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 ( /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 six times, with the macropods, kangaroo mice, dipodids, springhare , pangolins and homininan apes. In the Triassic period some groups of archosaurs (a group that includes the 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 dinosaurs.
A larger number of modern species are capable of bipedal movement for a short time in exceptional circumstances. Several non-archosaurian lizard species move bipedally when running, usually to escape from threats. Many animals rear up on their hind legs whilst fighting or copulating. A few animals commonly stand on their hind legs, in order to reach food, to keep watch, to threaten a competitor or predator, or to pose in courtship, but do not move bipedally.
There are two main types of bipedal locomotion: macropods, some smaller birds, and heteromyid rodents move by hopping on both legs simultaneously; other groups, including apes and larger birds, walk or run by moving one leg at a time.