Locomotion, the act of moving from one place to another, takes countless forms in the animal kingdom, but one of the rarest is bipedalism. This term literally means “two feet” and refers to any animal that uses only two limbs for terrestrial movement, whether walking, running, or hopping. While most terrestrial vertebrates rely on four limbs, bipedalism is a successful adaptation that has evolved independently across diverse classes, including mammals, birds, and reptiles. This unique movement requires anatomical restructuring and serves purposes like energy conservation or defense.
Defining Obligate and Facultative Bipedalism
The animals that use two legs for movement are categorized based on how consistently they employ this gait. Obligate bipedalism describes species whose anatomy is specialized for and requires two-legged movement as their primary mode of terrestrial locomotion. These animals cannot comfortably or efficiently move using four limbs on the ground. Humans are the most recognizable example of this category, relying almost exclusively on their two legs.
By contrast, facultative bipedalism refers to animals that are capable of moving on two legs for short periods or specific reasons, but whose primary locomotion is quadrupedal or arboreal. These creatures use their hind limbs temporarily, often in response to an immediate need like a burst of speed or carrying an object. Facultative bipeds retain the anatomical structure for four-limbed movement, making their two-legged stance an optional behavior.
Animals That Primarily Use Two Legs
Obligate bipeds represent the most specialized group, having undergone profound skeletal changes to support upright movement. Humans possess distinct adaptations, including an S-shaped curvature in the spine that acts as a shock absorber and a wide, bowl-shaped pelvis that helps balance the upper body over the hips. The angle of the femur, or thigh bone, slants inward to position the knees and feet directly under the center of gravity, a structure known as the valgus angle, which allows for a stable gait.
Birds constitute the largest and most diverse group of living obligate bipeds, a trait inherited from their dinosaur ancestors. All birds move on the ground using only their two hind limbs, whether they walk, run, or hop. Large, flightless birds like the Ostrich and Emu have powerful legs built for running long distances, while aquatic species such as Penguins waddle on their short, thick legs to navigate terrain. Their forelimbs evolved into wings, which are used for flight or balance but not for terrestrial propulsion.
The most famous extinct examples of obligate bipedalism were the theropod dinosaurs, such as Tyrannosaurus rex. These massive predators possessed enormous hind limbs and a heavy tail used for counterbalance, allowing them to chase prey without relying on their smaller forelimbs. The combination of specialized leg structure and counterbalancing anatomy defined these animals as creatures committed to a two-legged gait.
Animals That Use Bipedal Movement Occasionally
Many species employ bipedal movement only when a situation demands it, fitting the definition of facultative bipeds. Marsupials like Kangaroos and Wallabies use a unique form of bipedal locomotion, relying on powerful, elongated hind legs for hopping. When moving slowly or grazing, however, they often use their muscular tail as a third point of contact, creating a “pentapedal” or five-limbed stance for stability.
In the reptile class, certain lizards have developed the ability to run bipedally for short, rapid bursts. The Basilisk lizard, sometimes called the “Jesus Christ lizard,” can sprint across water surfaces using a two-legged gait at high speeds. This behavior, observed in several agamid and monitor lizards, is typically an escape mechanism driven by the mechanics of acceleration and their center of mass shifting backward.
Non-human primates also demonstrate facultative bipedalism, though they are primarily knuckle-walkers or arboreal. Chimpanzees and Gorillas will stand or walk upright when carrying food, engaging in threat displays, or traversing difficult ground. Gibbons are notable for being highly bipedal when on the ground, using their long arms for balance as they walk short distances. Even smaller mammals, like Prairie Dogs and Bears, will rear up onto their hind legs for a better view or as an intimidation display.
Why Bipedalism Evolved
The transition to moving on two legs provided several distinct advantages that likely drove its evolution in various lineages. For early hominins, standing upright offered a more energy-efficient means of traveling long distances across open landscapes compared to quadrupedal walking. This efficiency was beneficial when resources were scattered and required foraging.
Bipedalism provided several key evolutionary benefits:
- Increased field of vision, allowing animals to see over tall grasses to spot potential predators or distant food sources.
- Improved thermoregulation by reducing the body surface area directly exposed to the midday sun and catching cooler breezes higher off the ground.
- Freed forelimbs, enabling them to be used for carrying infants, transporting gathered food, or manipulating tools.