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All dinosaurs are thought to be descended from a fully bipedal ancestor, perhaps similar to Eoraptor.Bipedal movement also re-evolved in a number of other dinosaur lineages such as the iguanodons.Some 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.The word is derived from the Latin words bi(s) 'two' and ped- 'foot', as contrasted with quadruped 'four feet'.Several arboreal primate species, such as gibbons and indriids, exclusively walk on two legs during the brief periods they spend on the ground.
On the other hand, most macropods, smaller birds, lemurs and bipedal rodents move by hopping on both legs simultaneously.
Even this distinction is not completely clear-cut — for example, humans other than infants 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.
Zoologists often label behaviors, including bipedalism, as "facultative" (i.e.
optional) or "obligate" (the animal has no reasonable alternative).
Several lizard species move bipedally when running, usually to escape from threats.