Bipedal Animals
The great majority of living terrestrial vertebrates are quadrupeds. Among mammals, bipedalism is a normal method of ground locomotion in various groups of primates (e.g. lemurs, gibbons and Hominina), in the macropods (kangaroos, wallabies, etc.), and in a few groups of rodents, including kangaroo rats, gerbils and spring hares. All birds are bipeds when on the ground, a feature inherited from their dinosaur ancestors. Bipedalism evolved more than once in archosaurs, the group that includes both dinosaurs and crocodilians. Many species of lizards become bipedal during high-speed, sprint locomotion, including the world's fastest lizard, the spiny-tailed iguana (genus Ctenosaura). There are no known living or fossil bipedal amphibians.
Most bipedal animals move with their backs close to horizontal, using a long tail to balance the weight of their bodies. The primate version of bipedalism is unusual because the back is close to upright (completely upright in humans).
Humans, gibbons and large birds walk by raising one foot at a time. On the other hand most macropods, smaller birds, lemurs and bipedal rodents move by hopping on both legs simultaneously. Tree kangaroos are able to utilize either form of locomotion, most commonly alternating feet when moving arboreally and hopping on both feet simultaneously when on the ground.
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