An ostrich, one of the fastest of living bipeds
A Man Running - Eadweard Muybridge
Bipedalism is a form of terrestrial locomotion where an
organism moves by means of its two rear limbs, or legs.
An animal or machine that usually moves in a bipedal
manner is known as a biped (pronounced /ˈbaɪpɛd/),
meaning "two feet" (from the Latin bi for "two" and ped
for "foot"). Types of bipedal movement include walking,
running, or hopping, on two appendages (typically legs).
Relatively few modern species are habitual bipeds
whose normal method of locomotion is two-legged.
Within mammals, habitual bipedalism has evolved four
times, with the macropods, kangaroo mice, springhare
 and homininan apes. In the Triassic period some
groups of archosaurs (a group that
ancestors of crocodiles) developed bipedalism; among
their descendants the dinosaurs all the early forms and
many later groups were habitual or exclusive bipeds; the
birds descended from one group of exclusively bipedal
A larger number of modern species are capable of bi-
pedal movement for a short time in exceptional circum-
stances. Several non-archsaurian lizard species move bi-
pedally when running, usually to escape from threats.
Many animals rear up on their hind legs while fighting.
A few animals commonly stand on their hind legs, in or-
der to reach food or to keep watch, but do not move
There are two main types of bipedal locomotion:
macropods, smaller birds, and rodents move by hopping
on both legs simultaneously; other groups, including
apes and large birds, walk or run by moving one leg at a
The word is derived from the Latin words bi(s) ’two’ and
ped- ’foot’, as contrasted with quadruped ’four feet’.
Facultative and obligate bipedalism
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, b