Bipedalism is a form of movement on the ground. Some tetrapods have evolved the ability to move on two limbs. Those that do, use their rear limbs. Some animals move on two limbs all the time, they are called habitual bipeds. Optional bipeds can move on two or four legs.
Early quadruped animals used all four limbs for locomotion but many later ones are bipedal. The earliest dinosaurs were bipedal but millions of years later some reverted and became quadrupeds. Birds are descendants of bipedal dinosaurs, and are bipeds themselves. Their forelimbs have become wings.
Some optional bipeds stand on two legs to drive away competitors and predators, to see farther, or as body language. Their locomotion is on four limbs.
- Amongst mammals, apes, bears and pangolins are optional bipeds.
- Some small mammals can stand up on two legs quite well, but only move on four legs. Meerkats are a familiar example.
- Birds carry their weight vertical. This is especially obvious in the ostrich, which is the fastest running biped.
- Bipedal dinosaurs, on the other hand, carried the body more or less horizontal. The weight of the front part of the body was balanced by the weight of their tails. Dinosaurs were probably primitively bipedal, and the larger herbivores reverted to quadrupeds as their weight increased.
- Pangolins are bipedal when walking on a level surface. They keep their body horizontal, and the tail balances the front part.