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 //, meaning 'two feet' (from Latin bis 'double' and pes 'foot'). Types of bipedal movement include walking, running, and hopping.
Several groups of modern species are habitual bipeds whose normal method of locomotion is two-legged. In the Triassic period some groups of archosaurs (a group that includes crocodiles and dinosaurs) developed bipedalism; among the dinosaurs, all the early forms and many later groups were habitual or exclusive bipeds; the birds are members of a clade of exclusively bipedal dinosaurs, the theropods. Within mammals, habitual bipedalism has evolved multiple times, with the macropods, kangaroo rats and mice, springhare, hopping mice, pangolins and hominin apes (australopithecines, including humans) as well as various other extinct groups evolving the trait independently. A larger number of modern species intermittently or briefly use a bipedal gait. Several lizard species move bipedally when running, usually to escape from threats. Many primate and bear species will adopt a bipedal gait in order to reach food or explore their environment, though there are a few cases where they walk on their hind limbs only. Several arboreal primate species, such as gibbons and indriids, exclusively walk on two legs during the brief periods they spend on the ground. Many animals rear up on their hind legs while fighting or copulating. Some animals commonly stand on their hind legs to reach food, keep watch, threaten a competitor or predator, or pose in courtship, but do not move bipedally.