Bipedalism is a form of locomotion defined by movement on two rear limbs or legs, a trait that has evolved multiple times across the animal kingdom. Many different types of animals have adopted this method of terrestrial travel. The classification of two-legged walkers depends on whether the animal uses this gait as its primary mode of movement or only adopts it under specific conditions. This distinction separates animals into two main categories: obligate bipeds and facultative bipeds.
Animals That Must Walk on Two Legs
Obligate bipedalism describes animals whose primary or sole method of moving across the ground involves only two legs. Birds represent the largest and most diverse group of obligate bipeds, with their forelimbs having evolved into wings for flight. Species like the ostrich and emu rely on powerful two-legged running, utilizing a bent-knee and horizontal torso posture. Penguins are also obligate bipeds, utilizing a distinctive waddle to navigate on land while their specialized wings are adapted for swimming.
Macropods, which include kangaroos and wallabies, employ a specialized form of bipedal movement known as saltatorial locomotion, or hopping. Their powerful hind legs and large elastic tendons allow them to store and release energy with each bounce, making the gait highly efficient at speed. A kangaroo’s massive muscular tail acts as a dynamic fifth limb for balance during fast hops. When moving slowly to graze, the tail forms a stable tripod with the hind legs, supporting a significant portion of the body’s weight.
Other mammals, such as the pangolin, also show obligate bipedal walking on level ground, lifting their front limbs off the soil. They use their heavy tail to counterbalance the weight of their head and upper body while walking. This form of movement allows them to carry their long, sharp claws off the ground, preserving them for digging or defense.
Animals That Sometimes Walk on Two Legs
Facultative bipedalism refers to animals that typically move on four limbs but switch to a two-legged stance for temporary, circumstantial reasons, providing a momentary advantage. Many large primates, including chimpanzees and gorillas, adopt bipedal walking to carry objects, freeing their hands for manipulation. This type of bipedalism is often performed in a bent-hip, bent-knee posture, which is less energy-efficient than the upright human gait.
Bears often stand on their hind legs primarily for threat display or enhanced visibility, not for long-distance travel. Standing upright allows them to appear larger to rivals and helps them scan the landscape for food or danger. While they can walk short distances in this posture, they quickly revert to a stable, four-legged gait for speed or endurance.
Among reptiles, certain lizard species, such as the basilisk lizard, use bipedalism as a high-speed escape mechanism. When accelerating rapidly, the lizard’s body tilts backward, and it sprints exclusively on its hind legs to maintain momentum. This sudden shift in gait is triggered by the need for quick bursts of speed to flee a threat.
Structural Requirements for Bipedal Movement
Skeletal and muscular adaptations are necessary for any animal to effectively move on two limbs. The primary biomechanical challenge of bipedalism is managing the center of gravity over a narrow base of support. Animals like birds and macropods solve this by keeping their spine largely horizontal, using a tail or forward body lean to position the center of mass directly over the feet. Their gait is often a spring-like action, relying on powerful tendons for elastic energy storage.
The structure of the pelvis is a major distinguishing factor, especially for animals that walk upright. In a fully erect biped, the pelvis must be broad and basin-shaped to effectively transmit the entire upper body weight to the legs and provide attachment points for stabilizing muscles. This is a contrast to the long, narrow pelvis found in most four-legged animals.
The structure of the spine must be specialized for the specific form of bipedalism. Animals that maintain a horizontal body posture often have a straighter spine. Fully upright walkers have a series of spinal curves that act as shock absorbers.
Foot structure also varies among bipeds. Some use a digitigrade stance, walking on their toes. Others, like humans, use a plantigrade stance, placing the entire foot flat on the ground to improve stability and push-off power.