The Sumatran orangutan (Pongo abelii) is an ape that lives on the island of Sumatra, Indonesia. It is a critically endangered species. Sumatran orangutans are disappearing from all its major habitats and today they are found in small isolated pockets around Lake Toba. Orangutans occupy habitats in the north of the Lake. They are hunted to extinction in the south of Lake Toba but the hunting goes thousands of years back. Sumatran orangutans eat fruits.
Adult males are 1.4 m (4.6 ft) tall and weigh up to 90 kg (200 lb).
Males are larger than females. Females average 45 kg (99 lb) in weight and the length measuring up to 90 cm (3.0 ft).
The Sumatran orangutan makes home in the forests of peat swamps, alluvial bottomlands, freshwater habitats, and floodplains. The largest concentration of orangutans is found in these habitats. In the hill dipterocarp forest, submontane and montane forests, and lowland dipterocarp forest orangutans live in relatively small numbers.
Human hunting and food availability primarily determines the habitat range of orangutans. The overall population is continuously declining in the forests of Sumatra. Orangutans like to eat soft-pulped fruit and thus they live in habitats which are abundant in these fruits.
Although the feeding area of Sumatran orangutans is large yet they seem to be aware of the fruiting areas. Not only this, they also know what fruits are useful to them. Scientists suppose that there is a feeding map in the orangutans’ mind and that they are well aware of their eating habits. Evidence suggests that Sumatran orangutans are able to understand signs of fruits.
The availability of fruits not only determines the behavior of orangutans it also defines its territory. This indeed shows the extent to which orangutans rely on fruits. Sumatran orangutans mostly live at an altitude less than 1,000 meters above the sea level. At this level orangutans easily find soft-pulped fruits.
They do not rely on one or two types of fruits. Sumatran orangutans eat a wide variety of fruits. In Gunung Leuser National Park orangutans consume fruits from 92 tree species and liana species. Sumatran orangutans typically travel alone but sometimes they meet on the feeding site at the same time which makes them less solitary.
Favorite among its fruits are
Cyathocalyx sumatranus (Annonaceae)
Antiaris toxicaria (Moraceae)
Mallotus schaeorocarpus (Euphorbiaceae)
Rambutan Nephelium lappaceum
Tinomiscium phytocrenoides (Menispermaceae)
All the above fruits have soft juicy pulps. Sumatran orangutans are likely to feed on small fruits. Other fruits that make up the orangutan’s diet include jackfruit (Artocarpus elastic, Moraceae), and durians (Durio spp., Bombacaceae), Tetramerista glabra (Tetrameristaceae), Sandoricum beccarianum, Neesia.
Some fruits are available throughout the year while others are entirely seasonal. For instance in the Ketambe the eight species of Ficus spp. (Moraceae) are present eight months in a year. When fruits go short the orangutans rely on leaves or bark but these are not the preferred foods. Figs make up the most of the Sumatran orangutan’s diet. However few fig species are most readily eaten such as F. stupenda, F. benjamina, F. subulata, F. drupacea, and Ficus annulata.
Sumatran orangutans supplement their diet with seeds because seeds provide high calorie content. They can crush even the hardest of the seeds with their powerful teeth. Prominent among seeds is Heritiera elata (Steruliaceae).
Leaves make up 5 – 25% of the Sumatran orangutan’s diet. While they prefer to feed on new buds and shoots mature leaves of few tree species are also consumed. Orangutans are likely to eat leaves of stinging nettle such as Dendrocnide spp. (Urticaceae). They avoid eating leaves with their lips because stinging nettles could hurt them badly. Sumatran orangutan’s diet consists of epiphytic fungi, stems of climbers, aerial roots, leaf galls, and grass.
With the help of their powerful teeth orangutans are able to rip the bark off the trees. Even the hardest parts of the woods including xylem or phloem layers are regularly eaten. Orangutans also rely on insects. Study suggests that Sumatran orangutans eat up to 17 different species of insects. Alongside insects and leaves, orangutans eat soil because it is rich mineral nutrients.
Orangutans do not eat vertebrates. However they will consume bird eggs but if there are hatchlings in a nest, orangutans will eat them. Orangutans often raid squirrel’s nest. This behavior might be out of its opportunistic nature rather than going out deliberately. Similarly one adult female fed on young gibbon and lorises.
Sumatran orangutans will often share the same feeding site. A group of 20 – 25 orangutans may well use the forest site of 4 hectares.
Males have larger home ranges as compared to those of females. The male’s territory is 2 – 3 times the size of female’s. However adult males probably occupy small range because they must find females not only for mating but also to ensure their dominance.
Scientists are not clear if the home range is selected on the basis of food availability. In rugged habitats, Sumatran orangutans will move around to find soft pulped fruits. In plain ideal habitats, they do not move as much because they could consume food trees fruit within their large range. Female orangutans are thought to cover 8.5 square kilometers in Suaq Balimbing. In swamp forest, males occupy an area of 25 square kilometers.
Unique among orangutans is that they form loose groups. According to researchers, Sumatran orangutans move in coordination and they also get together on rare occasions. In group, there is a strong bond between mother and a child but it also fades away in time.
When orangutans reach adulthood they move alone. Young apes spend time playing together while staying close to their mother. When males become independent they either form new groups or become wanderers.
While in group orangutans produce high-pitched calls just to make sure that they are tied to each other. These calls are loud enough to be heard at distances. It makes the presence of adult males feel to young orangutans or females. Orangutan’s calls are ‘bubbling grunts’.
The group formation depends on two factors; one is food and the other one is mating females. Large groups are formed when food is abundant. Individuals within a group travel, sleep, and eat together. By so doing they make travel bands.
Although each orangutan gets a small portion of food (in a group) but it protects females from males of other groups. It’s worth living in groups. While living in a group orangutans also learn skills. Adult solitary males live a life without any skill.
Unlike Bornean orangutans, Sumatran species often live in groups.
Tool Use and Nesting
Sumatran orangutans are very handy in using tools. They are thought to use as many as 54 different tools for extracting insects. Orangutans use 15 – 20 tools for eating fruits.
They are often seen holding sticks which they use to extract termites out of their mounds. Besides, Sumatran orangutans also use tools to feed on malayana fruit seeds.
Sumatran orangutans are busy apes for they make a new nest every new day. They sleep in nests which are made of twigs, branches, and leaves. It takes them 20 minutes to build a complete nest. Mother and infants sleep in the same nest. While they make nests for night sleep sometimes orangutans relax during the day.
Male orangutans attain maturity at the age of 14 – 16 years. Females reach maturity at the age of 10 years.
A single infant is born after a gestation period of 245 days. The mother will carry the infant months after the birth. The young orangutan suckles for 5 – 6 years. When babies are 11 months old they begin to find food independently.
Young become fully independent at the age of 7 – 10 years.
The average life expectancy of a Sumatran orangutan is 45 years in the wild.
Conservation Status and Population
Sumatran orangutans are critically endangered animals but still there are 14,000 Sumatran orangutans living in the wild.