Bipedalism refers to the ability of an animal to move primarily on two hind limbs. This form of locomotion distinguishes certain species, as most terrestrial vertebrates are quadrupeds. A two-legged gait represents a significant adaptation, influencing an animal’s anatomy, behavior, and interaction with its environment. Understanding which animals utilize this movement provides insight into diverse evolutionary paths.
Animals That Primarily Walk on Two Legs
Humans are prominent examples of obligate bipeds. The human skeletal structure shows numerous adaptations for upright posture, including a shorter, broader pelvis that supports the upper body during walking. The femur, or thigh bone, angles inward, positioning the knees and feet directly under the body’s center of gravity, which aids balance. The human spine has an S-shaped curvature that helps absorb shock and maintain stability, while the foot features an arch for shock absorption and propulsion.
Birds demonstrate bipedalism, using their hind limbs for all terrestrial movement. Their unique skeletal and muscular configurations allow for efficient walking, running, and hopping. While flight is a defining characteristic for many birds, their strong hind limbs are adapted to support their entire body weight on land. The structure of a bird’s hindlimb facilitates bipedal locomotion despite variations in body size and habitat.
Animals That Occasionally Walk on Two Legs
Many animals are facultative bipeds, able to walk on two legs but typically using four or more.
Kangaroos
Kangaroos are well-known for their bipedal hopping, using powerful hind legs for rapid movement. When moving slowly, kangaroos often employ a “pentapedal” gait, using their tail as a fifth limb for support and propulsion. The tail also helps counteract pitching motions and stores elastic energy.
Primates
Primates like chimpanzees and gorillas engage in bipedal locomotion for specific purposes. Chimpanzees walk on two legs when carrying resources like food, allowing them to transport more items than walking on all fours. They also adopt an upright stance to reach food or survey their surroundings. Gorillas may stand bipedally during displays or when reaching for elevated branches.
Lizards
Some lizards exhibit facultative bipedalism, particularly when seeking bursts of speed to escape predators. The basilisk lizard, sometimes called the “Jesus Christ lizard,” runs across water on its hind legs. This feat involves rapid foot movements that slap and stroke the water, creating air pockets that prevent sinking, while the tail provides counterbalance. On land, many lizard species can transition to bipedal running for quick escapes.
Why Bipedalism?
Bipedalism offers advantages that have driven its evolution.
Energy Efficiency
For humans, walking on two legs is energy-efficient, especially for long-distance travel, using less energy than quadrupedal locomotion. This efficiency benefited early hominins covering distances between food sources.
Freeing Forelimbs
An upright posture frees the forelimbs for other tasks. In primates and humans, this includes carrying objects, using tools, or transporting infants. Manipulating objects with hands while moving has been linked to the development of complex behaviors and technologies.
Improved Vision & Thermoregulation
Bipedalism provides an improved field of vision, allowing animals to see over tall grasses or other obstacles to spot predators or locate food. Standing upright aids in thermoregulation by reducing the body’s surface area exposed to direct sunlight. This orientation allows for increased airflow, facilitating cooling.
Display/Intimidation
Bipedalism can serve as a form of display or intimidation. Standing on two legs can make an animal appear larger, deterring rivals or predators. This upright stance provides an advantage during conflict, allowing forelimbs to be used for striking with greater force.