Orangutan Biology

Orangutan Biology

The two orangutan species are classified in the order primates and the family Horninidae, along with humans and the other great apes-gorillas, chimpanzees and bonobos. They share 96.4 per cent of their DNA with humans, and with the other great apes they are our closest living relatives.

Orangutans, however, are the only great apes living in Asia.Orangutans are descended from a prehistoric species which lived throughout Asia. By the time they had evolved to their present form, they were spread from what is now Java, in the south, through Laos and into southern China.Today, the remaining two species are confined to the Southeast Asian islands of Borneo and Sumatra: Pongo pygmaeus (Bornean orangutan) and Pongo abelii (Sumatran orangutan).


Physical description 

As well as their reddish—brown hair, orangutans ate distinguished by the fact that their arms are longer than their legs and their feet resemble their hands. These are adaptations for climbing, as are their highly flexible hips. Adult males can weigh more than 120 kilograms when mature and their arm span exceeds two metres.

Females  are smaller, weighing on average 45 kilograms. Like other hominids, orangutans have eyes which face forward and small ears; sight is the more important sense. Their teeth are also similar to those of humans: orangutans have molars for grinding, incisors for chewing and canines for biting.One of the most distinctive features of orangutans are the adult males’ cheek pads.

Their exact function is not known but there are various theories: that they protect the face during fighting; they help to project the long call {see Behaviour below); or they are simply a sign of adulthood.  The Sumatran species has a narrower face and longer beard, while the Bornean is slightly darker in colour and its males have wider cheek pads. There are also behavioural differences: Sumatran orangutans tend to be more frugivorous (fruiteating) and there is greater evidence of their use of tools than in Borneo.  

Behaviour
Orangutans are the largest arboreal (tree living) animals in the world. In fact,they are entirely dependent upon trees for their existence and perfectly adapted to life in the forest. Up to 60 per cent of an orangutan’s life is spent foraging for food, mostly in the about orang utancanopy, and 40 pet cent nest every night.

Orangutans are expert climbers. using their feet as a second pair of hands, and travel with ease through the forest canopy Rather than jumping from tree to tree, they reach across and pull adjacent trees together If the space between two is too far to reach, they will rock the tree they are in and use it like a swing to bndge the gap. The Bornean species has been recorded travelling as far as four kilometres in a day and tends to travel farther than its Sumatran relative because its food sources are more spread out.

Orangutans, particularly the female, rarely descend to the forest floor. When they do, they do not “knuckle walk” like the other great apes; instead, they walk on clenched fists. They will occasionally walk on two legs and can wade across small stream, although they can neither swim nor even float.

Unlike other large mammals that have clearly defined territories, which are marked with their scent and defended from competitor, orangutans have “home ranges” which are not marked or defended. Indeed, an individual’s home range may overlap with that of another. An adult male’s home range can extend to 4,500 hectares and will encompass those of several females, which might covet as much as 1,500 hectares. Although male orangutans do not defend their ranges, they will fight over access to females.

While almost all other apes and monkeys are social and gregarious, orangutans tend to be solitary or semi-solitary In Borneo, the largest typical group is a mother and her two offspring: one dependent and the other an adolescent approaching independence. Scientists believe that this unusual lifestyle evolved as a result of erratic fruiting: with limited resources available, group-living would lead to competition for food. This theory is supported by evidence from Sumatra, where orangutans exhibit mote social behaviour when fig trees fruit simultaneously-something that is rare in Borneo.

Orangutans’ solitary lifestyle has reduced the need to develop elaborate vocal communication, and ‘the long call’ and the “kiss-squeak” are their 40 most distinctive sounds. Once dubbed ‘the lion’s roar of the  jungle’, an adult male’s long call is a deep, rumbling, grunting bellow that can carry for more than one kilometre. Its exact function is unknown, but it is likely that it serves to alert females to the presence of an adult male and to warn other males to keep their distance. As males frequently long-call in aggressive situations or when aroused, it may also be a means of releasing tension.

The kiss-squeak is used to convey displeasure or annoyance and has often been interpreted to mean “go away!” For example, an orangutan will frequently kiss-squeak when disturbed by human researchers. Interestingly if a subordinate orangutan is displaced by a more dominant one, the subordinate will often kiss-squeak as it moves away.The only time the two sexes come together is to mate. Courtship lasts between three and ten days, during which time the male will follow the female continuously They will mate frequently and the male will long-call to deter other males from approaching. It is the female who, not wanting to share her food resources, initiates the final separation. The male plays no part in bringing up his offspring.

Infants remain with their mother for several years in order to  learn the skills necessary to survive: climbing, nest building and, most importantly, creating a ‘mental map’ of the forest-learning where food trees are and their fruiting patterns.

But there is a cost: having a dependent offspring for so many years slows down the rate at which a female can reproduce. Orangutans are the slowest breeding of all primates and have one of the longest inter-birth intervals of any land mammal.

A female orangutan reaches puberty at ten or eleven years old, but will generally not have her first infant until she is at least 13. She will subsequently only give birth once every six to eight years. With a life expectancy of around 45 years. she will have no more than three to four offspring during her lifetime.

It is this extremely long birth interval that makes orangutans particularly vulnerable to extinction, especially those in small or fragmented populations. A rise in the mortality rate of adult females of as little as one or two per cent can drive a population to extinction because it doesn’t have the capacity to recover quickly enough.


Habitat
Orangutans’ preferred habitat is low-lying peat swamp and surrounding forest. They rarely live above an altitude of 800 metres. On both Borneo and Sumatra, their range is limited and increasingly fragmented.


Diet
Orangutans are primarily frugivorous. However, in times of fruit scarcity they will alter their eating habits, On such occasions they eat lower quality foods such as bark, leaves and termites. Although they have been recorded eating meat, it is a rare occurrence. In total, scientists have documented more than 500 food types in the species’ diet.

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