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Why do orangutans not knuckle-walk 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 do not knuckle-walk because they are primarily arboreal animals, spending more than 90 percent of their time in the trees. Their anatomy is specifically adapted for life in the rainforest canopy, where they move by scrambling through branches using all four limbs or by brachiation—swinging with their arms.1

Unlike African apes, which have evolved to travel on the ground using knuckle-walking, orangutans possess flexible hip joints that allow for similar movement in their legs and arms, facilitating their grip on vertical tree trunks. When orangutans do descend to the ground, they are slow and move by walking on their closed fists or extended palms rather than their knuckles. This locomotor style reflects their evolutionary adaptation to the Asian rainforests, contrasting with the ground-dwelling tendencies of the African great apes.12