Behaviour
Sumatran orangutan behavioural characteristics
Orangutans are well known for being relatively solitary compared to the other great apes. Orangutans are frugivores (fruit-eaters) and their food is highly patchy in the forest. If there are not many trees fruiting, it’s in an orangutan’s best interests to visit them on its own, or with its own young, and eating as much of the food as possible itself. The more orangutans that travel together, the more trees they will have to visit and the further they will have to travel each day to get enough to eat. But when food is abundant, orangutans will often gather and feed together in a single tree and even travel together for several days.
About 60% of an orangutan’s diet is made up of fruits. Other things they eat include young leaves, liana and palm stems, insects, and occasionally tree bark. In Sumatra especially they eat large numbers of ants and termites each day. In Sumatra too, they even eat meat, in the form of the slow nocturnal primate known as the Slow Loris (Nycticebus coucang). Bornean orangutans have never been seen to eat Slow Loris and this behavior is probably unique to the Sumatran species.
Orangutans make leafy nests in the trees to sleep in and rest. They make a new nest just about every night (occasionally they will repair an old one they find), and in Sumatra they make one most days too, around midday, for a kind of siesta. They make these nests by bending a few large branches into the middle, and then weaving smaller branches into this frame, until it makes a comfy, “bowl” like nest to sleep in. When they think it might rain they often also make a kind of roof, also from branches and leaves, and go over the top of the nest once they are in it.
A typical day for an orangutan involves waking up in its leafy nest at sunrise (just after 6.00 am), and then feeding for 3 hours or so in a nearby tree containing lots of fruits. They then start to travel a bit, picking up additional food items as they pass by, until deciding its time for their midday rest and building a nest to lie in. After about an hour’s rest, they then travel some more, again snacking on the way, often on termites or leaves or stems, before seeking out another fruit tree in which to gorge themselves for a few more hours until its time to build another nest for the night, eventually retiring to rest and sleep in the late afternoon, around 4 or 5pm (though sometimes earlier, or later too, of course).
A female orangutan has her first infant when she is around 15 years old, after a pregnancy lasting about 8 months and 20 days. She then has one infant every 8 or 9 years in Sumatra (6 to 8 years in Borneo), which is the longest inter-birth interval of any terrestrial mammal.
Given that a healthy female orangutan should live well into her 50’s, and perhaps even her 60’s in the wild, she can probably rear only 4 or 5 infants in her lifetime. Twins have been known with captive orangutans, but probably would not survive in the wild due to the difficulty a mother would have carrying 2 infants around in the treetops and feeding herself at the same time.
nfant orangutans grow and develop very slowly and people often underestimate their age as a result. For the first year an infant will cling too its mother’s fur virtually 24 hours a day. During the second and third years they will start to explore a bit more and learn to climb, but never more than a few metres away from the safety of their mothers arms and always clinging on to her again whenever they are travelling between trees. Only after about 3 years of age do they really start to move around by themselves with confidence and even then they still spend all day and night travelling, feeding and sleeping with mum. Only when a new brother or sister is born, when the first infant is about 9 years old, do they begin to travel out of eye contact with their mum on a regular basis, and sleeping in their own nests, but even then they will still visit her every few days and spend a lot of time together.
In this way, an orangutan infant learns the many skills it needs to survive in the forest. It learns where to find food, producing a kind of mental map of the food resources in the forest. It also learns how to make a decent nest, and how to process and eat food when it finds it. In the peat swamp forests of Sumatra they even learn how to make and use tools to get honey from bees nests and to get the seeds out of the rock hard and very itchy Neesia fruit.
Male orangutans are quite unique in the primate world in that there are two forms or “types” of breeding males. Wild female orangutans are sexually mature around 12 or 13 years of age, and first give birth around age 15. Males mature around the same time, when they are about the same size as adult females (around 35 to 45 Kg), meaning they are able to travel as fast as the females through the trees, to catch them, and to force them to mate (often termed “rape”). However, each forest area also has a few that are called fully mature or “cheek-padded” adult males (reflecting their large, flat “disc-like” faces) as well, who try to dominate the area and attract the females to mate with them when they are ready to become pregnant. These males are almost twice the size of an adult female (reaching some 80 kg, or even more) and are not able to chase and catch the females. It used to be thought that all males simply went through a small “subadult” phase and then matured into these bigger males after a few years. Recent research however suggests that some subadult males take many years before developing their large size and cheek-padded faces, even getting well into their twenties before it happens. Studies have now shown that around 50% of infants in an orangutan population are fathered by a number of these smaller males, and about 50% are fathered by whoever is the dominant, large, cheek-padded male in the area at the time.
The most commonly heard sound heard by people watching or following orangutans is the “kiss squeak”. This is something they do when they are anxious or afraid. When they get very upset, they may add some additional “grumpfs” to this.
Infant orangutans will sometimes winge and whine just like human kids. They often make a kind of “meeping” sound when they need help from their mums, especially when they want her to give them some food or to carry them from tree to tree.
By far the loudest and most audible orangutan vocalization is the adult male’s “long call”. Only the large cheek-padded males (and some larger, maturing unflanged males) make this call. It sounds like a serious of loud bellows or roars, followed by numerous quieter “bubbling” noises. The long call is thought to attract females to the dominant male of their choice, especially for breeding, and to inform other potential rival adult males to get out of the way, unless they want to make a challenge and fight for the dominance of the area.
Sumatran orangutans are almost exclusively arboreal, meaning they spend just about all of their lives off the ground, moving, feeding and sleeping up in the trees. They travel mostly by what is called “quadramanous clambering” which basically means they use all four “hands” (their feet are of course just like hands) to clamber through the branches. They do not very often “brachiate” which is the hand over hand mode of travel so typical of the gibbons. They also often travel in the smaller trees in the middle canopy layer of the forest, swaying them back and forth as they go in order to get a handhold on the next one, and then crossing on to that and starting again. This is a very energy efficient way to travel through the trees.
