Why are the gibbon and orangutan mainly arboreal while gorillas spend much time on the ground?

The primary factors influencing whether a primate is arboreal or terrestrial include body size, locomotor adaptations, and dietary requirements.123

• Body Size and Support: As primates evolved larger body sizes, their ability to navigate the periphery of tree crowns decreased due to the risk of falling from slender branches. While smaller primates can easily navigate the canopy, larger species face physical constraints. For example, female and young gorillas climb more frequently than adult males because much of the forest vegetation cannot support the weight of a mature male.24
• Locomotion: Primates have developed specific locomotor patterns to suit their environments. Gibbons are specialized for brachiation (arm-swinging), an arboreal adaptation. Orangutans are adapted for moving among vertical tree trunks, gripping them with all four limbs. In contrast, African apes like gorillas have evolved knuckle-walking, a form of quadrupedal locomotion that facilitates movement on the ground.156
• Dietary Needs: Increased body size is often associated with the addition of bulk foods to the diet, which influences habitat use. Gorillas, being the most massive primates, spend much of their time foraging on the ground, where they can access the large quantities of vegetation required to sustain their size.214