Orang-utan

Orang-utans are sexually dimorphic, with adult males being about twice the weight of females. The height of the male is about 54 in (137 cm), it weighs 130-200 lb (60-90 kg), and has an arm span of 7-8 ft (2.1-2.4 m). The long, coarse, coat of orang-utans is typically reddish brown, but is bright orange in juveniles, and can be maroon or dark brown in some adults. The fur is especially long and shaggy over the shoulders, where it may reach up to 18 in (46 cm) in length. Orangutans lack the strong brow ridges of chimpanzees and gorillas, and have a dish-shaped face that is hairless and usually black; young animals have pinkish skin on the muzzle and around the eyes. As the males mature, they develop deep throat pouches which extend under the arms and over the shoulder. Cheek flanges of fatty tissue are present in both species. Male orang-utans from Borneo have huge cheek flaps, resembling horse blinders, while males from Sumatra have relatively longer, oval faces with cheek flanges that extend sideways and a well-defined moustache and beard.

Orang-utans are largely arboreal and spend little of their time on the ground. Although these apes are slow-moving and cautious, their long arms and hooked hands and feet provide an effective means of moving rapidly through the forest canopy. Orang-utans are versatile climbers, using a modified over-arm brachiation as a mode of locomotion. Leaping or jumping over any distance is uncommon. Orang-utans use their flexible joints and powerful hands and feet to distribute their weight over several small branches, any one of which might not support them. When a gap in the canopy is encountered, they use their weight to swing trees back and forth until the distance can be bridged. Most orang-utans will occasionally descend from the trees to the forest floor, though the practice is most common among adult males. Once on the ground, orang-utans move using a form of knuckle-walking, with the weight carried on the bunched fists rather than on the knuckles. They rarely walk in a bipedal fashion, and if they do so it is stiff-legged and awkward.

At night, orang-utans sleep in nests built high above the ground. Most nests are built in the middle level of the forest canopy. Orang-utans prefer to nest in places that afford good visibility, and they change the nest location each night. During the day, less elaborate nests are built for resting and protection against heavy rainfall. Just as humans use umbrellas, orang-utans hold leafy branches over their head as a shield from rain.