Final answer:
Chimps expend more energy walking bipedally due to the demand for muscular exertion and balance not required in their optimized quadrupedal form. The lack of adaptations such as a broad pelvis and arched feet increases the effort needed for bipedalism.
Step-by-step explanation:
Walking bipedally requires chimps to expend 75 percent more energy than their usual mode of quadrupedal locomotion largely due to physiological factors. The evolution of bipedalism included significant anatomical changes such as a broader pelvis, longer legs, and arched feet, which are efficient for bipedal locomotion in humans, but these adaptations are absent in chimps. Chimps have a body structure that is optimized for quadrupedalism, with a center of gravity suited to all four limbs touching the ground. When they attempt bipedalism, they have to compensate for an unbalanced center of gravity and the lack of specialized limb proportions and joint constructions. This results in increased muscle exertion and energy consumption. Additionally, the mechanics of bipedal walking require one leg to be the 'stance leg,' while the other moves forward, a cycle that demands more balancing effort and muscular stabilization than chimps' natural gait, contributing to greater energy usage.