Despite being one of man’s closest living primate relatives, we know very little about the orangutan. Although it was once distributed throughout much of Asia in the Pleistocene, the orangutan is now only found in two places: Sumatra and Borneo (Goossens, 2009) and is the only great ape that lives exclusively on the Asian continent. The orangutan was first assigned the name Simia satyrus in the early seventeenth century, but in 1927 the name was changed to Pongo pygmaeus by the International Commission on Zoological Nomenclature (Goossens, 2009).
Orangutans are the only species within the genus Pongo. For many years there was no distinction between the Bornean and Sumatran orangutans. They were once thought to be subspecies of Pongo pygmaeus due to their morphological and behavioral differences (Wich, 2004; Goossens, 2009), but now the Sumatran orangutans and the Bornean orangutans are known to be distinct species because of their reproductive isolation and significant genetic differences (Zhang, 2000) with the Sumatran orangutans acquiring the Pongo abelii name.
The Bornean orangutans kept the Pongo pygmaeus name. There are no identified subspecies of Pongo abelii but Pongo pygmaeus has three subspecies.
Pongo abelii and Pongo pygmaeus differ most noticeably in hair length and color, facial hair, cheek size and shape, body build, and dental features. The two species also differ in feeding behaviors (NEED SOURCE). Sumatran orangutans also have a higher nucleotide diversity compared to their Bornean counterparts, yet both species of orangutan are more diverse than humans and other African apes (Goossens, 2009). One of the greatest differences between the two species of orangutan can be seen in their population size.
The Bornean orangutan population is estimated to be around 69,000 while the Sumatran orangutan population is much smaller at around 7,300 (Singleton, 2008).
Pongo abelii occupies the forests of the northern part of the island of Sumatra, Indonesia. (Singleton, 2008). This region poses many threats to the orangutan population. Both legal and illegal logging are risks to the population as they destroy the trees in which orangutan reside. Other threats include turning the forest into farmland and plantations as well as expansion of roads. Orangutans are hunted and captured for recreation and for the international pet trade on top of being killed as pests on farms. (Singleton, 2008) While Sumatran farmers are looking to implement effective crop protection strategies to rid their farms of orangutan pests, this could still harm the orangutan population as these crops comprise 21% of their diet (Campbell-Smith, 2012). So although efforts are in place to end the pest control, these efforts could prove to be counter-intuitive.
Due to these factors the population has declined over 80% in 75 years and continues to decline. Recent estimates put Pongo abelii’s numbers at around 7,300 (Singleton, 2008) divided into nine distinct populations with the largest composed of 2,611 individuals and the smallest composed of 43 individuals. Because of current threats these populations may be further subdivided in the future. (Singleton, 2008)
Actions are being taken to conserve the orangutan population in Sumatra. Pongo abelii is strictly protected under Indonesian law within the Leuser Ecosystem (which holds 75% of Sumatran orangutans) and the Gunung Leuser National Park. However, this level of protection in forest areas closer to sea level are also necessary to secure the population long term (Singleton, 2008). Due to the factors mentioned, Pongo abelii is critically endangered.
Orangutan species feed on a variety of foods found in their home ranges including fruit, insects, and vegetable matter, which they obtain through foraging techniques from within their solitary home ranges (Wich et al, 2006; Hardus et al, 2012). Sumatran orangutans feed on significantly more fruit than their Bornean counterparts (Wich et al, 2006). This is most likely because fruit availability is high year-round on the island of Sumatra (Hardus et al, 2012). Unlike most other primates, the overall food availability on Sumatra does not influence the composition of the orangutan’s diet, which usually remains consistent no matter the season.
There exists some variation within the fruit that Sumatran orangutans choose to eat. The first variation is between fig fruits and non-fig fruit species. Orangutans tend to prefer non-fig fruits over fig species. Wich et al (2006) states that orangutans only feed on figs when non-figs are less available, but Hardus (2012) contests that by saying there is no correlation between time spent feeding on figs and overall fruit availability. However, both researchers agree that figs are the second choice to other fruits. Diet also varies on an individual basis. In times of high fruit availability, the fruit preferences of individual orangutans can vary greatly (Hardus et al, 2012). Reproductive females have been known to prefer fig fruits to non-fig fruits even in times of high fruit availability (Wich et al, 2006).
Compared to the other great apes, orangutans are much more solitary, although they still have complex social behaviors. Their ranges are several hundred hectares with some overlap between individuals (Singleton, 2002). The large home range is consistent with their larger body size and foraging feeding behavior. The female orangutans have the most established and overlapping home ranges and are often accompanied by their offspring. Overlaps are frequently between immediate relatives, such as mother and daughter, because female orangutans most often settle in home ranges right next to their mothers. These and other non-familial female overlaps and their subsequent social interactions can be either antagonistic or amicable. (Singleton, 2002)
The home range of the dominant male encompasses one or several home ranges of the females. There can be some overlap between male home ranges and most interactions between the fully mature male orangutans are aggressive, which shows the territorial nature of the orangutan. Males that are not dominant are considered transient and move around until they can displace an existing dominant male. Many males therefore die at a young age due to this fighting, which adds to the orangutan adult sex ratio bias showing higher numbers of adult females than males. (Singleton, 2002)
Despite being deemed solitary, Pongo abelii has many social interactions and therefore can be called a “loose community” (Singleton, 2002). The home ranges are not exclusive to an individual, but some territorial behaviors do exist in the orangutan population.
The mating behavior of Pongo abelii is a very complex one characterized by extreme sexual selection and female choice, given the mostly solitary lifestyle of the orangutan. Because individuals are solitary there are limited opportunities for a member of one sex to find a member of the other, and even fewer opportunities for mating (Atmoko, 2009). Despite this, both male and female orangutans are fairly promiscuous in their mating behavior.
For Sumatran orangutans, mating interest begins at a young age, as early as two years old, starting with self-masturbation or masturbating others (Atmoko, 2009), which is extremely young within the apes, although full sexual development does not occur until much later. Females often reproduce for the first time on average at 15.4 years of age (Wich, 2004).
In times when fruit is readily available, individuals form bands to travel together in search of fruit. These travel bands may lead to consortships, a male-female pair traveling together for a time period ranging from days to months. This male-female pair will likely copulate (Atmoko, 2009). Most sexual interactions occur during consortships after a certain period of time where the pair builds up their relationship, as female orangutans tend to mate based on familiarity (Singleton, 2002). During consortships, males occasionally mate with females outside of the consortship as well. Besides the consortships, another way orangutans locate each other in the wild is through the “long calls” of fully mature males. Most orangutan mating behaviors revolve around male bimaturism.
Of all the apes, orangutans exhibit the most extreme form of male bimaturism. The two types of male orangutans are termed flanged males and unflanged males. Unflanged males may also be called “sub-adult,” although this name is controversial. While many people mistakenly identify being unflanged as an alternative tactic, these males are essentially in a transitional state into full maturation. Some orangutans can be in this state for up to 30 years (Atmoko, 2009). Some recent observations have noted that the presence of a flanged male in a home range can delay the development of other male orangutans for long periods of time, essentially increasing the time spent as a subadult (Atmoko, 2004).
Studies have found that this sort of developmental arrest is far more widespread on the island of Sumatra than Borneo (Dunkel, 2013). Unflanged males have reached sexual maturity and do have fully grown testes, but these orangutans lack every other secondary sex characteristic of fully mature male orangutans (Dunkel, 2013). Physically, unflanged males are not much larger than female adult orangutans and are hardly distinguishable from females. While fully mature, flanged males can be more than twice the size of females (Mitani, 1985). This is also the most extreme form of sexual dimorphism in primates, which can be attributed to the high sexual selection that takes place in orangutans.
Flanged males have larger cheek pouches and are covered in long hair (Atmoko, 2004). More significantly, flanged males have the ability to make “long calls” to alert other flanged males of their presence and to attract estrous females. Long calls are extremely loud vocalizations, which inform females of a dominant male orangutan’s location and allow female orangutans who are ready to copulate to approach the male (Singleton, 2002; Mitani, 1985). Long calls also help male orangutans to establish their territory and prevent another male from over taking the territory.
Unflanged males are unable to produce such long calls and instead must actively search for females over a large geographical area, leading to fewer reproductive opportunities (Singleton, 2002). And, due to their inability to make long calls, unflanged males cannot establish a set home range and instead are considered transient.
While flanged males are not tolerant of each other, responding to one another with aggression and fighting, and do not coexist in the same home range, flanged males are very tolerant of unflanged males in their territory (Atmoko, 2004). The best explanation for this points to their different mating strategies and unflanged males relative lack of success in siring offspring.
Both flanged and unflanged males are able to produce viable offspring but flanged males tend to have much higher reproductive success. Females tend to prefer flanged males over unflanged, leading to intense male-male competition within the species (Atmoko, 2009). Another explanation for why flanged males sire more children is because unflanged males cannot mate-guard female orangutans as effectively as flanged males (Atmoko, 2009) most likely due to their smaller body size.
The dominant male orangutan in a community is always flanged and he produces the most offspring. Non-dominant flanged males still produce more offspring than unflanged males because of their ability to make long calls, which means they do not need to actively search for a female to mate. Typically, for an unflanged male to reproduce, he must find a mate through coercion and violence, as females are more resistant to mating with unflanged males during fertile periods (Atmoko, 2009).
Female orangutans resist a large amount of copulation attempts by unflanged males, which may contribute to their long interbirth intervals (Wich, 2004). Because of this, forced matings occur frequently in orangutans, significantly more so than in other primates. Orangutan females resist mating attempts by unflanged males very strongly and males respond by inflicting violence; hitting, restraining, biting, and pushing; both on the female and those around her (Atmoko, 2009). Researchers believe these forced matings are reflective of the strong female choice in orangutan species. Because of this female choice, the reproductive success of flanged males is much higher than that of unflanged males. Within flanged males, females have shown preference to the dominant male in their population (Atmoko, 2009).
Despite this preference for flanged males to sire their offspring, female orangutans will still mate promiscuously with the unflanged males (Atmoko, 2009) in their home range during times of infertility (i.e. pregnancy, lactation) as well as mate promiscuously with non-dominant flanged males.
We find several problems with the arrested development of orangutans, but namely we question why it evolved in the first place. The closest living relatives of orangutans do not exhibit this kind of development or bimaturism. The hypotheses for why such dimorphism exists can vary. Some researchers argue that it is due solely to male-male competition while others state that it has to do with sexual selection and female choice. The likely explanation is a combination of both hypotheses. The existence of female choice in orangutan populations is what creates the male-male competition. (Utami Atmoko, 2009) As mentioned earlier, we also know that the presence of a dominant, flanged male will delay full development of other male orangutans. So there is also evidence that there is a psychosocial factor at play that could be influencing this transient phase.
Although most of their time is spent in solitude, Pongo abelii are not as solitary as what was once thought. Both subspecies of orangutans, the Sumatran Pongo abelii and the Bornean Pongo pygmaeus, occasionally exhibit social behavior when they aggregate with others in fruit trees (van Schaik, 1999) looking for food. Much of the social organization of the orangutan beyond these aggregations in trees remains a mystery, as there are very few cases of observed higher-level social units outside of mother-infant relationships. Despite their diurnality, orangutan-grouping patterns resemble those of nocturnal mammals such as their more primitive ancestors, prosimian primates. Some evidence exists for non-random association among female orangutans, which suggests a form of social unit. (Singleton, 2002)
Several researchers have suggested the possibility of fission-fusion behavior in female orangutans in Sumatra, particularly the Suaq population (Singleton, 2002; van Schaik, 1999). Van Schaik (1999) postulates that the difference in grouping likelihood between Sumatra and Bornea is due to the higher food availability in Sumatra, so the costs of grouping are not as high and thus Sumatran orangutans group more often in this fission-fusion manner. Typically, in fission-fusion primates, distinct social units termed “communities” are documented, but this has yet to be seen concretely in orangutans (Singleton, 2002). This is not to say that it does not exist.
The main benefits of these grouping behaviors are social. Grouping provides mating opportunities for orangutans, as sexual consortships usually spawn from the orangutan aggregations in fruit trees. There is also evidence to show that grouping by orangutans offers protection from harassment, dispelling the threats of both predation and infanticide. The costs for grouping are relatively low for Sumatran orangutans. (van Schaik, 1999)
In studying the grouping of Sumatran orangutans, Singleton (2002) notes clusters of females, where female orangutans settle in adjacent and overlapping ranges and preferentially socialize with one another. It is believed that members of female clusters of orangutans are relatives. It has been proposed that one or greater female clusters and the adult male with whom the females prefer to mate is a “community” seen in fission-fusion primates. However, this type of community is not as isolated as in other species. (Singleton, 2002)
Because these clusters are postulated to be groups of female kin, they give insight to the patterns of kinship in female orangutans. Among orangutans, food sharing is rare, but it can be seen between females in the same cluster (Singleton, 2002). Besides the spatial proximity and food sharing, another kin-correlated behavior in orangutans is the transmittance of behaviors and the culture that kin share. Behaviors such as tool use are passed from mother to infant, such as wiping of the face with leaves in the Suaq population. Another example is hiding for shelter in the rain (van Schaik et al, 2003). Most kin-correlated behavior is seen exclusively in mother-daughter relationships as adolescent females are consistently seen associating with their mothers while males return less and less until they disperse permanently (Singleton, 2002).
Pongo abelii females can be found grouping together in the overlapping areas of their individual ranges (Singleton, 2002). These groups are composed of female kin who preferentially associate with each other. There are not many kin-correlated behaviors in orangutans, but females in the same cluster have been known to share food with one another (Singleton, 2002), particularly between mother and offspring (Jaeggi, 2008). These behaviors can be interpreted in terms of inclusive fitness theory in the sense that although giving up food can be costly to the individual, it benefits the receiver.
Food sharing is most often seen between relatives in orangutans, excluding the instances where food sharing occurs between the sexes for other benefits (van Noordwijk, 2009), and therefore it can be concluded that the receiver of the food is typically a relative and thus fits the terms of inclusive fitness theory. Jaeggi (2008) postulates other explanations such as food sharing for nutritional and informational benefits, but they can all relate back to inclusive fitness theory in some way.
Patterns of dispersal in orangutans are largely determined by sex. During adolescence females tend to stay close to their mothers and typically find home ranges that are in close proximity to that of their mother (Singleton, 2002). This is what contributes to the clustering mentioned previously. Conversely, during adolescence, males tend to wander off from the home range of their mothers, returning occasionally with decreasing frequency until they reach reproductive age where they disperse far from their mothers (Delgado, 2000). These males go on to then try to displace the dominant males from their home ranges.
One explanation for this type of dispersal is the avoidance of inbreeding. If both males and females settled in home ranges close to their mothers, there would be much higher chances of inbreeding, but male dispersal helps to avoid that, and at the same time it increases genetic variation because male orangutans are new to the population in which they breed.
Competition has an incredibly strong effect on orangutans and it has played a large role in their evolutionary history. It is believed that male-male competition is one of the reasons for the sex. Flanged males tend to dominate unflanged males in all aspects of orangutan life (Atmoko et al, 2009). Because of this, flanged males tend to have more successful copulations than unflanged males and reproduce with much higher success due to being the preferential choice for female orangutans. The two types of males have been known to fight, but because of their larger body size flanged individuals win a majority of the fights. This leads to flanged males being the dominant male of a range while the unflanged males hang around the outside of the range, waiting for any breeding opportunity that they can get. (Atmoko et al, 2009)