Orangutans, Who Are They?

Jeanne Swanson

07/20/18

Orangutans

Orangutans are known to be strong, wise apes.

The first recorded sighting of Orangutans was in the early 1600s by Dutch physicians Jacob de Bondt and Nicholas Tulp, after which it was pronounced Simia satyrus by Carl von Linné, a name that lasted until 1927 when the International Commission on Zoological Nomenclature renamed it Pongo pygmaeus.

Since then, Orangutans have been discovered to be of two separate species, though some evidence contradicts this; Bornean Orangutans have been defined as Pongo pygmaeus, and Sumatran Orangutans, as Pongo abelii. Within Pongo pygmaeus are three commonly accepted sub-species, P.p. morio, P.p. pygmaeus, and P.p. wurmbii. Bornean Orangutans, Pongo pygmaeus, have reddish, deep maroon, or blackish brown fur, while Sumatran Orangutans, Pongo abelii, have rusty red or light cinnamon fur. The two groups also have differences in hair length and structure, facial hair pattern, body build, and the size and shape of cheek flanges and throat sacks in sexually mature males. Genetic data, however, is inconclusive on whether or not Orangutans show enough genetic diversity to be considered separate species.

Orangutans live across the jungles of Malaysia, a habitat that is threatened by deforestation and other human activities. Orangutans live in the Indonesian islands of Sumatra and Borneo. The island is a heavy forest, which helps the orangutans move around the forest from branch to branch.

Though Orangutans once had free rain across large parts of Asia, they now live exclusively on Borneo and Sumatra, with different species occupying each, but living in similar habitats on their own islands.

During the Pleistocene, Orangutans occupied the area between present-day Vietnam, Northern India, and Southern China. Later, they moved south, likely due to an ice age at the time pushing them south, and lowering sea levels, allowing passage to present-day Borneo and Sumatra. Fossils have been found on both Borneo and Sumatra dating from 30,000 years ago to 40,000 years ago, showing that Orangutans have been present there at least that long. However, genetic evidence indicates that Sumatran Orangutans (Pongo pygmaeus abelii) and Bornean Orangutans (Pongo pygmaeus pygmaeus, P. p. wurmii, and P. p. morio.) separated between 0.6 million and 6.4 million years ago, so they may have arrived earlier.

All Orangutans have large habitats, ranging from the forest company to its floor, and covering peat swamp, riverine, black swamp, and hill forest ecosystems; but the various sub-species each occupy a separate region, separated by mountains, rivers, or the ocean. Sumatra is occupied exclusively by Pongo pygmaeus abelii, or Pongo abelii, though this subspecies also has the greatest nucleotide diversity and general polymorphism of any Orangutan subspecies. This genetic evidence indicates that P. abelii historically had a larger population than their Bornean relatives, though differences in geography between the two islands may also account for the increased diversity.

Borneo is home to three different Orangutan or Pongo pygmaeus, subspecies, each of which occupies a separate geographical region; Pongo pygmaeus pygmaeus, Pongo pygmaeus morio, and Pongo pygmaeus wurmii. P.p. pygmaeus lives across Sarawak, and other parts of Western Kalimantan; P.p. wurmii lives across Central and Southern Kalimantan; and P.p. morio lives in Sabah and other parts of Northern and Eastern Kalimantan. Borneo is a less productive habitat than Sumatra, in that there are fewer fruits and other potential food sources grown, as a result of the less-fertile soil combined with a greater number of droughts; El Nino events may have more impact on Borneo than Sumatra, and lead to these droughts.

Diet and Dentition

Orangutans are omnivorous though predominantly herbivorous, with corresponding mandibular structures and dentition, though distinct differences between subspecies in diet, mandibular structure, and dentition have evolved due to the differential availability of food sources across their separate habitats.

Like their African relatives, Orangutans prefer to eat fleshy fruits when available but must supplement that diet with other vegetation, and potentially non-vegetarian sources of nutrition when the fruit is less available; the classical differentiation between frugivore and folivore species fails to capture the complexities of Orangutans diet and resulting dentition and mandibular structures.

Orangutans do have common habitats and diets despite the diversity of their subspecies. All Orangutans prefer ripe, pulpy fruits when they are accessible, though the degree to which they rely on such fruits does differ. Across both Borneo and Sumatra, the accessibility of fruit is largely based on masting cycles, which are unpredictable.

Masting refers to the synchronized flowering and fruiting of a given species or multiple species, which produces an abundance of fruit. During a mast year, Orangutans get to be relatively picky. They will select fully-ripened fruits with great flesh-to-seed ratios, or seeds that are immature and have little to no toxins or “chemical defenses”, and forage only in larger patches of fruit or seeds. They will discard or swallow whole mature seeds, letting them be pooped out or fall to the forest floor.

The flip side of masting species is that between mast years, the fruit will be very scarce. During these years, Orangutans will supplement their diet to a greater or lesser degree with fibrous or unripe fruit, leaves, epiphytes, lianas, bark, and figs, though they prefer to not eat figs with high levels of phenols, and prefer figs with plenty of digestible carbohydrates. However, each subspecies will supplement their diet to a different degree, and the specific diet which each subspecies will adopt during lean years also differs markedly; Sumatran Orangutans rely mostly on figs and insects, while Bornean Orangutans rely mostly on vegetation and inner bark, with which Orangutans in Eastern Borneo, P.p.morio, supplement more of their diet than their counterparts in Western Borneo, P.p. wurmbii.

Overall, when comparing Bornean Orangutan subspecies to their Sumatran counterparts, Bornean Orangutans display a more robust mandibular structure, in order to support their more folivorous diet. They have a deeper mandibular corpus and wider mandibular symphysis in proportion to the length of the mandible than P. abelii, which both studies and biomechanical theory have shown to assist in the stiffening of the balancing-side mandibular corpus during chewing, and the corpora during incision, or anterior biting, minimizing the parasagittal bending which puts tension on the upper corpus while compressing the lower corpus, and may eventually lead to fractures in the mandible. Orangutans here have strong mandibles for a reason; a 2008 study in Tuanan, Central Kalimantan, showed that the Orangutans there eat foods tougher than those eaten by Virunga Gorillas, and a 1994 study also shows that Orangutans can eat food that will require significant jaw strength, and as such put significant strain on their mandibles.

P.p. morio is the most folivorous out of all of the Orangutan subspecies, as its habitat in Northern and Eastern Borneo has the longest lean periods between mast years. A 2006 study that followed Orangutans in Ulu Segama showed them spending up to 77% of their feeding time on non-fruit, fibrous vegetation, with 37% of their total feeding time being devoted to consuming bark. The study simultaneously followed Orangutans in Mentoko, where they spent up to 67% of their feeding time on bark over the course of the month. The study concluded that P.p. morio will spend 50% of their time on non-fruit fibrous vegetation on average, the highest of any Orangutan subspecies.

P.p. wumbii, living in Southern Kalimantan, spends less time on fibrous vegetation than P.p. morio, though still more than P. abelii.

P. abelii maintains a fruit-based diet even during lean periods, though they still supplement their diet with non-fruit foods. Their preference is for energy-rich, ripe, soft fruit, which they will supplement with unripe or less energy-rich fruits, some fibrous vegetation, and insects if they must; however, unlike their Bornean counterparts, they spend the majority of their feeding time on fruit. They are able to do so because of the greater prevalence of figs, which are a consistent source of nutrition, and Sumatras' higher general productivity. If they are unable to find enough fruit to sustain themselves, they merely change their foraging area and are able to find more fruit.

Orangutans display strong sexual dimorphism, as well as two separate male morphotypes. Males continue growing after females have completed their growth, ending up approximately two times larger than females. Males also have a larger brain accompanied by a larger skull and face, larger canines, and more mandibular prognathism. Despite having a large size dimorphism for all other teeth, the incisors are the same between female and male Orangutans, possibly showing their importance in Orangutan life.

The two separate morphotypes displayed by male Orangutans differ in their secondary sex characteristics, such as cheek flanges, and may differ in skull morphology, though this is unclear- a 1996 study showed half of a group of Orangutans to have sagittal crests, but the data on sexually mature Orangutan males is too limited to be clear. Interestingly, different Orangutan populations have different ages of sexual maturity.

Social Structure and Reproductive Units

Orangutans live in loose communities, foraging across large home ranges and interacting with other Orangutans for social play or mating.

Female Orangutans have large and stable home ranges, from 150 Ha to over 850 Ha, depending on the presence of food in their range, and the frugivorous or folivorous nature of the species.

Neaves, Kelly, director. Sex in the Wild. Watch Gen Silent Now | Kanopy, 2014, www.kanopy.com/product/sex-wild.

Herzfeld, Chris. Wattana, an Orangutan in Paris. The University of Chicago Press, 2016.

Wich, Serge A. Orangutans: Geographic Variation in Behavioral Ecology and Conservation. Oxford University Press, 2010.

WATCH IT TONIGHT "The Ape Who Went to College" airs at 8 p.m. today on Georgia Public Broadcasting and WTCI. The hourlong Animal Planet documentary is about Chantek, a male orangutan who learned sign language while living on the University of Tennessee a, director. My Wild Affair. PBS, Public Broadcasting Service, 2014, www.pbs.org/video/my-wild-affair-ape-who-went-college-1/.