Final answer:
Many primates that move on the ground on all fours use a mode of locomotion called quadrupedalism, which allows them to efficiently navigate their environment and engage in activities like grooming.
Step-by-step explanation:
Many primates spend time moving on the ground on all fours, and this mode of locomotion is called quadrupedalism. While some primates exhibit other forms of movement, such as vertical clinging and leaping, swinging from branch to branch (brachiation), or walking on two legs (bipedalism), quadrupedalism is common among those who spend significant time on the ground. Primates have adaptations like a rotating shoulder joint, opposable thumbs, and stereoscopic vision, which are useful both for arboreal living and for quadrupedal movement on the ground. Activities such as grooming, which involve touch and are common in many primate species, are also easily accomplished when moving on all fours.