Orangutans belong to the great apes and live on the south-east Asian islands of Borneo and Sumatra. Weighing up to 120 kg (males) and 50 kg (females), they are the world’s heaviest arboreal animals. And they are as much dependent on the rainforest as the rainforest is dependent on them.

Orangutans play a significant role in rainforest ecology. As frugivores they distribute the seeds of eaten plants and, while brachiating through the canopy, they break off weak or dying branches, opening the canopy up to sunlight which can then reach the forest floor.

Besides their bright orange fur, their very long arms and relatively short legs - with feet very similar to hands - are distinctive features of their appearance. Fully grown, males can be recognized by their large cheek pads and long fur.

The function of these cheekpads is not yet clear: it has been theorized that they may serve as protection for the face, or as a sound projection aid for long calls, or simply as a visual signal to other adults. They usually grow around the age of 15 to 20 years, at which point orangutan males become fertile.

Orangutans spend most of their life in the tree canopy. When moving they do not actually cling to branches but hook their hands and feet onto branches. Also, they do not jump from one tree to the other but use their weight to bend branches and trees to reach neighboring trees.

Because fruit trees in Borneo – more than Sumatra – are somewhat distant from each other and do not provide much fruit, orangutans travel up to four kilometers per day to find fruit, bark, leafs and insects for food. It is probably because of this that orangutans –unlike other great apes - mostly live alone, but in overlapping territories. Smaller groups consist of one female with one or two offspring. Males only join females for mating.

After an eight-month pregnancy a female will give birth to a child which she will care for intensively for about four years before becoming ready for mating again. Normally five to seven years pass before the next child is born. This slow rate of reproduction contributes to the endangered status of the orangutans, because they are not able to replenish their population at the rate at which their numbers are being reduced.

During the first year of life, an orangutan infant is constantly carried by its mother and during this time is weaned. Only after the first year does it start moving away from its mother and feeding on solid food. Even after the first year it continues to be breast-fed and carried over long distances, until its mother gives birth again. Even then the child will still stay with its mother and only leave her at about nine years of age. By that time an orangutan has learned enough (about climbing, nest building, feeding, orientation, etc) to survive on its own. Around the age of 12 a female will give birth for the first time.