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Why are orangutans not knuckle walkers like African apes?

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An illustration of a female Bornean orangutan (Pongo pygmaeus) with long reddish-brown fur, hanging from tree branches with one arm extended upward and one leg hooked over a branch. A scale bar at the bottom shows lengths of 30 centimeters and 12 inches for size reference. — orangutan Female orangutans (*Pongo*) have the longest breeding interval of any mammal, giving birth on average once every eight years.

Orangutans are not knuckle walkers like African apes because they are primarily arboreal animals, spending more than 90 percent of their waking hours in the trees. While African apes—such as gorillas, chimpanzees, and bonobos—are adapted for terrestrial travel via knuckle walking, orangutans have evolved specialized anatomy for life in the forest canopy.12

When orangutans do travel on the ground, they are slow and do not use the knuckle-walking gait seen in their African relatives; instead, they walk on their closed fists or extended palms. Their unique locomotor adaptations include:1

Arboreal Scrambling: They typically move through the forest canopy by using all four hands and feet.1
Flexible Anatomy: They possess flexible hip joints that allow for similar movement in their legs and arms, as well as hooklike hands and feet with opposable big toes.1
Proportionate Limbs: They have the longest arms of any great ape, which facilitates both climbing and occasional brachiation.1