Creature Feature: Orangutans

Stare deep into the eyes of an orangutan, and it’s easy to understand why their Malay name means “person of the forest.”  The steady gaze of these shaggy red primates exudes a human-like intelligence and understanding, an impression supported by numerous behavioral studies.  Orangutans are critically endangered, threatened by destruction of their rainforest habitat for palm oil and poaching.  As August 19th is International Orangutan Day, we consider what we have learned and can still learn from these intelligent forest dwellers—including the evolutionary bases of our own culture and language—if we prioritize conservation.

Orangutans are a member of the great ape family (Hominidae), which also includes humans and our closest living relatives: chimpanzees, bonobos, and gorillas.  There are three distinct species of orangutans currently known.  Sumatran orangutans (Pongo abelii) and Bornean orangutans (Pongo pygmaeus) are named for the Southeast Asian islands they inhabit.  The third species, Pongo tapanuliensis, lives in the Tapanuli region of Sumatra and contains less than 800 individuals [1].  The three orangutan species are distinguished genetically, geographically, and by slight differences in behavior and appearance.

Sumatran orangutans, like the male pictured here, have longer and lighter-colored hair than Bornean orangutans. Males such as this one can develop prominent cheek pads, called flanges. [Source]

Orangutans hold many records: they have extremely long arms (longer than their legs and bodies!) and are the largest arboreal mammal, meaning they spend most of their time in the trees.  Their extensive appendages allow them to swing from branch to branch, meandering through the rainforest canopy munching on figs and other fruits, leaves, bark, honey, and insects like bees and ants.  Orangutans have the longest juvenile period for primates, apart from humans: young orangutans nurse for 4–7 years and stay with their mother for 7–8 years [2,3]!  This extra-long adolescence allows them to observe mom and fully master the art of locating and eating forest foods [4].  Furthermore, an orangutan female gives birth on average only every 8 years, the lowest rate of any primate.  This means they reproduce very slowly, contributing to the species’ critically endangered status.

Orangutans have a very long juvenile period. This young Bornean orangutan will hang out with its mom for around 7 years total. Infants first cling to their mother’s chest and later ride on her back. Eventually they become independent to climb on their own. [Source]

Faced with a problem, such as inaccessible food or unpleasant weather, orangutans have a knack for problem-solving.  When building their nest in the canopy each night, they sometimes build a second nest above as a sun shade or rain shelter [5].  They are creative inventors and adept tool-users, utilizing tools to perform tasks such as prying open foods, extracting insects from tree-holes, reaching inaccessible water, or creating swings or ladders [3, 6].  An orangutan may pick a leaf to wipe clean her mouth, or she may use multiple leaves as a glove to handle spiny plants [5].  Juveniles learn from their mothers how to modify and use objects from their environment to make tools, important skills needed to forage and survive on their own.

A Bornean orangutan hangs out in its treetop nest, which may be as high as 20 meters (65 feet) above the ground! [Source]

Despite their mostly solitary lifestyle, orangutans have evolved complex communication systems.  They are known to produce over 30 different types of calls from a grumble to a kiss squeak, a roar to a raspberry [7].  In addition, there is some evidence that orangutans communicate about past or future events, a characteristic of human language.  For instance, one study found that male Sumatran orangutans gave long-distance calls signaling the direction they planned to travel the next day, allowing other orangutans to keep track of the calling male’s location and adjust their own movements accordingly [8].  A second study suggested orangutans may produce alarm calls about predators that were recently present but no longer there [9].  While studying communication in non-human primates can be challenging, the more we research, the more we realize that properties of human language and society have roots in behaviors we share with other species.

Just as human groups around the world have cultures with behavioral norms and tool-making traditions, orangutans in separate geographic areas developed their own distinct cultures.  One way to study animal culture is to identify special behaviors termed cultural variants—behaviors practiced regularly by individuals at one or more sites, but not at other sites with similar ecological conditions [5].  Culture is transmitted socially, meaning it is passed on not by genes but through individuals learning from each other.  In orangutans, cultural variants include tool sets, feeding techniques, and social signals that are passed down with each generation [5].

A study of six wild chimpanzee populations in Borneo and Sumatra found that populations further apart geographically were more different in their behavior and tool-use.  Interactions between individuals facilitated a larger repertoire of cultural variants, meaning that in places where individuals interact frequently, the orangutans had a greater number of unique social signals, tools, and other behaviors [5].  Studies like this are particularly exciting because they reveal the rich behavioral diversity in the animal kingdom and also allow us to test hypotheses about the development and spread of human culture.

The rainforest habitat of orangutans has dwindled alarmingly in recent decades.  This, combined with the threat of poaching and their slow reproductive rate, contributes to the critically endangered status of all three species of orangutan.  Huge swaths of rainforest in Borneo and Sumatra have been destroyed and converted to plantations for oil palm (palm oil is a common ingredient in many processed foods), rubber, candlenut, or other uses [10].  Continued deforestation is predicted to lead to a decline in Sumatran orangutans from an estimated 14,600 individuals in 2016 to as few as 10,000 individuals by 2030 [10].  To help conserve wild orangutans and other rainforest creatures, you can reduce your consumption of palm oil, and look for sustainably-sourced paper and rubber products.

Map of Borneo showing the extent of deforestation, the biggest threat to orangutan survival. Once covered in species-rich tropical forest, much of Borneo has been cleared by logging or burning to make room for agriculture and palm oil plantations. [Source]

Studying creatures like orangutans provides incredible knowledge and insight about human evolutionary history, including the evolution of language, tool-use, and the origin of culture.  Sadly, the clock is ticking on our time to appreciate and learn from our red-haired cousins.  Like many of Earth’s diverse inhabitants, we may never have a chance to meet an orangutan in person.  However, we can contribute to saving the “people of the forest” through our conservation actions as global citizens of this planet we share with our fellow animals.


To see orangutans in action, check out this live animal cam from the San Diego Zoo.


Jessica Schaefer is a PhD student in the Animal Behavior Graduate Group at UC Davis. She studies the evolution of mating systems and behavior of species from limpets to lizards to birds.


References:

[1] Nater, A. et al. (2017). Morphometric, behavioral, and genomic evidence for a new orangutan species. Current Biology, 27, 3487–3498.

[2] San Diego Zoo (2019). Orangutan. San Diego Zoo Animals & Plants. https://animals.sandiegozoo.org/animals/orangutan

[3] Cawthon Lang, K.A. (2005). Primate Factsheets: Orangutan (Pongo) Behavior. University of Wisconsin–Madison Primate Info Net. http://pin.primate.wisc.edu/factsheets/entry/orangutan/behav

[4] Schuppli, C., Forss, S. I. F., Meulman, E. J. M., Zweifel, N., Lee, K. C., Rukmana, E. Vogel, E. R., van Noordwijk, M. A., & van Schaik, C. P. (2016). Development of foraging skills in two orangutan populations: needing to learn or needing to grow? Frontiers in Zoology, 13(43). https://doi.org/10.1186/s12983-016-0178-5

[5] Van Schaik, C.P., Ancrenaz, M. Borgen, G., Galdikas, B., Knott, C.D., Singleton, I., Suzuki, A., Utami, S.S., Merrill, M. (2003). Orangutan cultures and the evolution of material culture. Science, 299(5603), 102–105. https://doi.org/10.1126/science.1078004

[6] Fox, E.A., van Schaik, C.P., Sitompul, A., & Wright, D.N. (2004). Intra- and interpopulation differences in orangutan (Pongo pygmaeus) activity and diet: implications for the invention of tool use. American Journal of Physical Anthropology, 125, 162–174. https://doi.org/10.1002/ajpa.10386

[7] Hardus, M.E., Lameira, A.R., Singleton, I., Morrogh-Bernard, H.C., Knott, C.D., Ancrenaz, M., Utami Atmoko, S.S., & Wich, S.A. (2008). A description of the orangutan’s vocal and sound repertoire, with a focus on geographic variation. In: Orangutans: Geographic Variation and Behavioral Ecology and Conservation, Ed. S.A. Wich, S.S. Urami Atmoko, T.M. Setia, & C.P. van Schaik. Oxford University Press. https://doi.org/10.1093/acprof:oso/9780199213276.003.0004

[8] Van Schaik, C.P., Damerius, L., Isler, K. (2013). Wild orangutan males plan and communicate their travel direction one day in advance. PLOS ONE, 8, e74896. https://doi.org/10.1371/journal.pone.0074896

[9] Lameira, A.R. & Call, J. (2018). Time-space-displaced responses in the orangutan vocal system. Science Advances, 4(11). https://doi.org/10.1126/sciadv.aau3401

[10] Wich, S.A., Singleton, I., Nowak, M.G., Atmoko, S.S.U., Nisam, G., Arif, S.M., Putra, R.H., Ardi, R., Fredriksson, G., Usher, G., Gaveau, D.L.A., & Kühl, H.S. (2016). Land-cover changes predict steep declines for the Sumatran orangutan (Pongo abelii). Science Advances, 2(3), e1500789. https://doi.org/10.1126/sciadv.1500789

Source for main image

[Edited by Meredith Lutz]

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