A digitigrade is an animal that stands or walks on its digits, or toes. Digitigrades include walking birds (what many assume to be bird knees are actually ankles), cats, dogs, and many other mammals, but not plantigrades or unguligrades. Digitigrades generally move more quickly and quietly than other animals. Humans usually walk with the soles of their feet on the ground, in plantigrade locomotion. In contrast, digitigrade animals walk on their distal and intermediate phalanges. Digitigrade locomotion is responsible for the distinctive hooked shape of dog legs.