Page "Bipedalism" Paragraph 6

Zoologists often label behaviors, including bipedalism, as " facultative " ( i. e. optional ) or " obligate " ( the animal has no reasonable alternative ).

Even this distinction is not completely clear-cut — for example, humans normally walk and run in biped fashion, but almost all can crawl on hands and knees when necessary.

There are even reports of humans who normally walk on all fours with their feet but not their knees on the ground, but these cases are a result of conditions such as Uner Tan syndrome — very rare genetic neurological disorders rather than normal behavior.

Even if one ignores exceptions caused by some kind of injury or illness, there are many unclear cases, including the fact that " normal " humans can crawl on hands and knees.

This article therefore avoids the terms " facultative " and " obligate ", and focuses on the range of styles of locomotion normally used by various groups of animals.