Horses "lost" their side toes over millions of years, and ended up with just one toe per foot—the middle, or third, one. During embryonic development, a horse's second and fourth vestigial toes form splint bones that fuse to the back of their leg bone. The first and fifth toes don't form at all. Scientists theorize that horses and other ungulates evolved to have fewer digits so that they could run fast over long distances.
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Key Facts In This Video
Humans are plantigrade animals, which means that all of the bones on our feet touch the ground when we walk. 00:16
Digitigrade animals have an elongated heel bone. 01:06
Ungulates save energy by running on just the tips of their fingers and toes. 01:44