Y: Time to go again to the A Moment of Science mailbag. A listener writes:

    Dear A Moment of Science,

    Why do chimps and gorillas walk on their knuckles? And do any other animals do this?

D: Great questions! The answer to the second question is, no. Chimps and gorillas are the only creatures that walk on their knuckles.

Y: As to why they do this, that’s something that’s puzzled researchers for years. But scientists at Case Western Reserve University seem to have found an answer. The gist is that the apes evolved to both climb trees and get around on the ground.

D: See, according to Newton’s laws of motion, every action has an equal and opposite reaction. So, when chimps and gorillas walk on the ground, the ground pushes back up. And their bodies simply aren’t constructed to handle that energy.

Y: Right. Chimps and gorillas have long arms, short legs, stiff, upright backs, torsos shaped like cones, and flat feet. That’s great for climbing trees, but not so great for walking using only their feet.

D: Their flat feet make walking difficult … which is why they use their knuckles, which work like shock absorbers to deal with the energy of the ground pushing up.

Y: Plus, their cone-shaped bodies allow for efficient shoulder rotation, which also helps absorb energy when the apes travel on the ground.

D: We humans, of course, have arched feet, which is why we’re able to walk long distances without damaging our feet. Just like ape knuckles act like shock absorbers, our foot arches do the same.