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A Wilder View: Why apes walked around on their knuckles

Posted at 10:30 AM, Jan 04, 2023

MISSOULA - In the animal world walking on your knuckles seems just as strange as walking on two feet.

Until recently, no one really knew why apes walked around on their knuckles.

This edition of A Wilder Views takes a look at why our close cousins use this form of movement.

While some anteaters and orangutans are recognized to sometimes walk on their knuckles, knuckle walking is the primary form of movement for chimpanzees, bonobos, and gorillas.

As chimpanzees and bonobos walk around, they will extend their arms, flatten out their hands and curl in the knuckle while gorillas keep their arms straighter. An ape’s body is adapted to climb trees and walk on the ground.

To be able to do both of these forms of motion they have evolved to have long arms and short legs. The long arms allow them to grasp from branch to branch.

But chimpanzees — and more so gorillas — are now spending over 80% of their time walking on the ground. Even with such a major portion being spent below the vines their anatomy still has these long arms though because of their ancestors.

The reason for knuckle-walking lies in Newton's third law of motion — for every action, there is an equal and opposite reaction.

When we walk, we apply force backward on the ground by pushing the ground with our feet and following Newton’s Law, the same force is used by the ground in the opposite direction, so in this case, it’ll be the forward direction, and this makes us able to move.

If we take these same physics and apply them to apes walking with just two feet would be quite difficult. You see chimps and gorillas have large bodies. For instance, the average male gorilla weighs 400 pounds. Due to this heaviness, they have trouble dissipating the energy exerted from the ground.

Knuckle walking helps them move their weight around. Since their front legs are longer for climbing when they get up on their knuckles, it shortens up the front legs and provides better balance.

Now let’s go back to looking at us as an example. When we run, we are pushing down with a lot of force on the ground which again following Newton’s Laws means the ground is pushing up at an equal force.

That would be a lot of force for our body to take but we have a unique adaptation of having arched feet. That simple arch serves as a shock absorber. Without this, long walks or runs would greatly damage our bones and joints in our feet and legs.

Gorillas and chimps don’t have arches, so their feet are flat. But they have adapted torsos that are triangular shaped. This adapted shape is vital for knuckle-walking as it serves as their form of a shock absorber like the arches in our feet.

Whether they’re swinging on vines, jumping, knuckle-walking, or walking upright, every animal seems to know just the right way to get from one place to the other