolved in central Africa that had characteristics of both Old World monkeys and apes. This primate, named Proconsul, included four known species and had a posture similar to that of a monkey. However, its lack of a tail, facial structure, and strong grasping capabilities distinguish it as an ape. Studies on the growth and form of fossilized Proconsul teeth show that this primate's life history was similar to that of a modern-day gibbon, with additional features of other apes and monkeys (Palmer, 2010). It's likely that the first apes evolved from a monkey-like creature that had descended to the ground and, through the process of evolution, lost many of their ancestral features, acquiring new ones as they adapted to their new environment. For example, a tail would be useful for a life in the canopy, aiding movement and balance. On the ground, a long tail is a hindrance to terrestrial movement and would be one of the first adaptations to disappear (Morris, 2008). Whether or not Proconsul is an intermediate between monkeys and apes and a common ancestor of present-day apes in the evolutionary tree is still debated. Regardless of this uncertainty, Proconsul is one of earth’s first known apes.

dryopithecusBetween 9 and 17 million years ago, apes from the genus Dryopithecus were living not just in Africa, but were the first known species of ape to have migrated into Europe and Asia, giving clues to the migratory patterns of our ancestors (Palmer, 2010; Begun, 2004). Although there are variations in the five species of this genus, such as Proconsul, it resembled a monkey in many ways, but the bones in the forearm and elbow suggested it moved around in the treetops like an orangutan or gibbon. However, its skull formation is similar to that of the chimpanzee. Like all fossils, its evolutionary position is debated, but some scientists claim that Dryopithecus and its close relatives were the ancestors of all living great apes (Palmer, 2010).

During this period, between 8.5-12.5 million years ago, three species of the genus Sivapithecus were living in the rainforests of Asia. Sivapithecus was around 1.5 meters in height, with many of its physical attributes resembling those of a chimpanzee. However, like an orangutan, it had a concave face with projecting incisors and large canines (Palmer, 1999). Fossils of this species have been found in Turkey, China, and Pakistan. Analysis of bone structure indicates they were adept at moving both on the ground and in the trees and fed on a diet of savannah grasses and seeds (Palmer, 1999). The genus Sivapithecus is now acknowledged as the direct ancestor of modern-day orangutans (Fleagle, 1999), and scientists believe that this line, the lineage that descended to modern-day orangutans, branched off from the line that descended to modern-day gorillas, chimpanzees, bonobos, and humans around 12 million years ago (Palmer, 2010; Fleagle, 1999).

gigantopithecus_skullThe orangutan is the only non-human great ape found in Asia, but this development is relatively recent because another group of primates may have evolved from Sivapithecus (Fleagle, 1999) and lived at the same time as orangutans in what are now China, India, and Vietnam (Morris, 2008). Gigantopithecus lived between 300,000 to a million years ago, and was, as its name suggests, a giant. In fact, it's the largest ape ever known, with males weighing up to 540 kg - twice as much as a male gorilla (Morris, 2008). Scientists believe Gigantopithecus fed chiefly on plants and lived on the ground, walking on all four limbs like a gorilla (Morris, 2008; Galdikas, 1999). We don't know why Gigantophithecus died out, but it's possible that early man hunted it to extinction (Galdikas, 1999; Morris, 2008).

evolution tree of hominoidsWhile the orangutan lineage was evolving in Asia, the primates that led to the modern-day African great apes and humans were still in Africa. Fossil evidence of ancestral gorillas, chimpanzees, and bonobos is sparse, due to the acidity of rainforest soil, which tends to dissolve bones, but fossils of early humans found in open savannahs and genetic mapping of the human and non-human ape genome offer a clearer picture of how the other great apes evolved.

The line that descended to modern-day gorillas is believed to have diverted from the line that chimpanzees and humans followed around 8 million years ago, with the chimpanzee lineage diverting from the human lineage at around 4 million years ago. The line that led to modern-day bonobos probably split from the chimpanzee line around 2.5 million years ago (Palmer, 2010). However, some scientists date the split between the human and chimpanzee lineages at around 7 million years, based on Kenyan fossils named Orrorin from about 6 million years ago that have distinctly human characteristics (Senut et al, 2001).

The human (hominin) fossil collection is reasonably large, and although there is debate about the exact route the lineage from ancestral apes to humans took, the evolution from a quadruped covered in fur to the hairless bipedal ape we are today is well documented. Homo Sapiens emerged in Africa around 200,000 years ago, and spread throughout the rest of the world between 80,000-60,000 years ago (Palmer, 2010; Lockwood, 2007).



Sumatran male Orangutans are the largest tree-dwelling animal in the world, and the second largest non-human great ape, after gorillas. A mature female orangutan may weigh approximately 40kg and have a standing height of 110 cm. although this is about the same weight as a subadult male, orangutans are extremely sexually dimorphic, and this is never more apparent than in the disparity in size between the adult females and adult males. An adult male at maturity will weigh almost twice as much, at approximately 80kg, and dominant adult males who have developed cheek pads and enlarged throat sacs have been known to weigh as much as 120kg. They will have an average standing height of 130cm (van Schaik & Delgado, 2000; Morris, 2008).

Sumatran orangutans are the smaller of the two species and are easily identifiable by their longer, denser, lighter colored fur, which ranges from a light red to cinnamon color, and their narrower face, which is often framed by what looks like a beard. Generally considered more gracile looking than its Bornean cousin, the flanges of the males, jutting cheek pads made up of fibrous fatty tissue, tend to lie flat on the face of the Sumatrans, and males generally have smaller throat sacs. The Bornean orangutan is larger than the Sumatran, has darker fur of an often maroon or dark red color, wider faces, and the males of this species tend to have flanges that protrude forward (Goossens et al, 2009; van Schaik & Delgado, 2000; Morris, 2008).

Spending almost all of their time in the forest canopy, orangutans, despite their great size, are perfectly adapted to life in the trees. Their forearms are incredibly long and strong, with flexible shoulder joints and an intermembral index, the ratio of the fore-limb to hind-limb length, of 140, meaning that when orangutans walk upright on their hind legs, the fingertips touch the ground (Morris, 2008).

They have hook-like hands, long curved fingers and short thumbs, which are about one third the total length of the palm. Their long fingers give the orangutan the ability to create a ‘double lock’ grip, whereby the fingers are able to wrap around thin creepers, vines and branches so much that the last finger bone comes to lie parallel to the bones of the palm, allowing the fingernail to lock against the skin of the palm (Morris, 2008).

In comparison to their forearms, their hind limbs are short, and they have hand- like feet with curved fingers and shortened big toes, which diverge from the sole and can assist in a variety of gripping movements. Their hip joints are extremely flexible, as are their ankles, and their wrists are capable of swiveling through 150 degrees (Morris, 2008; van Schaik & Delgado, 2000).  

Orangutans possess the same dental morphology as humans, with two incisor teeth, one canine, two premolars and three molars in each side of each jaw, upper and lower. However, the dental arch is different, and, like other non human great apes, they have a gap, called a diastema, between the second incisor and the canine, which is projected forward. It has been argued that the orangutan dental morphology, the broad central incisors, the small lateral incisors and short maxillary canines, is related to historic bark stripping and feeding on hard objects (van Schaik & Delgado, 2000; Taylor, 2009).




durian!Orangutans are large-bodied animals that must eat large amounts of high-calorie foods. Largely frugivorous, when it is abundant, the fruit will make up as much as 90% of their diet, supplemented with leaves, shoots, seeds, buds, flowers, bark, insects and mineral-rich soil, and occasional instances of meat eating (Morris, 2008; Galdikas, 1988). However, the type and variety of food types eaten has been shown to be heavily influenced by a series other contributing factors, including island differences, seasons, climate, habitat type, and habitat quality, with differences seen in all major Sumatran and Bornean study sites (Russon et al, 2009a).

rambutan The complete observed orangutan food list contains 1693 species, which includes 1666 plant species, 16 invertebrate, 4 vertebrate and 7 other. Plant food species represent 453 genera and 131 families, while invertebrate species eaten include 4 species of ants, 4 species of termites, 2 species of caterpillars, leeches, wasps, maggots, bee larvae, crickets, and ticks. Vertebrates consumed include slow lorises, gibbons, birds’ eggs, young birds and tree rats (Russon et al, 2009a).

The fruits they eat most often include:

  • Durians
  • Figs
  • Lychees
  • Jackfruit
  • Breadfruit

Several fruits that only have scientific names, including Nessia, Sarcotheca, Nephelium, Tetramerista, Mallotus, Gironniera, Lithocarpus, Antiaris, Tinomiscium and Eugenia. 

They also eat flowers. For example, the blooms of Xanthophyllum rufuum are a favorite (Morris, 2008). Leaves make up a large part of an orangutans diet, particularly those of Gironniera nervosa, which is also an important source of bark. Leaves of various species of the breadfruit group Artcarpus are consumed, as are those of the trees of the genus Baccaurea. Although both ripe and unripe fruit will be eaten, orangutans prefer young, soft plant parts to older ones, especially leaves, which develop toxins as they grow to discourage leaf eaters (Morris, 2008).

 The islands of Borneo and Sumatra differ in both forest type and forest productivity, and because of this, marked differences are observed in the diets of the Bornean and Sumatran species’. Figs are found in abundance in Sumatra, but are absent from large parts of Borneo, and, as such, play a much larger role in the diets of Sumatran orangutans than they do Borneans (Galdikas, 1988). In one four year study in Tanjung Putting in Indonesian Borneo, only 0.4% of foraging bouts involved figs, in comparison to 54% of all foraging bouts in a similar study in Gunung Leuser in Sumatra (Galdikas, 1988). Seeds are also eaten more widely by Bornean orangutans than they are by Sumatrans, due to the high frequency of trees from the Diptercarp family in Borneo’s forests. Trees from this family fruit irregularly, but produce large quantities of oil rich seeds, which orangutans love (Russon et al, 2009a).

Sumatran forests are generally more productive in orangutan foods, but forests on both islands suffer from irregular fruiting and seeding patterns, with the most extreme fluctuation being the mast fruiting and corresponding food shortages which appear at 2-10 year intervals, and are linked to the El Nino Southern Oscillation weather phenomenon. Mast fruiting refers to periods of low fruit productivity that are punctuated by periods of high fruit availability, with 90% of canopy species producing fruits at the same time, followed by severe fruit scarcity. During mast fruiting, orangutans will gorge exclusively on fruit, build up fat reserves, and then diversify their diet when the mast is over, relying on different types of ‘fall-back’ foods. Because Sumatran forests produce higher numbers of energy rich figs and fruit on a more consistent basis, mast fruiting has a greater effect on Bornean orangutans; For example, at Gunung Palung in west Kaliamantan, Indonesian Borneo, 37% of an orangutans diet after a mast fruiting is made up of low quality food items such as bark (Morrogh-Bernard at al, 2009).

Differences have also been noted at study sites on the same island. Orangutans eat from Shorea leprosula at four different sites, but only eat its fruit at one site. They eat from Durio kutejensis at four study sites in East Borneo, but eat different parts of the plant at each different site. Pith has also been observed being eaten at all sites except that in Tuanan in Borneo (Russon et al, 2009a). While many of these anomalies may be explained by ecological factors and longer field studies, they nevertheless demonstrate the complexity of the orangutan diet.



Davida and David in treeAll great apes have slow life histories, and the orangutan is no exception. In fact, it has the longest birth interval of any land mammal, with one of the longest periods of infant dependency (Galdikas, 1999).

An orangutan is born after a gestation period of around 9 months (usually between 260 to 270 days) and is in almost constant body contact with its mother for the first few months of its life. After 12 months an orangutan will begin to venture away from its mother, contact declining to around 50% at the time of its second birthday. However, this only refers to times when a mother is stationary in a tree, as an infant orangutan will still be carried from tree to tree on its mothers body until it is between 2 -4 years old, and even after this age, a female orangutan will still use her body as makeshift bridge for her offspring to climb across if the gap between trees is too wide. Studies in both Sumatra and Borneo have shown that an infant will spend more than 50% of its time within 10 meters of its mother until is at least 6 years of age, and in some areas will remain in the same tree as its mother until the age of 8 (van Noordwijk et al, 2009).

Due to their arboreal lifestyle, the rate and frequency of nursing has been difficult to ascertain, but it is believed an infant orangutan will receive milk from its mother until the age of around 5-7, with studies suggesting the age of complete weaning is later in Sumatra than it is in Borneo (van Noordwijk et al, 2009). An orangutan will begin begging for solid food from its mother after about a year, either taking the food directly from her hand or holding its hand to her mouth. Mothers are extraordinarily tolerant of this behavior, but this tolerance will decline as the infant matures, and usually completely evaporate after weaning (Galdikas, 1999, van Noordwijk et al, 2009).

During this period of close association and dependence, immature orangutans carefully observe their mothers feeding, social and nest building behavior. By the age of 3-5, orangutans are able to process everything their mothers eat, and even in populations in Sumatra that display complex tool use, orangutans were able to process food for which tools are used before they were weaned.

Like other non-human great apes, orangutans in the wild construct night nests out of branches and leaves. Despite evidence that orangutans as young as three are able to construct their own nests, pre-weaning offspring will always share their mother's nest.

At approximately 5-8 years old, an orangutans mother will begin a consortship with a male orangutan and produce a new offspring, at which time the weaned orangutan will begin a period of wandering, leaving their mother for prolonged periods. The amount of contact between an orangutan and their mother at this stage varies from study site to study site, but Sumatran orangutans appear to remain in association with their mother until a later age (6-8 years) than do Borneans, with orangutans at study sites at Kinabatangan and Tuanan spending considerable time away from their mothers at age 7, and an orangutan of 5 venturing more than 50 meters away in the Sebangau study site (Wich et al, 2009; van Noordwijk et al, 2009).

After completely leaving their mother, an orangutan will begin the process of establishing a home range. Home ranges vary in size from site to site, and also between sexes, and seem to be influenced strongly by forest productivity and fruit availability. Female home ranges overlap considerably with other females (Galdikas, 1999; Singleton et al, 2009; Wich et al, 2009), while adult male home ranges are so large that all current estimates are unproven, due to the inability of researchers to conduct long-term observations of males traveling over such long distances (Delgado et al, 2009). In Sumatra, female home ranges have been observed as being as little as 150-200 hectares at the Ketambe research site and as large as 850 hectares at Suaq Balimbing. In Borneo, female home ranges have been estimated at between 350-600 hectares at Tanjung Puting to 180 hectares at Kinabatangan. Male home ranges at Gunung Palung in Borneo were estimated at higher than 650-700 hectares, larger than 560 hectares at Sebangau and over 2500 hectares at Suaq Balimbing in Sumatra (Utami et al, 2009a). Orangutans practice female philopatry, meaning weaned female orangutans establish home ranges in close proximity to their mothers, while male orangutans disperse farther (Galdikas, 1999).

Female orangutans reach sexual maturity between the ages of 10 and 15, but will not have their first offspring until they are at least 15. Unlike chimpanzees, females do not have the conspicuous pink swelling that advertises ovulation, and will usually conceive after a consortship, usually lasting a few days, with the dominant male orangutan in the area, although rapes by non-dominant flanged and unflanged males do occur, and recent DNA testing in the Kinabatangan study area has shown that 40% of infants are sired by non-dominant males (Utami et al, 2009b). Female orangutans will usually give birth to no more than 3 or 4 offspring, and their life expectancy is around 45 in the wild.

After leaving their mothers, male orangutans enter a period that has been referred to as sub-adult wandering, dispersing and traveling over long distances, often associating with orangutans of both sexes of a similar age. Adult males in captivity reach sexual maturity at 8 to 10 years of age, although this will most likely happen at a slightly later age in the wild. Orangutans exhibit extreme sexual dimorphism and are also unusual in having two kinds of sexually mature males (Harrison & Chivers, 2006).

Upon sexual maturity, some male orangutans will develop into ‘flanged’ males. Flanged males are twice the size of adult females and are distinguishable by their throat sacs, wide cheek pads, and long coat. Flanged males are the dominant males in any area, and tend to migrate through large home ranges, monopolizing access to females and repelling other males with large, booming long calls. Un-flanged males, also often referred to as ‘sub-adult’ or ‘non-dominant’ male orangutans, are usually the same size as adult females, and lack the physical characteristics of flanged males. Subordinate to flanged males, they can often remain in this stage of arrested development for as much as 20 years, while remaining sexually mature and able to sire offspring, but will develop the characteristics of flanged males before they die (Utami et al, 2009; Harrison & Chivers, 2006; van Schaik & Delgado, 2000). The life expectancy of wild male orangutans is similar to females, at 45-50 years.



Female and offspring Long thought to be a solitary ape, long-term studies of orangutan behavioral ecology beginning in the 1960s and 1970s have shown that orangutans are best described as semi-solitary, with a social system just as complex as that of the more gregarious gorillas and chimpanzees.

The social organization of all primates is strongly influenced by the availability of food, and it has been argued that the current social and mating behavior of orangutans has evolved from one similar to that of modern-day gorillas, as climatic changes between 3-5 million years ago led to periods of irregular fruiting, forcing orangutans to range over larger areas to find enough food to sustain their large bulk, and necessitating a solitary lifestyle (Harrison & Chivers, 2006). 

The most often observed relationship in orangutan society is between a mother and her offspring. As discussed above, orangutans have the longest birth interval of any primate and an extraordinarily long infant dependency. These pairings, known as ‘dyads’, last from between 6-9 years, and begin to end at the point of weaning, with separation occurring after the birth of a new offspring. This dependency has been attributed, in part, to an infant orangutans need to learn complex fruiting patterns and processing techniques of often toxic rainforest foods, and may explain the low infant mortality rate of wild orangutans, which is estimated at 5-8% in the first year of life (van Noordwijk et al, 2009: Galdikas, 1999). Studies of weaning ages also suggest food plays a key part in the levels of tolerance shown by mothers to weaned offspring, with mothers showing tolerance when forest conditions are favorable, but showing marked intolerance as soon as the cost of association becomes too high for her and her dependent offspring (van Noordwijk et al, 2009).

These ecological factors also seem to influence the sociality of adult females. Female orangutans practice philopatry, whereby daughters establish home ranges in the vicinity of their mothers, and home ranges overlap considerably (Galdikas, 1999). Although ‘clusters’ have been observed, where related females show preferential range overlap and association, and there are anecdotal observations of a mother intervening in the attack of an adult daughter at the Tuanan research site, female orangutans almost always avoid contact with other females, with research at Gunung Palung in West Borneo suggesting females actively avoid each other, a foraging strategy used mutually to reduce the likelihood of encountering food patches that are being, or have been, depleted by another orangutan. Although aggressive encounters do happen, they are rare, with just 29 such encounters during this 9-year study (Knott et al, 2008).

juveniles play in tree Although sub-adult males will occasionally form traveling bands, and play between juveniles is common, adult male competition is hostile and has been cited as a major determining factor in orangutan social organization, caused by both ecological pressures and access to sexually active females. As discussed, there are two morphs of adult male orangutans, the ‘flanged’ and ‘unflanged’ male.

Although sizes are difficult to ascertain, home ranges of all adult male orangutans are believed to be at least 3 times as large as those of females. Like females, ranges overlap, and will usually contain one dominant flanged male, other non-dominant flanged males and any number of unflanged males (Utami et al, 2009a). Despite these high range overlaps, encounters between flanged males are relatively rare, diverted by mutual avoidance, and any encounters are usually triggered by the presence of a sexually active female or coveted food source. At the Ketambe research center, 64% of aggressive encounters took place near a fig tree, and although only 5 cases of serious fighting were observed, when they do occur, they can result in serious injury or death (Utami et al, 2009a).

Relations between flanged and unflanged males are marked by imposed tolerance. While flanged males avoid each other, unflanged males often stay near flanged males, particularly during consortships, usually at a distance of 40-50 meters. Studies at Suaq Balimbing in Sumatra suggest unflanged males will spend more time in association with flanged males if they are in a consortship with a fertile female than not. At this site, unflanged males tended to stay closer to the dominant male, who monopolized fertile females in the area. Although flanged males will often give chase to these marauding unflanged males, fights are rare and there are no reports of physical attacks (Utami et al, 2009a).

Unflanged adult males generally show tolerance towards each other, and will often form associations, with play and homosexual encounters observed, although associations will decrease as they get older. Aggression rates are low amongst these males, although increase if one of the males is in consort with a female (Utami et al, 2009a).

The reason for two different morphs of the male orangutan, each with different reproductive strategies, is one of the most fascinating aspects of orangutan behavior, but despite years of study, many questions remain unanswered.

The orangutan mating system is based on a mixture of female choice and male harassment and coercion. Scientists believe that this fierce competition for female mating opportunities has resulted in the extreme sexual dimorphism and arrested development in orangutan societies.

Females mate promiscuously and have been observed at different research sites exhibiting preferences for both flanged and unflanged males (Setia et. al., 2009). Generally, though, females demonstrate preferences towards the dominant flanged male in their home range, and will actively seek out this male, who will advertise his location by giving long calls. Consortships and matings between females and dominant flanged males are generally cooperative, and dominant flanged males will sire the majority of offspring. In comparison, matings between females and unflanged males are generally forced and usually only come about after the unflanged male has actively traveled through an area to locate the potentially sexually active female. Although female preference and mate choice is an important factor in orangutan mating systems, it has meant males have had to develop two contrasting strategies; call-and-wait vs. sneak-and-rape (Harrison & Chivers, 2006; Galdikas, 1999).

The evolution of large-bodied flanged males is relatively easy to understand. Similar to gorilla societies, where male silverbacks fight for access to females and defend family groups, size has its benefits, and millions of years of mating with dominant flanged males explain the extreme dimorphism seen between orangutan males and females. But why, if size means everything, don’t all male orangutans develop cheek pads and fight for access to females in the same way? Several hypotheses have been put forward. One, developed recently after paternity testing revealed unflanged males have a higher success of siring offspring than do non-dominant flanged males, suggests unflanged males, as a result of their decreased body size and increased speed and agility, have a greater chance of sneaking forced copulations with sexually active females than do slow-moving non-dominant flanged males, so arresting development and delaying the onset of flanged sexual characteristics is advantageous in the long run (Utami et al, 2009b).




The popular image of an orangutan is of a serene loner, quiet, calm, mysterious, difficult to see from the ground, as it moves, alone, through the forest canopy. While this image is somewhat accurate and the orangutan does appear quieter than its more gregarious ape cousins, the gorillas, chimpanzees, bonobos, and gibbons, it nevertheless has a surprisingly varied vocal repertoire, with geographic differences in size, development, and acoustic structure, indicating a form of language in orangutan society and cultural variations between populations.

Long callThe most famous and distinct of the orangutan vocalizations is the long call. Emitted by male orangutans that are showing the secondary sexual characteristics of maturity, known as flanged males, the call usually begins softly with a series of ‘bubbly’ grumbles, before increasing in intensity to form a long sequence of bark-like pulses or roars, ending with a long series of sighs. The call is the loudest acoustic signal in the orangutan vocal repertoire, and although variations occur from male to male, it is produced several times a day, with each call lasting between 15 seconds and 4 minutes. The call is enhanced by the male’s cheekpads, which are believed to help project the sound forward, and their large hollow air sac, called laryngeal chambers, which act as resonators and help carry the sound as far as a kilometer away (Delgado et al, 2009; Hardus et al, 2009). 

The complete function of the long call is difficult to determine, but its main function appears to be as both a spacing mechanism and mating strategy (Delgado et al, 2009). Orangutans live in large, heavily overlapping home ranges, where visibility is limited in the dense rainforest canopy, and long distance vocalizations play an important role in regulating relationships and interactions. By producing a powerful long call, the flanged males advertise their presence to other males in the area, and males respond differently to these calls depending on their relative status of dominance. Studies in Sumatra have shown that the dominant flanged male in any area is the orangutan most likely to approach other long calls, whereas lower ranked males tend to move away from other long calls (Delgado et al, 2009; Utami et al, 2009a). Experimental playbacks of recorded long calls at Kutai National Park in Borneo confirm this, with low ranking flanged males and unflanged males avoiding calls by the highest ranking male (Mitani, 1985). Research in Malaysian Borneo has also revealed flanged males produce long calls more often along the periphery their range, and when disturbed by potential rivals (Utami et al, 2009a)

The male long call also acts a mate attraction function. Flanged males produce more long calls when not in association with females, and research has shown that females, regardless of their reproductive status, either approach these calls or coordinate their ranging pattern with the locally dominant flanged male. Current data for the role of long calls as a female attraction function are more extensive for study sites in Sumatra, where research has shown that females selectively approach after long calls by the current dominant male, rather than those of non-dominant males, and spend 90% of their time in earshot of the dominant flanged male, as opposed to the 70% of their time spent in audible range of lesser status flanged males (Delgado et al, 2009). Long-term studies in Borneo have also provided examples of females running to flanged males when threatened by unflanged males, indicating again that, not only does the long call function as a female attraction and guarding strategy, but that orangutans are able to determine the identity and status of flanged males based on their long call (Delgado et al, 2009; Utami et al, 2009b).

In addition to the long call, orangutans display a wide variety of other vocalizations, from middle range calls and grumbles to kiss squeaks and raspberry sounds. The documented number of vocalizations could be as high as 32, although this is difficult to confirm because orangutan vocalizations tend to be employed in short distance communication during largely infrequent social interactions, and are difficult to hear by human observers on the ground. Additionally, much of the focus on sound repertoire recently has focused on the complexities of the long call, so other vocal types have been neglected. However, studies do suggest variations in vocal repertoire displayed at each site, and these differences have been interpreted as evidence of culture in different orangutan populations (Hardus et. al., 2009).

Other notable orangutan vocalizations include:

  • Kiss squeak: The kiss squeak has been described as a sharp intake of air through pursed trumpet-like lips, which causes a sharp kiss sound. This vocalization is given by orangutans of all ages and both sexes and is often used to display annoyance, at human observers, other orangutans or predators.
  • Raspberry: A spluttering sound orangutans of all ages make during nest building. Orangutans sometimes run twigs through their mouth after this vocalization, using the saliva to assist nest building.
  • Soft hoot: Emitted by infants, this vocalization is similar to a whimper made by a human child and is most often used to display fear or displeasure.
  • Rolling call: Emitted by adult males and females during intimidation displays, these tend to consist of several low-frequency guttural noises.




 Siswi makes an umbrella with leaves Intelligence in great apes has historically been determined by the extent of tool use shown by wild populations, particularly as tool use and tool manipulation was once thought to be a distinctly human ability. Hence, for a long time, chimpanzees, which exhibit a dizzying array of complex tool use at various research sites in Africa, have often been thought of as the most intelligent non-human ape, and the one most like ourselves, while gorillas, on account of the virtual absence of observed tool use in the wild, have long been denigrated in the popular media for their lack of smarts. Orangutans have usually been placed somewhere between the two, but laboratory tests on captive orangutans and research in the last few decades on levels of tool use and innovation in semi-wild and ex-captive orangutans have revealed them to be astonishingly intelligent animals.

Tool use in wild orangutans was usually thought to be restricted to two behavioral contexts: nesting/ covering and agonistic displays (Galdikas, 1982). Orangutans manipulate branches, saplings, and leaves to build their night nests, and have been observed breaking off and dropping or throwing branches and sticks at other orangutans, species or human observers during agnostic displays (Galdikas, 1995). Males have also been observed pushing over snags while performing their long calls, and during the first 9 years of a long-term wild study at Tanjung Puting in Borneo, orangutans were also observed rubbing their faces with crumpled leaves pulled off adjacent branches, before dropping them or throwing them at human observers. Only once during this period of the study was an instance of tool use observed in a context other than agnostic displays or nesting, when an adult male broke off the end of a dead branch and scratched himself in the vicinity of his anus for 30-35 seconds, before putting the stick in his mouth and biting a piece off (Galdikas, 1982).

More recent studies at Suaq Balimbing in Sumatra have found evidence of orangutans using tools to extract honey, ants or termites from tree holes, manipulation of vines to swing across gaps in the canopy and using leaves to fashion gloves to handle prickly fruits (Russon et al, 2009).  Further similar levels of innovative feeding techniques have been observed at the Tuanan site in Borneo (van Schaik et al, 2003).

In comparison to the relative rarity of tool use observed in wild populations, such activities are common in rehabilitant, ex-captive, and semi-wild orangutans. At Tanjung Puting in Borneo, semi-wild ex-captive orangutans were observed using sticks to dig holes, jab at other orangutans, stir liquids, rake objects from fires, prying loose objects and as an arm extender to reach far away objects (Russon et al, 2009; Russon, 2004). Sticks and branches seem to be an important part of ex-captive innovation, with similar activities at other rehabilitation sites having been observed. At Ketambe in Sumatra orangutans were observed using sticks to open fruit, disturb ant nests, probe rat burrows and poke other animals in cages (Russon et al, 2009), and at a rehabilitation center in Ketapang, West Borneo, a female adult orangutan was observed using a stick to try and pry open the lock on her cage.

Human-made objects available are often used, and orangutans raised in their vicinity have proved to be expert imitators, with orangutans at Tanjung Puting being observed putting together make-shift clothes out of leaves and rags, putting rice on to ‘plates’ of bark, trying to put mosquito nets overnight nests, breaking in to buildings and commandeering dugout canoes (Galdikas 1982 & 1995). American Sign Language (ASL) lessons conducted by OURF president Gary Shapiro on ex-captive females Princess (Shapiro, 1982) and Rinnie (Shapiro & Galdikas, 1999) at this site demonstrated that orangutans were just as able to communicate with humans via this medium as were the famous signing chimpanzees and gorillas, although Princess and other orangutans did seem to be generally more reluctant to converse about topics other than food.

A number of reasons have been put forward to explain the discrepancy in the level of tool use, tool manipulation and innovation observed between wild and ex-captive orangutan populations. Why, if orangutans at rehabilitation centers are intelligent enough to make and manipulate tools for their own gain, why do they generally not do so in the wild?

Studies have shown social learning and cultures contribute substantially to orangutans’ level of innovation in the wild, with many of the different types of tool use observed being passed down through generations. Ex-captive orangutans, deprived of their mothers’ guidance, have to invent their own way of doing of things, and, lacking the social constraints of life in the wild, are free to explore their surroundings and take advantage of objects wild orangutans would generally ignore. The levels of imitation shown by ex-captives are also a likely result of humans having replaced orangutans as parental figures. As already discussed, orangutans have an extraordinarily long period of infant dependency, with infants spending between 5- 9 years learning every aspect of orangutan life from their mothers. Deprived of this, ex-captive orangutans seem to pick up atypical, or unusual, behavior from watching humans, and this contributes to the level of imitative behaviors displayed.  Ex-captive orangutans at most sites also receive supplemental feeding, and freed from the endless quest to find food, which underpins almost all aspects of wild orangutan behavioral ecology, ex-captive orangutans have the time to experiment and innovate in different ways (Galdikas 1982; Russon et al, 2009).

It has also been argued that orangutans in the wild do not exhibit complex tool use simply because they do not have to. Millions of years of evolution have equipped them with the dental and physical morphology to withstand life as frugivores in the forest canopy, and, although animals in captivity and semi wild environments have proved that orangutans have the intelligence and cognitive ability to construct and manipulate tools, orangutans have survived in the wild for millions of years without needing to.



Shapiro teaches signs to Rinnie Language is an important part of being human.  Many have long assumed (indeed, many still assume) that our use of language is one of the most important things that sets us apart as a species.  But are human beings really unique in this regard?  In the last half-century, there has been a great deal of scientific curiosity regarding the ways in which the great apes are similar to human beings, and to what extent those similarities may be observed and studied.  A number of studies have been conducted exploring the potential for orangutans, chimpanzees, gorillas, and bonobos to understand and utilize human language and symbols.  The results have been impressive.

OURF President Gary Shapiro reports that early efforts to examine the language learning capabilities of great apes focused on the acquisition of vocal language.  Attempts to teach vocal language to both chimpanzees and orangutans proved to be largely unsuccessful.  In one study, a chimpanzee was trained for six years in a human cultural environment and was only able to produce four words.  These were poorly vocalized and clearly difficult for the chimpanzee to reproduce. Various studies were conducted throughout the nineteenth century all with a similar lack of success. This was interpreted by some as evidence of humanity’s uniqueness in linguistic capability.  However, as Shapiro points out, this “assumes that vocal speech is equivalent to linguistic competence.  It can be argued similarly that the negative results of the experiments do not mean the apes have no competence for language per se, but only that vocal language is apparently not within the animals’ neurobiological capacity.”  The fact that orangutans and other great apes have great difficulty acquiring and producing spoken vocabulary does not mean that they have no capacity to learn to communicate, but that they are suited to a different form of communication than humans (Shapiro, 1985).  

Studies have shown that orangutans are naturally inclined to at least some level of gestural communication.  Cartmill and Byrne were able to identify 64 gestures through extensive observation of 28 individual orangutans in 3 zoos.  Of those 64 gestures, their analysis established “tight meanings” for 29.  That is, those 29 gestures were used more or less consistently to convey a particular meaning.  These meanings were very broad in nature (Affiliate/Play, Stop action, Share food/object, etc), and do not appear to approach the specificity humans are familiar with in their own communication, (Cartmill and Byrne, 2010).  That being said, this is already strong evidence that the orangutan capacity for language goes far beyond their vocal limitations. 

Shapiro has been one of the pioneering researchers in the area of orangutan sign learning ability.  His research in this area began in 1973 at the Fresno city zoo, where his eighteen-month study of symbolic capability with an orangutan named Aazk became the basis of his master’s thesis.  Through her relationship with Shapiro, Aazk was able to learn to arrange symbols meaningfully on a board in order to build sentences (Thompson, 2010).  A few years later, Shapiro traveled to Borneo to conduct more extensive studies teaching sign language to orangutans in their native environment. 

Princess signs to Gary at dinner One of Shapiro’s more remarkable Bornean students was a juvenile orangutan named Princess.  Princess developed a very strong attachment to Shapiro, selecting him to fill a parental role in their relationship.  Shapiro likewise adopted Princess as his daughter and raised her himself for lack of an orangutan mother to take her in.  This provided a unique setting for Princess’s training.  She was raised by Shapiro in a home environment, but that home was in the middle of her native habitat, where she was permitted considerable freedom.  Princess was taught a variety of Ameslan and modified Ameslan signs, and was able to acquire 37 signs over the course of 19 months of training, in a manner comparable to that of Washoe the chimpanzee and Koko the gorilla.  In fact, Princess acquired vocabulary somewhat more quickly than her gorilla and chimpanzee counterparts, probably because she was older and more mature at the beginning of her training.

Following this, Shapiro (1985) conducted a more extensive study involving Princess and three other orangutans, further documenting and comparing the species’ ability to communicate via sign language. The four subjects were chosen to represent a broad range of variables; two were male, two were female, two were caged, two were free-ranging, two right-handed, two left-handed.  The subjects were trained in the use of ten signs, all of which referred to physical objects, over the course of a fifteen month period.  At the end of this period, all of the orangutans had shown proficiency in at least some of the signs, with a wide range of variance between individual orangutans.  The caged subjects in the study learned better than the uncaged ones, though Shapiro points out that this is most likely due to the caged male’s superior individual performance, rather than conclusive evidence that holding condition has a large effect on an orangutan’s learning ability.  Across the board, the orangutans seemed more interested to learn those signs that were related to food.  This is not surprising since an orangutan in the wild spends a great deal of its time looking for its next meal.  In addition to signs referring to food, the subjects appeared to have an easier time learning those signs which required them to touch a specific location on their body, as opposed to those in which the active hand performs some gesture in midair.  This preference has also been observed in both gorillas and chimpanzees.

Further demonstrating the remarkable language learning abilities of Orangutans is an individual named Chantek (Miles, 1994; PBS, 2014), currently residing in a Zoo Atlanta orangutan habitat.  Beginning in 1978, Chantek was raised and taught to sign—beginning at the age of nine months—by anthropologist Lyn Miles at the University of Tennessee at Chatanooga.  In order to most fully explore Chantek’s capacity for human-like language and cognitive ability, he was raised as much like a human child as possible, fully immersed in the human cultural experience.  The results of this project were remarkable.  Over the course of eight years at the university, Chantek was able to produce more than one hundred and fifty signs, and recognize considerably more.  He also developed a significant understanding of spoken English.  In addition, he was able to combine signs he knew into new signs to designate items not already in his vocabulary, calling ketchup “tomato-toothpaste,” and a cheeseburger “meat-cheese-bread.”  Chantek is not alone among primates, or even among orangutans, in his ability to combine his vocabulary.  During Shapiro’s studies, Princess also displayed this ability.

The special circumstances of Chantek’s upbringing and his extensive language skills give him a unique outlook on himself and the world.  When asked who he was, he has described himself as an “orangutan person”. By contrast, when asked about his fellow orangutans in the Atlanta zoo habitat, he described them as “orange dogs,” indicating that he conceives of himself as being in a different category than others of his species. Still more fascinating—though heartwrenching—was Chantek’s reaction to his confinement immediately after the university project was terminated. Chantek was returned to the Yerkes Primate Research Center.  At the center, he was confined to a small cage after having enjoyed a good quality of life and a high level of freedom at Chattanooga. Miles asked Chantek on one occasion how he was, to which he signed “hurt.” Asked to indicate the location of his injury, he responded “feelings.”  Any animal is likely to be visibly frustrated upon being caged after a period of freedom, but Chantek’s language skills give him an awareness of his own condition and an ability to express his emotions in ways that are not so clearly observable in any animal besides humans.

In addition to language, Chantek developed an interest in another symbolic system important to human culture: currency. Chantek was given a daily allowance of metal washers with which he could purchase certain treats. If he had “money,” he could pay some of it for a soda, or some other reward he found valuable. If he didn’t have money, he would ask for chores to do to earn some. He was even able to understand that if he saved up for several days, he could purchase more desirable rewards, such as a ride in the car to McDonald's, where he could pay for the highly valued “meat-cheese-bread.” Chantek’s extensive language skills, as well as his unique sense of self and his ability to grasp human cultural constructs like simple economics, provide a vivid illustration of just how similar the great apes are to human beings.

The general interest of the scientific community has shifted in recent decades. Language experiments like those of the 70s and 80s have taken a back seat to efforts more focused on orangutans living as naturally as possible. Studies are still being done. Zoo Atlanta, for example, conducts studies in language and cognitive abilities that heavily involve orangutan interaction with a computer located inside the exhibit. Something as aggressive as Chantek’s complete immersion in human culture is unlikely to be repeated in the current scientific climate. However, it continues to be seen that orangutans are immensely intelligent and have great communicative potential. What we have learned from these studies should profoundly impact the way in which we view orangutans, and, perhaps moreso, the way in which we view ourselves. 

- This article was written by Schuyler Lewis via VolunteerMatch.com.

 mosquito the most deadly animal in the worldParasites belong to the group of pathogens that includes viruses, bacteria, fungi, and prions. Although they have a rather unfortunate reputation, the majority of living organisms are parasitic and they play an important role in orangutan population dynamics and ecosystem diversity. With habitat fragmentation and increasing exposure to human settlements, the dynamic of parasites and their effect on remaining wild orangutan populations are becoming the focus of increasing research (Foitova et. al., 2009).

Parasitic infections can have a devastating impact on the health of any individual, and orangutans, who in some regions suffer prolonged periods of lean fruit availability and decreased nutrition, can be particularly susceptible. Studies have shown there are a number of external factors that influence the susceptibility of populations to parasitic infections, including density, contact with humans and climatic factors which affect temperature and humidity. For example, some species of parasites were found to affect chimpanzees more significantly during periods of higher rainfall, and comparison of parasite levels in Bukit Lawang, a popular tourist destination in Sumatra, and Ketambe, where human contact is low, has found that levels are significantly higher in Bukit Lawang, where several species of pinworm have also been documented (Foitova et. al., 2009).

Research published in 2002 recorded the presence of 10 parasites in 371 fecal samples from 24 wild and semi-wild orangutans, and similar studies in 2005 and 2007 showed similar levels, with 16 parasite species found in 421 samples and 13 parasite species in 376 samples respectively, including Chilomastix mesnelii, Endolimax nana, Balantidium coli, Dientamoeba fragilis and Entamoeba hartmanni. Variations were found in parasites found most commonly in wild and semi-wild populations, indicating that exposure to humans and outside influences can result in orangutans hosting parasites not typically found in wild populations (Foitova et. al., 2009).

Although this area of research is in its infancy, it is assumed orangutans, like chimpanzees, eat plants containing bioactive substances, which reduce the level of parasitic infections, and orangutans at Sebangu in Borneo have been seen self-medicating (Morrogh-Bernard, 2008).