Orangutans are the only exclusively Asian genus of extant great ape. The largest living arboreal animals, they have longer arms than the other, more terrestrial, great apes. They are among the most intelligent primates and use a variety of sophisticated tools, also making sleeping nests each night from branches and foliage. Their hair is typically reddish-brown, instead of the brown or black hair typical of other great apes.
Native to Indonesia and Malaysia, orangutans are currently found only in rainforests on the islands of Borneo and Sumatra, though fossils have been found in Java, the Thai-Malay Peninsula, Vietnam and Mainland China. There are only two surviving species, both of which are endangered: the Bornean Orangutan (Pongo pygmaeus) and the critically endangered Sumatran Orangutan (Pongo abelii). The subfamily Ponginae also includes the extinct genera Gigantopithecus and Sivapithecus. The word "orangutan" comes from the Malay words "orang" (man) and "(h)utan" (forest); hence, "man of the forest".
- Genus Pongo
The populations on the two islands were classified as subspecies until recently, when they were elevated to full specific level, and the three distinct populations on Borneo were elevated to subspecies. The population currently listed as P. p. wurmbii may be closer to the Sumatran Orangutan than the Bornean Orangutan. If confirmed, abelii would be a subspecies of P. wurmbii (Tiedeman, 1808). Regardless, the type locality of pygmaeus has not been established beyond doubts, and may be from the population currently listed as wurmbii (in which case wurmbii would be a junior synonym of pygmaeus, while one of the names currently considered a junior synonym of pygmaeus would take precedence for the northwest Bornean taxon). To further confuse, the name morio, as well as various junior synonyms that have been suggested, have been considered likely to all be junior synonyms of the population listed as pygmaeus in the above, thus leaving the east Bornean populations unnamed.
In addition, a fossil species, P. hooijeri, is known from Vietnam, and multiple fossil subspecies have been described from several parts of southeastern Asia. It is unclear if these belong to P. pygmaeus or P. abeli or, in fact, represent distinct species.
Anatomy and physiology
An orangutan's standing height averages from 4 to 5 ft (1.2 to 1.5 m). On average, an orangutan weighs between 73 to 180 pounds (33 to 82 kg). Males can weigh up to 250 lb (110 kg) or more. Orangutan hands are similar to humans' hands; they have four long fingers and an opposable thumb. Their feet have four long toes and an opposable big toe. Orangutans can grasp things with both their hands and their feet. The largest males have an arm span of about 7.5 ft (2 m).
Orangutans have a large, bulky body, a thick neck, very long, strong arms, short, bowed legs, and no tail. They are mostly covered with long reddish-brown hair, although this differs between the species: Sumatran Orangutans have a more sparse and lighter coloured coat.
The orangutan has a large head with a prominent mouth area. Adult males have large cheek flaps (which get larger as the ape ages) that show their dominance to other males and their readiness to mate. The age of maturity for females is approximately 12 years. On average, orangutans may live about 35 years in the wild, and up to 60 years in captivity. Both sexes have throat pouches located near their vocal chords that make their calls resonate through the forest, although the males' pouches are more developed. There is significant sexual dimorphism: females can grow to around 4 ft 2 in or 127 cm and weigh around 100 lb (45 kg) while flanged adult males can reach 5 ft 9 in or 175 cm in height and weigh over 260 lb (118 kg).
The arms of orangutans are twice as long as their legs, and an adult orangutan's arms can be well over seven feet from fingertip to fingertip. Much of the arm's length has to do with the length of the radius and the ulna rather than the humerus. Their fingers and toes are curved, allowing them to better grip onto branches. Orangutans have less restriction in the movements of their legs than humans and other primates, due to the lack of a hip joint ligament which keeps the femur held into the pelvis. Unlike gorillas and chimpanzees, orangutans are not true knuckle-walkers, and are instead fist-walkers.
Ecology and behavior
Orangutans live in primary and old secondary forests, particularly dipterocarp forests and peat swamp forests.  Both species can be found in both mountainous and lowland swampy areas.  Sumatran orangutans live in elevations as high as 1500 m (4921 ft), while Bornean orangutans live no higher than 1000 m (3281 ft). The latter will sometimes enter grasslands, cultivated fields, gardens, young secondary forest, and shallow lakes.  Orangutans are the most arboreal of the great apes, spending nearly all of their time in the trees. Most of the day is spend feeding, resting, and moving between feeding and resting sites. They start the day feeding for 2-3 hours in the morning. They rest during midday followed by traveling in the late afternoon. When evening arrives, they begin to prepare their nest for the night. Tigers are the major predatory threat to orangutans in Sumatra. Orangutans may also be preyed on by clouded leopards and crocodiles. The former can kill adolescents and small adult females but have not been recorded killing adult males. In Borneo, orangutans are not threatened by tigers and seem to descend to the ground more often than their Sumatran relatives. Orangutans do not swim. At least one population at a conservation refuge on Kaja island in Borneo have been photographed wading in deep water.
Fruit makes up 65–90 percent of the orangutan diet. Fruits with sugary or fatty pulp are favored. Ficus fruits are commonly eaten, because they are easy to harvest and digest. Lowland dipterocarp forests are preferred by orangutans because of their plentiful fruit. Bornean orangutans consume at least 317 different food items that include young leaves, shoots, bark, insects, honey and bird eggs.
Orangutans are thought to be the sole fruit disperser for some plant species including the climber species Strychnos ignatii which contains the toxic alkaloid strychnine. It does not appear to have any effect on orangutans except for excessive saliva production.
Geophagy, the practice of eating soil or rock, has been observed in orangutans. There are three main reasons for this dietary behavior; for the addition of minerals nutrients to their diet; for the ingestion of clay minerals that can absorb toxic substances; or to treat a disorder such as diarrhea.
Orangutans live a more solitary lifestyle than the other great apes. Most social bonds occur between adult females and their dependent and weaned offspring. Adult males and independent adolescents of both sexes tend to live alone.  The society of the orangutan is made up of resident and transient individuals of both sexes. Resident females live with their offspring in defined home ranges that overlap with other adult females, who may be their relatives like mothers and sisters. One to several resident female home ranges are encompassed within the home range of a resident male, who is their primary breeder.  Transients males and females range broadly.  They usually travel alone, but as sub-adults they may travel in small groups. However this behavior does not extend to adulthood. The social structure of the orangutan can be best described as solitary but social. As the ranges of males and females overlap, they commonly encounter each other while traveling and feeding and may have brief social interactions. Interactions between adult females range from friendly, to avoidance to antagonistic.  Resident males may have overlapping ranges and interactions between them tend to be hostile.
During dispersal, females tend to settle in home ranges that overlap with their mothers. However, they do not interact when them any more than the other females and they do not seem to form bonds though affiliation, grooming, or agonistic support. Males disperse much farther from their mothers and enter into a transient phase. This phase lasts until a male can challenge and displace a dominant, resident male from his home range. There are dominance hierarchies between adult males that regularly encounter each other with the most dominant males being the largest and having the best body conditions. Adult males dominate sub-adult males. Both resident and transient orangutans aggregate on large fruiting trees to feed. The fruits tend to be abundant, so competition is low and individuals may benefit from social contacts. Orangutans will also form travelling groups in which members coordinate travel between food sources for a few days at a time. These groups tend to be made of only a few individuals. They also tend to be mating consortships, each made of an adult male and female traveling and mating.
Reproduction and parenting
Male orangutans exhibit arrested development. They mature at around 15 years of age by which they have fully descended testicles and can reproduce. However they do not develop the cheek pads, pronounced throat pouches, long fur or long-calls of more mature males until they gain a home range, which occurs when they are between 15 and 20 years old. These sub-adult males are known as unflanged males in contrast to the more developed flanged males. The transformation from unflanged to flanged can occur very quickly. Unflanged and flanged males have two different mating strategies. Flanged males use long calls to advertise their location which attract estrous females. Unflanged males wander widely in search of estrous females and upon finding one, will force copulation on her. Both strategies are successful, however females prefer to mate with flanged males and will seek them out for protection from unflanged males. Resident males may form consortships with females that can last days, weeks or months after copulation.
Female orangutans experience their first ovulatory cycle between 5.8 and 11.1 years. These occur earlier in larger females with more body fat than in thinner females.  Like other great apes, female orangutans have a period of adolescent infertility which may last for 1-4 years. Female orangutans also have a 22-30 day menstrual cycle. Gestation lasts for nine months with females giving birth to their first offspring between 14 and 15 years old. Female orangutans have the longest interbirth intervals of the great apes, having eight years between births.
Male orangutans play almost no role in rising the young. Females are the primary caregivers for the young and are also instruments of socialization for them. A female often has more than one offspring with her, usually an adolescent and an infant, and the older of them can also help in socializing their younger sibling. Infant orangutans are completely dependent on their mothers for the first two years of their lives. The mother will carry the infant during traveling, as well as feed it and sleep with it in the same night nest. The infant doesn’t even break physical contact with its mother for the first four months and is carried on her belly. The amount of physical contact soon wanes in the following months. When an orangutan reaches the age of two, its climbing skills are more developed and will hold the hand of another orangutan while moving through the canopy, a behavior known as "buddy travel". Orangutans are juveniles from about two to five years of age and start to exploratory trips from their mothers.  Juveniles are usually weaned at about four years of age. Adolescent orangutans seek peers and play and travel with peer groups while start having contact with their mothers.
Tool use and culture
Like the other great apes, orangutans are among the most intelligent primates. Wild chimpanzees have been known since the 1960s to use tools. Tool use in orangutans was observed by Birutė Galdikas in ex-captive populations.
Evidence of sophisticated tool manufacture and use in the wild was reported from a population of orangutans in Suaq Balimbing (Pongo pygmaeus abelii) in 1996. These orangutans developed a tool kit for use in foraging that consisted of insect-extraction tools for use in the hollows of trees, and seed-extraction tools which were used in harvesting seeds from hard-husked fruit. The orangutans adjusted their tools according to the nature of the task at hand and preference was given to oral tool use. This preference was also found in an experimental study of captive orangutans (P. pygmaeus).
Carel P. van Schaik from the University of Zurich and Cheryl D. Knott from Harvard University further investigated tool use in different wild orangutan populations. They compared geographic variations in tool use related to the processing of Neesia fruit. The orangutans of Suaq Balimbing (P. abelii) were found to be avid users of insect and seed-extraction tools when compared to other wild orangutans. The scientists suggested that these differences are cultural. The orangutans at Suaq Balimbing live in dense groups and are socially tolerant; this creates good conditions for social transmission. Further evidence that highly social orangutans are more likely to exhibit cultural behaviors came from a study of leaf-carrying behaviors of ex-captive orangutans that were being rehabilitated on the island of Kaja in Borneo. The above evidence is consistent with the existence of orangutan culture as geographically distinct behavioral variants which are maintained and transmitted in a population through social learning.
In 2003, researchers from six different orangutan field sites who used the same behavioral coding scheme compared the behaviors of the animals from the different sites. They found that the different orangutan populations behaved differently. The evidence suggested that the differences in behavior were cultural: first, because the extent of the differences increased with distance, suggesting that cultural diffusion was occurring, and second, because the size of the orangutans’ cultural repertoire increased according to the amount of social contact present within the group. Social contact facilitates cultural transmission. Carel P. van Schaik suggests that young orangutans (P. abelii) acquire tool use skills and cultural behaviors by observing and copying older orangutans.
Orangutans do not limit their tool use to foraging, displaying or nest-building activities. Wild orangutans (P. pygmaeus wurmbii) in Tuanan, Borneo, were reported to use tools in acoustic communication. They use leaves to amplify the kiss squeak sounds that they produce. Some have suggested that the apes employ this method of amplification in order to deceive the listener into believing that they are larger animals.
A two year study of orangutan symbolic capability was conducted from 1973-1975 by Gary L. Shapiro with Aazk, a juvenile female orangutan at the Fresno City Zoo (now Chaffee Zoo) in Fresno, California. The study employed the techniques of David Premack who used plastic tokens to teach the chimpanzee, Sarah, linguistic skills. Shapiro continued to examine the linguistic and learning abilities of ex-captive orangutans in Tanjung Puting National Park, in Indonesian Borneo, between 1978 and 1980. During that time, Shapiro instructed ex-captive orangutans in the acquisition and use of signs following the techniques of R. Allen and Beatrix Gardner who taught the chimpanzee, Washoe, in the late-1960s. In the only signing study ever conducted in a great ape's natural environment, Shapiro home-reared Princess, a juvenile female who learned nearly 40 signs (according to the criteria of sign acquisition used by Francine Patterson with Koko, the gorilla) and trained Rinnie, a free-ranging adult female orangutan who learned nearly 30 signs over a two year period. For his dissertation study, Shapiro examined the factors influencing sign learning by four juvenile orangutans over a 15-month period.
The first orangutan language study program, directed by Dr. Francine Neago, was listed by Encyclopædia Britannica in 1988. The Orangutan language project at the Smithsonian National Zoo in Washington, D.C., uses a computer system originally developed at UCLA by Neago in conjunction with IBM.
Zoo Atlanta has a touch screen computer where their two Sumatran Orangutans play games. Scientists hope that the data they collect from this will help researchers learn about socializing patterns, such as whether they mimic others or learn behavior from trial and error, and hope the data can point to new conservation strategies.
A 2008 study of two orangutans at the Leipzig Zoo showed that orangutans are the first non-human species documented to use 'calculated reciprocity' which involves weighing the costs and benefits of gift exchanges and keeping track of these over time.
Sexual interest in human females
Male orangutans have been known to rape human women. The cook of noted primatologist Birutė Galdikas was raped by an orangutan. An orangutan tried to have sex with Actress Julia Roberts but was prevented by a film crew.
The Sumatran species is critically endangered and the Bornean species of orangutans is endangered according to the IUCN Red List of mammals, and both are listed on Appendix I of CITES. The total number of Bornean orangutans is estimated to be less than 14% of what it was in the recent past (from around 10,000 years ago until the middle of the twentieth century) and this sharp decline has occurred mostly over the past few decades due to human activities and development. Species distribution is now highly patchy throughout Borneo: it is apparently absent or uncommon in the south-east of the island, as well as in the forests between the Rejang River in central Sarawak and the Padas River in western Sabah (including the Sultanate of Brunei). The largest remaining population is found in the forest around the Sabangau River, but this environment is at risk. A similar development have been observed for the Sumatran orangutans.
A 2007 study by the Government of Indonesia noted in 2004 it was estimated that there was a total wild population of 61,234 orangutans, 54,567 of which were found on the island of Borneo. The table below shows a breakdown of the species and subspecies and their estimated populations from the report:
|Pongo abelii||Sumatran Orangutan||Sumatra||6,667|
|Pongo pygmaeus||Bornean Orangutan||Borneo|
|P. p. morio||Northeast Bornean Orangutan||Sabah||11,017|
|P. p. morio||Northeast Bornean Orangutan||East Kalimantan||4,825|
|P. p. wurmbii||Central Bornean Orangutan||Central Kalimantan||>31,300|
|P. p. pygmaeus||Northwest Bornean Orangutan||West Kalimantan and Sarawak||7,425|
This indicates a decline from some estimates between 2000 and 2003 which found 7,300 Sumatran Orangutan individuals in the wild and between 45,000 and 69,000 Bornean Orangutans. Thousands of orangutans don't reach adulthood due to human disruption. Orangutans are killed for food while others are killed because of disruption in people's property. Mother orangutans are killed so their infants can be sold as pets. Many of the infants die without the help of their mother. Since recent trends are steeply down in most places due to logging and burning, it is forecast that the current numbers are below these figures.
Orangutan habitat destruction due to logging, mining and forest fires, as well as fragmentation by roads, has been increasing rapidly in the last decade. A major factor in that period of time has been the conversion of vast areas of tropical forest to oil palm plantations in response to international demand (the palm oil is used for cooking, cosmetics, mechanics, and more recently as source of biodiesel). Some UN scientists believe that these plantations could lead to irreparable damage to orangutan habitat by the year 2012. Some of this activity is illegal, occurring in national parks that are officially off limits to loggers, miners and plantation development. There is also a major problem with hunting and illegal pet trade. In early 2004 about 100 individuals of Bornean origin were confiscated in Thailand and 50 of them were returned to Kalimantan in 2006. Several hundred Bornean orangutan orphans who were confiscated by local authorities have been entrusted to different orphanages in both Malaysia and Indonesia. They are in the process of being rehabilitated into the wild.
Conservation centres and organisations
A number of organisations are working for the rescue, rehabilitation and reintroduction of orangutans. The largest of these is the Borneo Orangutan Survival Foundation, founded by Dr. Willie Smits, which employs between six hundred and a thousand people at a hundred sites. It operates a number of large projects, including the Samboja Lestari Forest Rehabilitation Program and the Nyaru Menteng Rehabilitation Program managed by Lone Drøscher Nielsen. Other major conservation centres in Indonesia include those at Tanjung Puting National Park and Sebangau National Park in Central Kalimantan, Kutai in East Kalimantan, Gunung Palung National Park in West Kalimantan, and Bukit Lawang in the Gunung Leuser National Park on the border of Aceh and North Sumatra. In Malaysia, conservation areas include Semenggoh Wildlife Centre in Sarawak and Matang Wildlife Centre also in Sarawak, and the Sepilok Orang Utan Sanctuary near Sandakan in Sabah.
Orangutans have 48 diploid chromosomes, and its genome was sequenced in January 2011. Following humans and chimpanzees, the Sumatran orangutan has become the third species of hominid to have its genome sequenced. The draft of the genome sequence is based on a captive female named Susie.
The researchers also published less complete copies from ten wild orangutans, five from Borneo and five from Sumatra. It was found that genetic diversity was lower in Bornean orangutans (Pongo pygmaeus) than in Sumatran ones (Pongo abelii), despite the fact that Borneo is home to six or seven times as many orangutans as Sumatra. The comparison has shown that these two species diverged around 400,000 years ago, more recently than was previously thought. It was also found that the orangutan genome has evolved much more slowly than chimpanzee and human DNA.
- ^ a b c Groves, C. (2005). Wilson, D. E., & Reeder, D. M, eds. ed. Mammal Species of the World (3rd ed.). Baltimore: Johns Hopkins University Press. pp. 183–184. OCLC 62265494. ISBN 0-801-88221-4. http://www.bucknell.edu/msw3/browse.asp?id=12100803.
- ^ a b c Bradon-Jones, D., A. A. Eudey, T. Geissmann, C. P. Groves, D. J. Melnick, J. C. Morales, M. Shekelle, and C. B. Stewart. 2004. Asian primate classification. International Journal of Primatology. 23: 97-164.
- ^ "National Geographic". National Geographic Society. http://animals.nationalgeographic.com/animals/mammals/orangutan.html. Retrieved 2009-07-25.
- ^ a b Patricia L. Miller-Schroeder (2004). Orangutans. The Untamed World. p. 64. ISBN 1553880498. http://books.google.com/?id=EOb5Ca5DjTQC&lpg=PP1&dq=Orangutans&pg=PP1#v=onepage&q=. Retrieved 2009-07-07.
- ^ "Orangutan Anatomy Page". Red-ape.co.uk. http://www.red-ape.co.uk/orangutan/orang_anatomy.html. Retrieved 2009-07-03.
- ^ "Orangutans of Indonesia". Cornell University Southeast Asia Program. http://www.einaudi.cornell.edu/southeastasia/outreach/resources/orangutans.ppt.
- ^ a b c "Primates: Orangutans - National Zoo| FONZ". Nationalzoo.si.edu. http://nationalzoo.si.edu/Animals/Primates/Facts/FactSheets/Orangutans/default.cfm. Retrieved 2011-01-27.
- ^ "Sumatran Orangutan Society". Archived from the original on 2006-11-14. http://web.archive.org/web/20061114233340/http://www.orangutans-sos.org/faq.php. Retrieved 2007-04-01.
- ^ "Orangutans at the Atlanta Zoo, Georgia". Galenfrysinger.com. http://www.galenfrysinger.com/georgia_atlanta_zoo_orangutans.htm. Retrieved 2011-01-27.
- ^ Schwartz, Jeffrey (1987). The Red Ape: Orangutans and Human Origins. Cambridge, MA: Westview Press. p. 286. ISBN 0813340640.
- ^ a b Rijksen HD, Meijaard E. (1999) Our vanishing relative: the status of wild orangutans at the close of the twentieth century, Dordrecht (The Netherlands): Kluwer Acad.
- ^ Rodman PS. (1988) "Diversity and consistency in ecology and behavior", In:Orangutan biology. p 31-51 Schwartz JH, (ed).
- ^ a b Galdikas BMF, Insley SJ. (1988) "The fast call of the adult male orangutan", J Mammal 69(2): 371-82.
- ^ a b c d e f g h i j Rijksen HD. (1978) A field study on Sumatran orangutans (Pongo pygmaeus abelii Lesson 1827): ecology, behaviour and conservation, Wageningen (The Netherlands): H. Veenman Zonen BV.
- ^ Swimming orangutans' spearfishing exploits amaze the wildlife experts.
- ^ Cawthon Lang KA (2005-06-13). "Primate Factsheets: Orangutan (Pongo) Taxonomy, Morphology, & Ecology". http://pin.primate.wisc.edu/factsheets/entry/orangutan. Retrieved 2007-10-12.
- ^ a b Galdikas, Birute M.F. (1988). "Orangutan Diet, Range, and Activity at Tanjung Puting, Central Borneo". International Journal of Primatology 9 (1): 1. doi:10.1007/BF02740195.
- ^ Rijksen, H. D. (December 1978). "A Field Study on Sumatran Orang Utans (Pongo pygmaeus abelii, Lesson 1827): Ecology, Behaviour and Conservation". The Quarterly Review of Biology 53 (4): 493. doi:10.1086/410942. http://links.jstor.org/sici?sici=0033-5770(197812)53%3A4%3C493%3AAFSOSO%3E2.0.CO%3B2-4.
- ^ Highfield, Roger (1 April 2008). "Science: Monkeys were the first doctors (Telegraph.co.uk)". The Daily Telegraph (London). http://www.telegraph.co.uk/earth/main.jhtml?xml=/earth/2008/04/01/scimonkey101.xml. Retrieved 2010-05-20. [dead link]
- ^ Matt Walker Wild orangutans treat pain with natural anti-inflammatory New Scientist 28 July 2008.
- ^ a b c te Boekhorst IJA, Schürmann CL, Sugardjito J. (1990) "Residential status and seasonal movements of wild orangutans in the Gunung Leuser Reserve (Sumatera, Indonesia) ", Anim Behav 39(6): 1098-1109.
- ^ a b Rodman PS. (1993) " Diversity and consistency in ecology and behavior". In: Orangutan population and habitat viability analysis workshop: briefing book; Jan 18-20; Tilson R, Traylor-Holzer K, Seal U, (eds); Medan, North Sumatra, Indonesia. Apple Valley (MN): IUCN/SSC Captive Breeding Specialist Group. p 31-51.
- ^ a b Galdikas BMF. (1984) "Adult female sociality among wild orangutans at Tanjung Puting Reserve". In: Female primates: studies by women primatologists, Small MF (ed), New York: Alan R. Liss. p 217-35.
- ^ Singleton I, van Schaik CP. (2002) "The social organisation of a population of Sumatran orangutans", Folia Primatol 73: 1-20.
- ^ a b Delgado RA Jr., van Schaik CP. (2000) "The behavioral ecology and conservation of the orangutan (Pongo pygmaeus): a tale of two islands", Evol Anthro 9(1): 201-18.
- ^ a b c van Schaik CP, Preuschoft S, Watts DP. (2004) "Great ape social systems". In: The evolution of thought: evolutionary origins of great ape intelligence, Russon AE, Begun DR (eds) Cambridge (England): Cambridge Univ Pr. p 190-209.
- ^ a b Fox EA. (2002) "Female tactics to reduce sexual harassment in the Sumatran orangutan (Pongo pygmaeus abelii) ", Behav Ecol Sociobiol 52: 93-101.
- ^ Schürmann CL, van Hooff JARAM. (1986) "Reproductive strategies of the orangutan: new data and a reconsideration of existing sociosexual models", Int J Primatol 7(3): 265-87.
- ^ a b c Utami SS, Goossens B, Bruford MW, de Ruiter JR, van Hooff JARAM. (2002) "Male bimaturism and reproductive success in Sumatran orangutans", Behav Ecol 13(5): 643-52.
- ^ Kaplan G, Rogers LJ. (1994) Orangutans in Borneo, Armidale (Australia): Univ New England.
- ^ a b Knott C. (2001) Female reproductive ecology of the apes: implications for human evolution. In: Reproductive ecology and human evolution, Ellison PT, (ed) New York: Aldine de Gruyter. p 429-63.
- ^ a b Galdikas BMF. (1995) Social and reproductive behavior of wild adolescent female orangutans. In: The neglected ape, Nadler RD, Galdikas BFM, Sheeran LK, Rosen N, (eds) New York: Plenum Pr. p 163-82.
- ^ a b c Munn C, Fernandez M. (1997) Infant development. In: Orangutan species survival plan husbandry manual, Sodara C,( ed) Chicago (IL): Orangutan Species Survival Plan. p 59-66.
- ^ Deaner RO, van Schaik CP, Johnson V. 2006. Do some taxa have better domain cognition than others? A meta-analysis of nonhuman primate studies. Evol Psych 4: 149-196
- ^ Goodall J. 1970. Tool-using in primates and other vertebrates. In: Lehrman DS, Hinde RA, Shaw E, editors. Advances in the study of behavior. New York, Academic Press. Vol. 3: p. 195-249
- ^ Galdikas BMF. 1982. Orang-Utan tool use at Tanjung Putting Reserve, Central Indonesian Borneo (Kalimantan Tengah). J Hum Evol 10:19-33
- ^ van Schaik CP, Fox EA, Sitompul AF. 1996. Manufacture and use of tools in wild Sumatran orangutans – implications or human evolution. Naturwissenschaften 83: 186-188.
- ^ Fox EA, Sitompul AF, Van Schaik CP.1999. Intelligent tool use in wild Sumatran orangutans. In: Parker S, Mitchell RW and Miles HL, editors. The Mentality of Gorillas and Orangutans. Cambridge, UK : Cambridge University Press. p: 99-116
- ^ O’Malley RC, McGrew WC. 2000. Oral tool use by captive orangutans (Pongo pygmaeus). Folia Primatol 71:334–341
- ^ a b van Schaik CP, Knott CD. 2001. Geographic variation in tool use on Neesia fruits in orangutans. American Journal of Physical Anthropology 114:331-342
- ^ van Schaik CP, van Noordwijk MA, Wich SA. 2006. Innovation in wild Bornean orangutans (Pongo pygmaeus wurmbii). Behaviour 143: 839-876
- ^ Russon AE, Handayani DP, Kuncoro P, Ferisa A. 2007. Orangutan leaf-carrying for nest-building: toward unraveling cultural processes. Animal Cognition 10:189–202
- ^ a b van Schaik CP. Among Orangutans: Red Apes and The Rise of Human Culture. 2004. Cambridge: Harvard University Press ISBN 9780674015777
- ^ a b van Schaik CP, Ancrenaz M, Borgen G, Galdikas B, Knott CD, Singleton I, Suzuki A, Utami SS, Merrill M. 2003. Orangutan cultures and the evolution of material culture. Science 299: 102-105
- ^ a b Hardus ME, Lameira AR, van Schaik CP, Wich SA. 2009. Tool use in wild orang-utans modifies sound production: a functionally deceptive innovation? Proceedings of the Royal Society B: Biological Sciences 276 (1673):3689-3694
- ^ Dissertation.
- ^ "Orangutan Language Project". Think Tank Research Projects. Smithsonian National Zoological Park. http://nationalzoo.si.edu/Animals/ThinkTank/ResearchProjects/OLP/default.cfm. Retrieved 2006-12-20.
- ^ Turner, Dorie (2007-04-12). "Orangutans play video games at GA. zoo". http://news.yahoo.com/s/ap/orangutan_video_games;_ylt=ArxSgpzeet8z1adszU5pfW7q188F. Retrieved 2007-04-12. [dead link]
- ^ Humans aren’t alone in giving gifts.
- ^ Chimps, Other Apes Laugh Like People Discovery channel, Jennifer Viegas' interview with study project leader Marina Davila Ross
- ^ Demonic Males: Apes and the Origins of Human Violence By Richard W. Wrangham, Dale Peterson, page Page 137
- ^ ""Beauty and the Beasts" by Carole Jahme". Salon.com. 2001-08-23. http://www.salon.com/books/review/2001/08/23/primates. Retrieved 2011-01-27.
- ^ a b c d e f g h Singleton, I., Wich, S.A. & Griffiths, M. 2008. Pongo abelii. In: IUCN 2010. IUCN Red List of Threatened Species. Version 2010.4. www.iucnredlist.org Downloaded on 28 Jan. 2011.
- ^ a b c d e f g h i j k Ancrenaz, M., Marshall, A., Goossens, B., van Schaik, C., Sugardjito, J., Gumal, M. & Wich, S. 2008. Pongo pygmaeus. In: IUCN 2010. IUCN Red List of Threatened Species. Version 2010.4. www.iucnredlist.org Downloaded on 28 Jan. 2011.
- ^ Density and population estimate of gibbons (Hylobates albibarbis) in the Sabangau catchment, Central Kalimantan, Indonesia[dead link].
- ^ "Orangutan Action Plan 2007-2017" (in Indonesian) (PDF). Government of Indonesia. 2007. p. 5. http://www.yorku.ca/arusson/Papers/GoI%20OU%20action%20plan%2007-17.pdf. Retrieved 1 May 2010.
- ^ Rijksen, H.D. and Meijaard, E. (1999). Our Vanishing Relative: The Status of Wild Orang-utans at the Close of the Twentieth Century. Boston: Kluwer Academic Publishers. ISBN 079235754X.
- ^ Cynthia Turnage and Mark McGinley. 2010. Orangutan. Encyclopedia of Earth. topic ed. C.Michael Hogan. ed. Cutler J. Cleveland. NCSE, Washington DC
- ^ The oil for ape scandal.
- ^ Smith, David (2007-03-25). "Five years to save the orang utan". World (London: The Observer). http://observer.guardian.co.uk/world/story/0,,2042243,00.html. Retrieved 2010-05-20.
- ^ "The Last Stand of the Orangutan". United Nations Environment Programme. February, 2007. http://www.unep-wcmc.org/resources/publications/LastStand.htm. Retrieved 2007-12-03.
- ^ Thompson, Shawn (2010). The Intimate Ape: Orangutans and the Secret Life of a Vanishing Species. Citadel Press. p. 180. ISBN 978-0806531335.
- ^ Sharshov, Alexander. "New Page 1". SB RAS Novobrisk. Institute of Cytology and Genetics. http://www.bionet.nsc.ru/labs/chromosomes/maps/orangutan.htm. Retrieved 28 January 2011.
- ^ a b c d Singh, Ranjeet (2011-01-04). "26 January 2011 | Nature | doi:10.1038/news.2011.50". Nature. http://www.nature.com/news/2011/110126/full/news.2011.50.html. Retrieved 2011-01-27.