Pongo spp consists of three species of orangutan, the largest arboreal species, and a member of the Great Apes. The first two articulated species were P. abelii (Lesson 1827)(Sumatran Orangutan) and P. pygmaeus (Linneus 1760) (Bornean Orangutan) who were recognized as distinct species since 2001 based on mitochondrial DNA analysis. However, a small southern population of the Sumatran Orangutan, previously thought of as P. abelii, was recognized as their own species in 2017 as P. tapanuliensis (Tapanuli orangutan). This was due to a combination of small morphological differences like skull size, first molar size, hair texture and genomic diversity. The morphological and behavorial differences between P. abelii and P. pygmaeus are more apparent, with Bornean orangutans (P. pygmaeus) coming to the ground less frequently than P. abelii and having more fatty flanges in the adult males. ("Morphometric, Behavioral, and Genomic Evidence for a New Orangutan Species", 2017; Xu and Arnason, 1996)

Geographic Range

All three species of genus Pongo are found in the Oriental Region, specifically the islands of Indonesia. P. abelii and P. tapanuliensis are found in Northern Sumatra. P. pygmaeus is found exclusively in Borneo, from the northern provinces of Sabah to Kalimantan. ("Bornean Orangutan", 08 February 2016; "Sumatran Orangutan", 16 October 2017; "Tapanuli Orangutan", 11 October 2017)

  • Biogeographic Regions
  • indian ocean


Members of Pongo spp. reside mainly in tropical evergreen forests, but their highest population densities occur in swamps, fertile valleys, and lowland soils. Orangutans prefer places with the most fertile soil, as this will yield the most fruit. Fruit trees in the Meliacedae (mahogany) family represent a common feature of many of their chosen habitats. Generally, it is assumed that Bornean orangutans have smaller ranges than those on Sumatra, but this is not the case. Males typically have larger ranges than females, and both male and female ranges will overlap with other orangutans. Range size is debated and varies from 850ha. to 2500ha, and perhaps even more.

Orangutans also utilize disturbed and fragmented forests, a trait that is priceless with the rapid deterioration of their old-growth habitat. They prefer areas of increased canopy closure, vertical stability, and uniform height. Canopy layering and shape was of little importance for selection. Orangutans inhabiting highly fragmented areas are important for connectivity of metapopulations and can still persist in these areas, like palm-oil forests.

There are vast areas of uninhabited tropical forest in both Borneo and Sumatra as well as swamps, and researchers think this could come from a vast array of reasons. Thousands of years of hunting in combination with occasional droughts causing starvation could have contributed to the vacancy in these now high-quality habitats. (Ancrenaz, et al., 2021; Davies, et al., 2017; Payne and Prudente, 2008; Seaman, et al., 2019)

Systematic and Taxonomic History

Orangutans are classified as members of the placental mammals, or infraclass Eutheria. They are members of family hominidae, or the family containing Great Apes and Humans. Great Apes include bonobos, eastern and western gorillas, chimpanzees, and the orangutans.

In 2009, researchers found that at least 13 million years ago, humans and orangutans shared a common ancestor although species from genus Pongo are the least related great ape to humans.

Based on historical biogeographical reconstruction, the ancestral population of orangutan most likely came from mainland Asia and settled on the Northern end of Sumatra. Within the Sumatran population, recently a new species has been articulated separate from P. abelii. Genetic evidence in combination with developmental differences in skull size distinguish P. tapanuliensis as a species and thus, one of the most endangered species of Great Ape, with perhaps only 800 individuals in existence today. However, P. tapanuliensis is also the oldest species of orangutan, diverging about 3.38 million years ago. About 674 thousand years ago, genetic modeling indicates that the Sumatran and Bornean populations split. (Grehan and Schwartz, 2009; "Morphometric, Behavioral, and Genomic Evidence for a New Orangutan Species", 2017; Xin, et al., 2013; Xu and Arnason, 1996)

  • Synonyms
    • Faunus (Oken, 1816)
    • Lophotus (Fischer, 1813)
    • Macrobates (Bilberg, 1828)
    • Satyrus (Lesson, 1840)
  • Synapomorphies
    • Exclusively Arboreal (unless male is too large for branches)
    • Cheek Flanges in males

Physical Description

Orangutans are characterized by their ape-like appearance and distinctive red, shaggy hair that covers almost their entire body, with males and some female possessing a beard of the same color. Their long arms, up to 7 or 8 feet in length, also characterize this genus. Males weigh around 160 pounds with females around 80. They have extremely flexible joints with full rotation of their hips. Both their long arms and flexible hips aid in their quadrumanous scrambling, which differs from swinging from branches, and features individuals shifting their weight to force branches to bend over to the next branch. Their weight is also noteworthy, with males weighing upwards of 300 pounds and females at 150.

Most historic literature is currently aimed at distinguishing physical characteristics of the Bornean and Sumatran populations of orangutan. There are general trends between the two islands, with Sumatran individuals having a less broad of a face and being slightly lighter than Bornean Orangutans. Perhaps the biggest difference in the two regional populations is the shape and texture of the male cheek flanges (pads). Male orangutans possess cheek flaps later in development as a form of sexual selection, with bigger flanges indicating a more successful mate. Sumatran males are noted to have a diamond like shape of their flanges, with tufts of short yellow or white hair. Bornean counterparts have a more square shape of the flanges, developing more laterally and rounded. It has few hairs and wrinkles the brow and creases the nose, giving Bornean males a mostly grumpy appearance. (Abrams, 2017; Mackinnon, 1975)

  • Sexual Dimorphism
  • male larger
  • sexes shaped differently


Orangutans are known for having the slowest life history of not only all great apes, but mammals in general. It takes an average of 15.4 years for a female to become reproductively active, with a range of 13-18 years. Females, on average, birth young about every 9 years.

Females that are reproductively active are usually traveling alone or with their offspring. If there is a male and a female traveling together, it may be due to a sexual relationship or a coincidental closeness in proximity from feeding. Males, flanged or unflanged, pursue females that are reproductively receptive, and will fight for a reproductively active female. Flanged males have cheek pads and longer vocalizations, as well as usually being more dominant and sexually successful. However, they are extremely intolerant of other males, using their vocalizations from throat pouches as a way to alert other males of their presence.

The bimaturism of male orangutans have implications on their courting behavior for a female. As noted, flanged males use primarily their vocalizations that deter males and attract a female. Unflanged males resort to searching for a female.

Female choice is the main factor in mating success. Older females prefer more flanged, dominant males. Sometimes, forced mating occurs, and this is accompanied by resistance and/or a fight. Females can be cooperative at the beginning and resistant towards the end or vice versa.

Once the female is receptive to reproduction, an involuntary consortship is formed for anywhere from a few days to a month. During this time, the male and female travel together, and both flanged and unflanged males exhibit mate guarding behavior to keep other males away. However, usually more dominant, flanged males are more successful in mate guarding, while unflanged males being kicked out by a larger male if encountered. (Payne and Prudente, 2008; Shumaker, et al., 2008; S.S, et al., 2009)

Once impregnated, gestation for female orangutans lasts about 8 and a half months. There is no breeding season for this genus, and females are able to have offspring year-round up until they die. As stated previously, females have offspring about every 9 years almost always to one, with very rarely having twins.

Newborn orangutans are extremely dependent upon their mother, and weigh 3.5-4.5 pounds. Body contact is continuous for the first 3 months, but entire dependence on food lasts for the first two years. At around 3 years of age, young orangutans make play nests and start exploring their habitat while always in view of the mother.

Full weaning of milk does not occur until about 3.5 years old. Young orangutans will spend the next few years imitating their mother's behavior through trial and error. The mother will teach her young everything from swinging through trees, using tools for food, and overall how to live in the forest.

At about six or seven years of age, offspring are considered adolescents and spend more and more time away from their mother. At this age, females express an interest in sexual selection and can be reproductively active, although males tend to pursue older, more established females. Females may not give birth until they are 10-15 years of age. Neither males or females are fully grown at this age, and will occasionally come into contact with their mother during this period. Their mother may have given birth to a sibling by then, giving these adolescents someone to play with. Males will not reproduce until they are 13-15 years old, despite being reproductively fertile well before that. (Payne and Prudente, 2008; Shumaker, et al., 2008; S.S, et al., 2009)

Orangutans are semi-solitary in nature, so males have almost no investment or care into offspring.

Females develop a strong bond with their young, and timing and investment are stated in the previous section. When she gives birth to a newborn, she often shields it from view. A wild female orangutan may only have 3-5 offspring in her entire life. (Payne and Prudente, 2008; Shumaker, et al., 2008; S.S, et al., 2009)

  • Parental Investment
  • altricial
  • female parental care
  • pre-fertilization
    • protecting
      • male
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • female
  • pre-independence
    • provisioning
      • female
    • protecting
      • female
  • extended period of juvenile learning


Orangutans are fairly long-lived apes in the wild living up to 58 years for males and 53 for females. In captivity, they are known to live as long, but the oldest captive orangutan is thought to have been 61 when she died.

Orangutans most commonly die of old age in the wild, but current human-induced threats include starvation, hunting by humans as pests, capturing babies to be sold on the black market as pests, and wildfires. (Tacutu, et al., 2017)


As stated before, members of the genus pongo are semi-solitary for most of their adult life. They are motile, and their day-to-day movements are largely a response to spatiotemporal variability in food availability. They eat mainly ripe fruits and food distribution is variable, so most orangutans travel alone unless it is a female with her offspring. Adult flanged males are the most intolerant of other members of their species unless they are actively trying to form a consortship with another female, thus they are rarely territorial. Besides when mating, orangutans are tolerant of other members of their species in their home range, however males tend to stay away from each other more than females. They are motile, and their day-to-day movements are largely a response to spatiotemporal variability in food availability.

There is a social hierarchy, but flanged males are at the top. They can outcompete unflanged males for a mate and will typically choose an older female rather than an adolescent one.

Their locomotion through the trees is a distinguishing characteristic of these Great Apes. They climb, clamor, and brachiate through the canopy, ingesting over 90% of their diet in this region.

Tool use also defines orangutans. Most studies have focused on the behavioral differences between the two regional populations of orangutans, or Bornean and Sumatran Island populations. Most orangutan offspring learn all of their behaviors from their mother, including tool use. Interestingly, research has found that different populations of orangutan display different behaviors based off of their geographic region, an indication of a culture. Roof building of nests (to stop rain) was observed in Borneo, but not on Sumatra. There were other behaviors, like covering yourself in leaves to protect from the sun, that were observed in some of the populations on Borneo and Sumatra, but not all.

Orangutans on the island Sumatra have generally been observed using tools more than that of Borneo. These orangutans are known to use sticks to poke out insects from trees and leaves to hold fruits with spines, as well as many others. Bornean orangutans are generally more social, and thus in captivity, have learned a variety of behaviors from orangutans and humans that are obviously not their species or direct mother. They have been known to make paintings in captivity as well as learn sign language, or associate specific signs with specific human meanings. Rehabilitating orangutans in Tanjung National Park in Kalimantan were seen imitating human behavior like clothes washing, teeth brushing, hammering nails, and voluntarily riding in boats. (Payne and Prudente, 2008; Schaik, et al., 2008)

Communication and Perception

Communication among members of genus Pongo is perhaps one of the most complex and diverse characteristics of these animals. The most distinct and charismatic form of communication for orangutans is the "long call," performed by adult flanged males only. Heard from over a mile away, these long calls invite behavioral responses from all ages and sexes, but other adult males respond the most. The flanged male uses their throat sac to emit a long, sometimes minutes long, series of "roars." Their cheek pads funnel the sound in a particular direction, and studies have shown males will sometimes make these calls in the direction they are traveling to warn other males they are on the move towards them as early as one day in advance. Other males use their long calls in response to trees falling, mostly because some males will knock down trees as a form of dominance. Some males use their long calls to attract females, and it has been noted that Bornean female orangutans can recognize the calls of individual males.

Besides long calls, there are many other forms of orangutan vocal communication. Bornean orangutans were observed in 2019 to have 11 vocal signal types. These vocal signals can be anything from displeasure, to playful infants fighting, to warning of rainfall. In fact, Sumatran orangutans were observed suppressing alarm calls up to 20 minutes when waiting for threats to pass. This means they are capable time-space-displaced responses or giving information about an event that has happened in the past.

Gestures are perhaps the most widely-used orangutan form of communications. Over 21 gesture types were identified in a Bornean population of orangutans in 2019. They have a range of meanings, from requesting objects, sharing tools, warning of harmful behavior, climb over/on me, resume play, etc. Gestures can be made by hands or feet, though hands are preferred. Orangutans have also been known to repeat gestures when their message was only partially understood, and in some cases can change their gestures if they believe their message was not understood at all. (Cartmill and Byrne, 2010; Galdikas and Insley, 1988; Lameira and Call, 2018; Leavens, 2007; Payne and Prudente, 2008)

  • Other Communication Modes
  • mimicry

Food Habits

Orangutans are highly opportunistic foragers, with over 317 food sources noted in 1988 with a complete species list numbering over 1600 different species preyed upon by orangutans. When resources are abundant, they prefer to consume fruits as 60% of the orangutan diet is fruit, but this varies month to month. When fruit is available, they consume a plethora of different species like ficus and durian, the bulk of their diet, but they will eat apples, oranges, bananas, berries, lychees, breadfruit, and jackfruit. With decreasing habitat available, many orangutans are raiding farmers fruit crops, with females being more likely to raid than males. In addition, it's been found that the greatest factor to their distribution, which is linked to foraging, is resting time. While much of their time is spent foraging, the periods of rest in between are the main determinant of distribution.

When fruit isn't an option, many orangutans will increase from 22% of their foraging time for bark to 44%. In addition to tree bark, species like Gironniera nervosa are also an important source of leaves and flowers. Most members of pongo prefer younger leaves and flowers, as many adult leaves develop toxins that can be harmful for herbivores. They also consume nectar and honey from trees and plants.

Orangutans don't only eat plants, consuming invertebrate species include 4 species of ants, 4 species of termites, 2 species of caterpillars, leeches, maggots, ticks, and larvae. They poke out many of these invertebrates using a stick in the burrows or crevices where the invertebrates reside. Orangtuans will also eat vertebrates including the eggs of many bird species, tree rats, small lizards, and primates like loras or gibbons.

There are a few regional differences between the orangutan diet. Subpopulations of orangutans each vary widely on what they eat, as their foraging behavior determines what they eat, and their behavior varies by population. For example, on the island of Sumatra, figs are much more prevalent, so Sumatran orangutans have a higher fig density in their diet. (Campbell-Smith and Campbell-Smith, 2011; Carne, et al., 2015; Galdikas and Insley, 1988; Russon, et al., 2009)


There are few predators for members of genus pongo besides humans. Tigers and leopards are the main predator of orangutan, but it is very rare as most of these predators have been extirpated from their original home ranges. Young orangutans are at most risk of predation, but being arboreal, orangutans are rarely predated upon. Bornean orangutans do not have to worry about tigers, as they have been eradicated from the island for decades. If a predator is spotted, many orangutans will raise an alarm call for neighboring orangutans, often after the predator has passed. (Lameira and Call, 2018; Schaik, et al., 2008)

  • Known Predators
    • Leopard
    • Tiger

Ecosystem Roles

With hundreds of plant and tree species predated upon by the genus g. pongo, orangutans are important seed dispersers for the forests they reside in. These forests house many other spectacular and rare species, like the Sumatran tiger or Sumatran elephant. (Galdikas, 1988)

  • Ecosystem Impact
  • disperses seeds
  • creates habitat

Conservation Status

All 3 species in this genus are considered critically endangered by the IUCN. Projections aren't up to date but estimate around 102,000 Bornean orangutans, 13,800 Sumatran orangutans, and fewer than 800 Tapanuli orangutans. All major threats to members of pongo* are human-induced or human-related, with the largest of these threats is habitat loss. Conversion of forest to agriculture is the main form of habitat loss, with the Sumatran orangutan being heavily affected by palm-oil plantations. Legislation like the Aceh Provincial Spatial Plan of 2013 disregards Sumatran orangutan habitat in favor of expansion of palm oil plantations into their territory. Housing, tourism, and recreation development also threaten orangutan habitat. Logging for timber, creation of roads, and mining operations all contribute to habitat fragmentation. Orangutans are also subject to the illegal pet trade, with intentional killing of adult female orangutans and the selling of her orphans on the black market. Fires on Borneo affect the Tapanuli and Bornean orangutans, like when 90% of Kutai National Park was burned and killed thousands of orangutans in the process.

Conservation Action is currently lacking for orangutans. There is no systematic monitoring scheme for any species of orangutan. There are action recovery plans for all but the species P. pygmaeus, or the Bornean Orangutan. Conservation sites have been identified for all 3 species of orangutan, but P. pygmaeus is lacking sites over their entire range. While over 90% of the Sumatran population is protected, a mere 35% is protected for Bornean species, and only 1-10% for the Tapanuli orangutan. However, there is hope, as all species have been included in international legislation and are subject to international trade control. There have also been recent education and awareness programs for all 3 species. ("IUCN Red List", 16 October 2017; "IUCN Red List", February 8 2016; "IUCN Red List", October 11 2017)

  • IUCN Red List [Link]
    Not Evaluated


Alexander Hey (author), Colorado State University, Tanya Dewey (editor), University of Michigan-Ann Arbor.



uses sound to communicate


young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.


Referring to an animal that lives in trees; tree-climbing.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


an animal that mainly eats meat


uses smells or other chemicals to communicate

  1. active during the day, 2. lasting for one day.
dominance hierarchies

ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates


animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

female parental care

parental care is carried out by females


an animal that mainly eats leaves.


an animal that mainly eats fruit


an animal that mainly eats seeds


An animal that eats mainly plants or parts of plants.


An animal that eats mainly insects or spiders.


imitates a communication signal or appearance of another kind of organism


having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.


an animal that mainly eats nectar from flowers


an animal that mainly eats all kinds of things, including plants and animals


Referring to a mating system in which a female mates with several males during one breeding season (compare polygynous).


rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.


reproduction that includes combining the genetic contribution of two individuals, a male and a female


lives alone


a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.


uses touch to communicate


the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.


uses sight to communicate


reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

year-round breeding

breeding takes place throughout the year


08 February 2016. "Bornean Orangutan" (On-line). IUCN Red List. Accessed September 15, 2021 at https://www.iucnredlist.org/species/17975/123809220.

February 8 2016. "IUCN Red List" (On-line). Bornean Orangutan. Accessed October 27, 2021 at https://www.iucnredlist.org/species/17975/123809220.

16 October 2017. "IUCN Red List" (On-line). Sumatran Orangutan. Accessed October 27, 2021 at https://www.iucnredlist.org/species/121097935/123797627.

October 11 2017. "IUCN Red List" (On-line). Tapanuli Orangutan. Accessed October 27, 2021 at https://www.iucnredlist.org/species/120588639/120588662.

2017. Morphometric, Behavioral, and Genomic Evidence for a New Orangutan Species. Current Biology, 27: 3487-3498.e10. Accessed September 15, 2021 at https://www.sciencedirect.com/science/article/pii/S0960982217312459.

16 October 2017. "Sumatran Orangutan" (On-line). IUCN Red List. Accessed September 15, 2021 at https://www.iucnredlist.org/species/121097935/123797627.

11 October 2017. "Tapanuli Orangutan" (On-line). IUCN Red List. Accessed September 15, 2021 at https://www.iucnredlist.org/species/120588639/120588662.

Abrams, S. 2017. "New England Primate Conservancy" (On-line). Sumatran Orangutan. Accessed September 29, 2021 at https://www.neprimateconservancy.org/sumatran-orangutan.html.

Ancrenaz, M., F. Oram, N. Nardiyono, M. Slimi. 2021. Importance of Small Forest Fragments in Agricultural Landscapes for Maintaining Orangutan Metapopulations. Frontiers in Forests and Global Change, 4: 5. Accessed September 15, 2021 at https://www.frontiersin.org/articles/10.3389/ffgc.2021.560944/full.

Campbell-Smith, G., M. Campbell-Smith. 2011. Raiders of the Lost Bark: Orangutan Foraging Strategies in a Degraded Landscape. PLoS ONE, Volume 6, Issue 6: e20962. Accessed October 20, 2021 at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3120831/.

Carne, C., S. Semple, J. Lehmann. 2015. Investigating Constraints on the Survival of Orangutans Across Borneo and Sumatra. Tropical Conservation Science, Volume 8, Issue 4: 940-954. Accessed October 20, 2021 at https://doi.org/10.1177/194008291500800405.

Cartmill, E., R. Byrne. 2010. Semantics of primate gestures: intentional meanings of orangutan gestures. Animal Cognition, 13/6: 793-804. Accessed October 13, 2021 at 10.1007/s10071-010-0328-7.

Davies, A., M. Ancrenaz, F. Oram, G. Asner. 2017. Canopy structure drives orangutan habitat selection in disturbed Bornean forests. Proceedings of the National Academy of Sciences, 114: 8307-8312. Accessed September 15, 2021 at https://www.pnas.org/content/114/31/8307.

Galdikas, B. 1988. Orangutan diet, range, and activity at Tanjung Puting, Central Borneo. International Journal of Primatology, Volume 9, Issue 1: 1-35. Accessed October 20, 2021 at https://doi.org/10.1007/BF02740195.

Galdikas, B., S. Insley. 1988. The Fast Call of the Adult Male Orangutan. Journal of Mammalogy, 69/2: 371-375. Accessed October 13, 2021 at https://www.jstor.org/stable/1381390.

Grehan, J., J. Schwartz. 2009. Evolution of the second orangutan: phylogeny and biogeography of hominid origins. Journal of Biogeography, 36: 1823-1844.

Lameira, A., J. Call. 2018. Time-space–displaced responses in the orangutan vocal system. Science Advances, 4/11. Accessed October 13, 2021 at https://www.science.org/doi/10.1126/sciadv.aau3401.

Leavens, D. 2007. Animal Cognition: Multimodal Tactics of Orangutan Communication. Current Biology, 17: R762-R764. Accessed October 13, 2021 at https://www.sciencedirect.com/science/article/pii/S0960982207016880.

Mackinnon, J. 1975. Distinguishing characters of the insular forms of orang-utan. International Zoo Yearbook, 15: 195-197. Accessed September 29, 2021 at https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1748-1090.1975.tb01398.x.

Payne, J., C. Prudente. 2008. Orangutans: Behavior, Ecology, and Conservation. North America: The MIT Press.

Russon, A., S. Wich, M. Ancrenaz, T. Kanamori, C. Knott. 2009. Geographic variation in orangutan diets. Orangutans: geographic variation in behavioral ecology and conservation: 135-156. Accessed October 20, 2021 at https://www.zora.uzh.ch/id/eprint/31255/.

S.S, U., T. Setia, B. Goossens, J. S, C. Knott. 2009. Orangutan mating behavior and strategies. Orangutans: Geographic Variation in Behavioral Ecology and Conservation, 1: 235-244. Accessed October 06, 2021 at https://www.researchgate.net/publication/230823426_Orangutan_mating_behavior_and_strategies.

Schaik, C., A. Marshall, S. Wich. 2008. Geographic variation in orangutan behavior and biology. Orangutans: 351-361. Accessed October 13, 2021 at https://oxford.universitypressscholarship.com/view/10.1093/acprof:oso/9780199213276.001.0001/acprof-9780199213276-chapter-24.

Seaman, D., H. Bernard, M. Ancrenaz, D. Coomes. 2019. Densities of Bornean orang‐utans (Pongo pygmaeus morio) in heavily degraded forest and oil palm plantations in Sabah, Borneo. American journal of primatology, 81: e23030–n/a.

Shumaker, R., S. Wich, L. Perkins. 2008. Reproductive Life History Traits of Female Orangutans (Pongo spp.). Primate Reproductive Aging, 36: 147-161. Accessed October 06, 2021 at https://www.karger.com/Article/FullText/137705.

Tacutu, R., D. Thornton, E. Johnson, A. Budovsky, D. Barado, T. Craig, E. Diana, G. Lehmann, D. Toren, J. Wang. 2017. "Human Ageing and Genomic Resources" (On-line). AnAge entry for Pongo pygmaeus. Accessed September 30, 2021 at https://genomics.senescence.info/legal.html#citation.

Xin, M., J. Kelley, K. Eilertson, S. Musharoff, J. Degenhardt. 2013. Population Genomic Analysis Reveals a Rich Speciation and Demographic History of Orang-utans (Pongo pygmaeus and Pongo abelii). PLOS ONE, 10: e77175. Accessed September 22, 2021 at https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0077175.

Xu, X., U. Arnason. 1996. The mitochondrial DNA molecule of Sumatran orangutan and a molecular proposal for two (Bornean and Sumatran) species of orangutan. Journal of Molecular Evolution, 43: 431-437. Accessed September 15, 2021 at https://pubmed.ncbi.nlm.nih.gov/8875856/.