Everything else was left behind
Why Do Horses Run on One Toe?
A horse runs on the tip of one enormous fingernail. The other toes are still there, shrunk to splinters of bone.
-
Step 1 of 5
Weight at the bottom of a leg is expensive.
The far end of a leg swings furthest and fastest, so weight there costs far more energy than the same weight higher up. Every running animal is light at the ankle.
-
Step 2 of 5
So the toes were cut down to one.
A horse's ancestors had several toes. Over millions of years the outer ones shrank and the middle one took the whole load, leaving a foot with almost nothing to carry.
-
Step 3 of 5
The muscle moved up.
The lower leg is almost all tendon, with the muscles that pull it sitting high near the body. It works like a puppet string: the motor is up top, the moving part is bare.
-
Step 4 of 5
The tendons store energy as well.
Those long tendons stretch as the hoof lands and snap back as it leaves, returning a large part of the energy of each stride. Some of a horse's running is free.
-
Step 5 of 5
The trade is that it only does one thing.
A hoof cannot grip, dig, climb or hold anything. It is a superb tool for running fast on firm open ground and no use whatsoever for anything else.
A goat's hoof is split into two halves that spread apart independently, which is why one animal's hoof is for speed and the other's is for cliffs.
The short version
A hoof is one huge toenail, and stripping the foot down to a single toe is what makes a horse's leg light enough to be fast.
Try it yourself
Swing your arm with a book in your hand, then hold it against your shoulder. Same weight, completely different effort.