About Orangutans

About Orangutans

Habitat and Classification

HabitatOrangutans are only found in Southeast Asia on the island of Sumatra and Borneo. (In Indonesia and Malaysia).They live in a dipterocarp forest (with some lowlands), lowland montane forest, riparian forest, and wetland forest.

ClassificationSumatra Island: Sumatran Orangutan (Pongo abelii), Tapanuli Orangutan (Pongo tapanuliensis)
Borneo Island: Bornean Orangutan (Pongo pygmaeus), classified into 3 subspecies (P.p.pygmaeus, P.p.wurmbii, P.p.morio)

生息図

オランウータンの分類

Morphology

Orangutans are known to be the largest arboreal mammal. In the wild, female weigh around 40kg and male weigh around 80kg. Male orangutans weigh twice as much as the female orangutans. (Rayadin and Spehar, 2015)

Orangutans are complete arborists, so their skeletons are well suited for moving from branch to branch. Their arms (forelimb length) are exceedingly long that its length is 1.5 times the length of their legs (hindlimb length). We humans walk on our feet, but orangutans walk and climb trees with their arms, so they need a long and strong arms to support and live.

Orangutans

Foraging

Orangutans’ diets are mainly plants such as fruits, leaves, barks, flowers, epiphytes, ginger, and mushrooms. About 60% of their diet comes from fruits. Orangutans eat different variety of plants and as far as we are aware, orangutans eat 831 kinds of plants that are belonged to 107 families, 316 genera (from total of 12 research sites), and this number is still increasing.

Foraging graph

Orangutan food(fruit)
Orangutan food (leaves, bark, etc.)
Leaf
Bark
Flower
Other
The reasons why variety of plants are diverse

Orangutans have such a wide variety of food because there are many kinds of species in the rainforest. For example, it is not uncommon to find plant in site A that are not in site B. In fact, out of 502 species reported from 3 survey sites, only 32 species were commonly foraged at 2 or more sites. (Russon, A et al 2010). Furthermore, insects such as ants and termites are eaten and there are also rare reports that orangutans eat small mammals like slow loris and squirrels.

Orangutan

Sociality

Orangutans are said to be the most solitary animal of all primates. Mother and its baby live together but other than that orangutans live basically alone. However, they are not in completely solitary because the orangutan know where each orangutans are at, thereby we can understand that orangutans keep social distances between each other.

Orangutans

The reasons why orangutans do not live as a group

Chimpanzees live in a forest of Africa, where fruits are abundant and every one of them can get full, even in groups. However, orangutans that live in Southeast Asia will quickly eat up all the fruits, if a large group of orangutans rushed to the fruits as it is scarce. In the season when fruits are limited, there are not enough fruits to satisfy everyone. Therefore, in an environment with few fruits, orangutans spread and work alone so that everyone can eat a satisfactory amount. Hence, during the fruit season when the fruits are abundant 2-3 orangutan feed together on the same tree.

Summary diagram

Habitat Density of Orangutan

A factor that determines the density of the orangutan’s population is the fruits availability in the forest. Studies have shown that Sumatran orangutans are denser than Bornean orangutans. This is because there are more fruits produced in the Sumatra island compared to the Borneo island.

Sumatra Island

On the island of Sumatra, there is a volcanic belt that provides the land with nutrients, leading to higher production of fruits compared to Borneo Island (due to the richness of soil). Thereby, the period of fruits abundance lasts longer in Sumatra Island. With many fruits in the forest, orangutans do not need to disperse to feed as they can forage and live together in a group. This is the reason why habitat density is high in Sumatra island.

スマトラ島図

Borneo Island

In Borneo island, the soil is not rich in nutrients compared to Sumatra island, so this results in low fruit production and long period of fruit scarcity. As a result, Bornean orangutans have a low habitat density as Bornean orangutans must disperse and feed for limited fruits available.

ボルネオ島図

Life History of a female

Female orangutans reach sexual maturity between the ages of 11-15 and give birth to their first child, between the ages of 15 to 18. Pregnancy causes the vulva to swell slightly, so it can be seen from the outside. Pregnancy lasts around 270 days (9 months), and from the age of 3, they start eating the same food as their mother, but lactation still proceeds until the age of 6-8 years old until they are on its own.

Parenting Methods

They only give birth to one child at a time as female orangutans live in the forest carrying their baby, so it is extremely rare to see female orangutan carrying a twin. After the age of 3, young orangutans can digest the same food as their mother. By spending time with their mother even after being able to eat the same food, young orangutans will learn and absorb what its mother is doing by identifying fruit trees / its location and recognize when the fruit is ready. Fruits might bear every year or once every few years so it would take a long period of time for young orangutans to learn what is edible and what is not. Following mother’s next birth (a younger brother or sister), the elder brother/sister will gradually move away from their mother and eventually become independent. The average duration that a mother and a child stay together is around 7.6 years for both orangutans (Sumatran orangutan and Bornean orangutan) which is the longest among primates (van Noordwijk et al. 2018). The lifespan of an orangutan in the wild is around 50-60 years old, so a female orangutan will give birth to around 4-6 babies in total.

Parenting Methods

Life History of a Male

Sexual maturity varies widely among individuals, with the earliest being around 7 and latest around 12 years old. When males reach sexual maturity, only the most dominant one has a large cheek folds on both sides of the face called “flanges”, which means “protruding part”.

Flange and Development

Flanges are used to intimidate other males who tries to invade their territory (migratory zone), or to show females their reproductive capabilities. Furthermore, flanges have the effect of making them look bigger than their actual size. Males with flanges are technically called “flanged males” and those without flanges are called “unflanged males”. It is interesting that the flange in males develops not according to age, but according to social dominance. It is not the case that all unflanged males eventually grow up and develop flanges. (Maggioncalda, et al. 1999). For example, suppose that there is a several male orangutans in that area. The dominant flange male in the area is no longer there due to migration from its area or due to death etc. Then one male orangutan in that area, who is aware that he will be the next dominant male, will develop its flange. In this process, the level of testosterone, a type of male hormone/growth hormone spikes. On the other hand, males who are aware of their inferiority continue to suppress the development of the flange. Thereby, some male orangutans remain unflanged for rest of their lives.

Flanges・Male

UnFlanges・Male

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