Evolutionary Variation
In the long-footed mammals, both the hoofed species (unguligrade) and the clawed forms which walk on the toes (digitigrade), the heel is well above the ground at the apex of the angular joint known as the hock. In plantigrade species it rests on the ground. In birds, the heel is the backward-pointing joint which is often mistaken as the "knee" (the actual knee of birds is hidden under the plumage).
Read more about this topic: Heel
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