The only great apes that live outside of Africa are orangutans, of which there are two species. The Sumatran orangutan is more endangered then the Bornean orangutan. The two differ in appearance and behavior, with Sumatran orangutans being slightly smaller and usually having hair that is lighter colored and orange-red, as well as a longer beard. Males develop throat pouches and fleshy pads on their cheeks but these are narrower and less pronounced than those of male Bornean orangutans.
Sumatran orangutans occur on Sumatra, in Indonesia, and are restricted to just the northern tip of the island. They live in primary lowland tropical forests, including mangrove, riparian forests and swamp forests.
Sumatran orangutans lead very solitary lives, during the day moving slowly in search of food through the trees. Up to 60% of an orangutan’s time is spent foraging and eating, and, although they can occupy large home ranges, half a mile a day is the most they usually travel. At night, they build a nest for sleeping up high in the canopy out of folded branches. They are not very territorial, sharing their home ranges with others, sometimes feeding alongside other orangutans around especially abundant fruit trees. Males will stake their claim to their territory by emitting long-calls, deep calls from their throat poach which echo through the forest to attract females and also to warn off potential rivals.
The diet of Sumatran orangutans varies seasonally, depending on the fruiting season of trees in the local area, the animals feeding when the fruit is ripe. Figs are very important in their diet. When fruit is not so available in dry seasons, Sumatran orangutans will eat vegetation such as young leaves, bark and flowers, and insects, particularly termites, ants, and crickets, and sometimes eggs.
These animals are polygynandrous; this means that two or more males mate with two or more female orangutans. Most mating occurs during rainy seasons, the heaviest fruiting months (December to May). After gestation of about nine months, the female builds a new nest high in a tree, where her single infant is born. The young clings to its mother for safety and remains at her side during the first few years. Infants may be weaned by the age of three, but they will stay with their mother until at least the age of 8 or 9 years, while being taught special skills for forest survival. Sumatran orangutans are slower breeders than other primates, with females bearing a maximum of three babies during their lifetime. Females are sexually mature by the age of 12 year and males at an average of 19 years old.
The primary threat to Sumatran orangutans is loss of habitat. These animals have a unique vulnerability to exploitation due to their extremely long inter-birth period, usually eight years, which makes them the world’s slowest breeding primates. Recent political instability in the area of their habitat has led to increased illegal logging in areas that are protected, and increased capture of infants to be sold for the illegal pet trade.
According to the IUCN Red List, the total Sumatran orangutan population is around 7,300 individuals. In addition, a population that is being established in Jambi and Riau Provinces in the Bukit Tigapuluh National Park, numbers around 70 animals and is reproducing. Overall, Sumatran orangutans’ numbers are decreasing today and they are classified as critically endangered (CR) on the list of threatened species.
Sumatran orangutans have a big role in the Sumatran lowland rainforests and are therefore considered a keystone species. Orangutans are important for dispersing seeds, being consumers of a wide variety of fruit, and they also help to maintain the diversity of woody plants of the rainforest. In addition, they prune and assist in regenerating plant growth, as they choose to eat only green leaves and stalks.