Humans are the most well-known bipedal animals, but they’re far from the only ones. Birds, kangaroos, some dinosaurs, certain lizards, and even a couple of octopus species move on two legs. The range is wider than most people expect, and the reasons these animals walk, hop, or run on two limbs vary dramatically across the animal kingdom.
Obligate vs. Facultative Bipeds
Animals that move on two legs fall into two broad categories. Obligate bipeds walk on two legs as their primary, full-time mode of getting around. They’re built for it, and their skeletons reflect that commitment. Humans and birds are the clearest examples. Facultative bipeds normally walk on four legs (or more) but switch to two under specific circumstances, like chasing prey, carrying food, or making themselves look bigger. Bears, chimpanzees, and some lizards fall into this group.
The structural differences between these two types are significant. Human skeletons show a bowl-shaped pelvis for support, an S-curved spine for shock absorption, thick lower-back vertebrae, large heel bones, and a rigid foot with a big toe aligned forward for push-off. Nonobligate bipeds tend to have longer upper limbs, a simpler C-curved spine, a narrower pelvis, more flexible joints, and feet built for grasping rather than striding.
Birds: The Largest Group of Bipeds
Every one of the roughly 10,000 living bird species is bipedal. That makes birds, as a group, far more numerous than any other category of two-legged animal. Their bipedalism is inherited from theropod dinosaurs, which were already running on two legs long before flight evolved. Because bird body shape was later sculpted by the demands of flight, even flightless species like ostriches, emus, and penguins retain the same basic two-legged design.
Bird hind limbs are remarkably versatile. The same leg structure that powers a flamingo’s wading, a penguin’s swimming, and an ostrich’s 70 km/h sprints also handles the shock of landing and the thrust of takeoff. The skeletal differences between bird species are more like fine-tuned adjustments than fundamental redesigns, which helps explain why birds have successfully adapted to nearly every habitat on Earth.
Theropod Dinosaurs
The theropod dinosaurs, including Tyrannosaurus rex, Velociraptor, and thousands of other species, were obligate bipeds. They balanced their heavy heads and torsos with long, muscular tails, keeping their center of mass over their hips. As theropods evolved toward the bird lineage, their tails shortened, their center of mass shifted forward, and their legs became progressively more crouched. Research tracing this transition from large theropods like Daspletosaurus through smaller species like Troodon and into modern chickens shows these postural changes happened gradually, not all at once. Today’s birds are, in a very real sense, the last surviving bipedal dinosaurs.
Kangaroos and Hopping Bipeds
Kangaroos are bipedal, but in a way that looks nothing like human walking. They hop, using a gait called saltatorial locomotion. What makes this efficient is elastic energy storage: as a kangaroo lands, its leg tendons stretch like rubber bands, absorbing energy and releasing it on the next hop. The tendons in large living kangaroos stretch remarkably close to their breaking point, which maximizes the energy returned with each stride. This is partly why giant extinct kangaroos, which had thicker and stiffer tendons, may not have been able to hop the same way. Their tendons couldn’t store and return enough elastic energy to make hopping worthwhile, and some researchers believe these larger species walked with a more conventional stride instead.
Lizards That Run on Two Legs
Several lizard species shift to bipedal running at high speeds, but the basilisk lizard takes it to an extreme by sprinting across the surface of water. Juvenile basilisks run at speeds between 1.3 and 1.6 meters per second, using a two-phase step. First, the foot slaps straight down into the water, generating a vertical force that exceeds the lizard’s body weight and creating an air pocket around the foot. Then the foot sweeps backward through that air pocket, producing forward thrust. The downward slap is critical: it generates nearly three times as much vertical force as the backward stroke.
This isn’t like any form of running on land. On solid ground, legs act like springs, flexing and rebounding. Basilisk legs act more like pistons, punching downward to generate force in a single direction. The lizards also produce surprisingly large sideways forces, up to 79% of their body weight, which they have to counterbalance to stay upright. As basilisks grow heavier, this trick becomes harder to pull off, which is why it’s mostly juveniles you see running across ponds.
Primates That Occasionally Walk Upright
Chimpanzees, our closest living relatives, do walk on two legs sometimes, but far less often than you might assume. In one study tracking wild chimpanzees for nearly 247 hours, researchers recorded only 179 instances of bipedal posture lasting five seconds or longer, a rate of about 0.73 bouts per hour. Ninety-six percent of those bouts happened while foraging, specifically when a chimpanzee stood up on a branch to pluck fruit overhead. Almost all of this bipedalism was postural (standing in place) rather than locomotor (actually walking). And it happened exclusively in trees, not on the ground.
This matters for understanding human evolution. The shift from occasional, tree-based bipedal standing to full-time, ground-based bipedal walking was one of the defining transitions in our lineage. Walking upright freed the hands for carrying food, tools, and infants. It made early humans more energy-efficient when crossing open landscapes. It improved thermoregulation in hot environments by reducing the body surface exposed to direct sun. And it may have made individuals appear larger and more intimidating to predators.
Bears, Apes, and Other Part-Time Bipeds
Bears are the animal most people picture when they think of a non-human standing on two legs. They rear up to get a better view of their surroundings, to assess threats, and during fights with other bears. Research into the evolution of bipedalism has highlighted combat as a plausible advantage of upright posture: standing tall lets an animal strike downward, see farther, and present a larger profile. But bears’ spines, hips, and feet aren’t built for sustained walking on two legs. Their bipedalism is a temporary posture, not a mode of travel.
Gorillas, orangutans, and other great apes also stand and occasionally walk bipedally, particularly when wading through water or carrying large objects. Like chimpanzees, though, their anatomy is optimized for quadrupedal knuckle-walking or climbing, not upright locomotion.
Octopuses: An Unexpected Biped
Two species of octopus have been documented walking on just two of their eight arms. The coconut octopus (Octopus marginatus) and Abdopus aculeatus use a rolling gait, alternating between two arms while wrapping the other six around their bodies for camouflage. The coconut octopus disguises itself as a coconut rolling along the seafloor; Abdopus aculeatus mimics a clump of drifting algae. Each “foot” stays in contact with the sand for more than half of each stride, which technically qualifies the movement as walking rather than running. This is one of the only known examples of bipedal locomotion in an invertebrate, and it appears to serve a defensive purpose: by bundling most of their arms into the disguise, these octopuses can move away from danger without breaking their camouflage.