A biped is any animal that moves on two legs. The word comes from the Latin roots “bi” (two) and “ped” (foot), and it applies to a surprisingly wide range of creatures. Humans are the most familiar example, but birds, kangaroos, some rodents, and even certain lizards and insects qualify, each using two-legged locomotion in different ways and for different reasons.
Obligate Versus Facultative Bipeds
Not all bipeds use two legs all the time, and this distinction matters. An obligate biped is an animal whose primary and default way of getting around is on two legs. Humans and most birds fall into this category. A facultative biped, by contrast, normally moves on four (or more) limbs but switches to two under certain circumstances. Chimpanzees are a good example. In the wild, about 96% of observed bipedal bouts in chimps occur while foraging, typically when a chimpanzee stands upright on a branch to pluck fruit from surrounding limbs.1PubMed. Arboreal bipedalism in wild chimpanzees: implications for the evolution of hominid posture and locomotion That means chimp bipedalism is task-driven and temporary, not their go-to way of covering ground.
Many lizards also fall on the facultative end of the spectrum. Several species of Australian agamids and basilisk lizards shift to running on their hind legs at higher speeds, apparently because the forward shift in their center of mass during acceleration lifts the front end off the ground. The basilisk lizard takes this a step further and runs bipedally across the surface of water, a feat that has made it famous as the “Jesus Christ lizard.” Cockroaches, too, rear up onto their hind legs at their fastest running speeds.2PubMed Central. Bipedal animals, and their differences from humans So bipedalism is not one thing with one cause. It is a locomotor strategy that has evolved independently many times, in many lineages, for many reasons.
The Many Ways to Move on Two Legs
When people picture bipedal locomotion, they usually picture walking. But walking is only one of several distinct two-legged gaits. Walking, running, hopping, and skipping are all forms of bipedal movement, and different animals favor different ones.
Human walking works roughly like an inverted pendulum. During each step, the body vaults over the stance leg, converting kinetic energy into potential energy and back again, which makes the whole process surprisingly efficient.3PubMed. The six determinants of gait and the inverted pendulum analogy: A dynamic walking perspective This pendulum-like exchange works well through the middle portion of each step but breaks down during the transition between steps, when one leg takes over from the other. That transition is where most of the energy cost of walking actually comes from.4PubMed. The strengths and weaknesses of inverted pendulum models of human walking
Kangaroos and kangaroo rats use a very different strategy: bipedal hopping. Rather than swinging over one leg at a time, they spring off both hind legs together, storing energy in their tendons like a pogo stick. In large kangaroos, this elastic energy storage is substantial and makes hopping at high speeds remarkably efficient. In smaller bipedal hoppers like kangaroo rats, the picture is different. Their ankle tendons do store and return elastic energy, and this storage increases at faster speeds, suggesting a functionally similar mechanism across body sizes.5PubMed. Elastic energy storage across speeds during steady-state hopping of desert kangaroo rats (Dipodomys deserti) But the total energy savings are much smaller than in kangaroos.6Journal of Zoology. Elastic energy storage in the hopping of kangaroo rats (Dipodomys spectabilis) Kangaroo rats seem to have evolved their oversized hind legs less for efficient travel and more for explosive predator-escape jumps, with the elastic savings being a secondary benefit.7PubMed. Kangaroo rat locomotion: design for elastic energy storage or acceleration?
Then there is skipping, which combines elements of walking and hopping in an asymmetric pattern. Jerboas (small desert rodents) and crows both use skipping gaits.2PubMed Central. Bipedal animals, and their differences from humans Birds in general display the widest repertoire of bipedal gaits of any group: different species walk, run, hop, or skip depending on their size, habitat, and lifestyle.
Bipeds Beyond Mammals
Birds are the most species-rich group of obligate bipeds on Earth. Every one of the roughly 10,000 living bird species is a biped, even flightless ones like ostriches and penguins. Their bipedalism is inherited from theropod dinosaurs, which walked on two legs long before birds existed. Research using chickens fitted with artificial tails (to shift their center of mass backward, mimicking the heavy tails of their dinosaur ancestors) has shown that as the center of mass moved rearward, the birds adopted a more upright femur and shifted their limb mechanics from knee-driven to hip-driven movement. This supports the idea that the gradual reduction of the tail through theropod evolution drove the shift toward the crouched, knee-driven walking style that modern birds use today.8PubMed Central. Walking like dinosaurs: chickens with artificial tails provide clues about non-avian theropod locomotion
Basilisk lizards deserve special mention for pushing bipedal locomotion into a medium no other vertebrate can manage at their size. Juvenile basilisks are light enough to run across water by slapping their large, fringed feet downward with enough force to generate support. The feet produce the greatest upward and forward forces during the first half of each step, when the foot plunges vertically into the water. They also generate large side-to-side forces that appear to help stabilize the lizard’s body mid-stride.9PubMed Central. Running on water: Three-dimensional force generation by basilisk lizards Their leg movements while running on water look dramatically different from those of land-running lizards: the feet sweep much farther to the sides, the stride is heavily biased toward the back half of the step, and the ankle and knee do not flex during early stance the way they normally would on solid ground. The limb seems to act more like a force-producing piston than a spring.10PubMed. Three-dimensional hindlimb kinematics of water running in the plumed basilisk lizard (Basiliscus plumifrons) As basilisks grow larger and heavier, they gradually lose the ability to stay atop the water, which is why adult basilisks sink partway through and end up swimming rather than running.11PubMed. Size-dependence of water-running ability in basilisk lizards (Basiliscus basiliscus)
Western and Clark’s grebes perform a similar feat during their elaborate courtship rushes, sprinting across lake surfaces in coordinated pairs. These birds take between 13 and 20 steps per second during their rushes, slapping their lobed feet against the water at speeds averaging around 3.8 meters per second. Even at those speeds, each foot slap produces only about 30 to 55 percent of the impulse needed to support the bird’s weight, meaning the grebes rely on a combination of foot forces, wing flapping, and forward momentum to stay above the surface.12The Company of Biologists (Journal of Experimental Biology). Western and Clark’s grebes use novel strategies for running on water
How Bodies Are Built for Two Legs
Walking upright places specific demands on the skeleton, and animals that have committed to bipedalism share certain anatomical signatures, even when they are not closely related. One of the most studied is the position of the foramen magnum, the opening at the base of the skull where the spinal cord exits. In bipedal mammals, this opening tends to sit further forward on the skull’s underside, positioning the head more directly over the spine. A comparative analysis across mammals found that a more forward foramen magnum evolved in concert with bipedalism at least four separate times: in kangaroo-like marsupials, in two different families of hopping rodents, and in the primate lineage leading to humans.13PubMed. Another look at the foramen magnum in bipedal mammals
Paleoanthropologists have used this trait to identify bipedalism in fossil hominins for decades, and work on modern species supports the general principle that foramen magnum position is a useful indicator of upright posture.14PubMed. Foramen magnum position in bipedal mammals That said, the picture is messier than it first appears. Some researchers have found that the specific indices used to measure foramen magnum placement do not reliably distinguish between all locomotor categories in primates, suggesting the relationship between skull anatomy and bipedalism involves more variables than a single measurement can capture.15PubMed. Locomotion, posture, and the foramen magnum in primates: Reliability of indices and insights into hominin bipedalism
Below the skull, the human pelvis and spine tell a complementary story. The lumbar curve of the lower back, called lordosis, is a hallmark of habitual bipeds. It positions the upper body’s weight over the hips rather than in front of them. In humans, this curve develops as an infant learns to walk, and the degree of lordosis is tightly linked to the shape of the pelvis.16PubMed. How Did the Pelvis and Vertebral Column Become a Functional Unit during the Transition from Occasional to Permanent Bipedalism? Even Japanese macaques trained to walk bipedally for months or years develop a pronounced lumbar lordosis that persists even when the animals return to all fours, driven by physical reshaping of the vertebral bodies themselves.17PubMed. Curvature of the lumbar spine as a consequence of mechanical necessities in Japanese macaques trained for bipedalism The spine, in other words, remodels itself in response to the demands of upright posture.
Why Walk Upright at All
If bipedalism demands so many skeletal changes, what advantage does it confer? For humans, at least, the answer has a lot to do with energy. Human bipedal walking is at least as efficient as typical four-legged walking in other mammals and is far more efficient than either bipedal or quadrupedal locomotion in chimpanzees.18PubMed. Bioenergetics and the origin of hominid bipedalism A biomechanical model of early hominin walking estimated that even relatively small changes in leg length, posture, and muscle architecture would have lowered bipedal walking costs in an ape-like ancestor below those of knuckle-walking. By the time of Australopithecus afarensis (the species that includes the famous fossil “Lucy”), walking was likely cheaper than it was for any contemporaneous quadrupedal ape.19PubMed. The metabolic cost of walking in humans, chimpanzees, and early hominins
How dramatic is the gap? A recent study had humans walk on all fours at the same speed they normally walk bipedally and measured the energy cost. Quadrupedal walking consumed roughly 250% more energy than bipedal walking.20PubMed. Reevaluating the energy cost in locomotion: quadrupedal vs. bipedal walking in humans Of course, humans are built for bipedalism, so that comparison is somewhat unfair. A more meaningful test involves animals that naturally use both gaits. In Japanese macaques, bipedal walking costs about 20 to 30 percent more energy than quadrupedal walking.21PubMed. Energetic costs of bipedal and quadrupedal walking in Japanese macaques Macaques that adopt a more crouched, bent-knee posture pay an even steeper price, up to about 140% of the quadrupedal cost.22PubMed. Energy expenditure of bipedal walking is higher than that of quadrupedal walking in Japanese macaques The upshot is that bipedalism is only efficient when the body is properly adapted for it. For a short-legged, bent-hip primate, walking on two legs is expensive. For a tall, long-legged, straight-kneed human, it is a bargain.
Trained Japanese monkeys illustrate why anatomy matters so much. Even after extensive bipedal training, these monkeys never developed the double-peaked vertical force curve that characterizes human walking, in which body weight transfers smoothly from one leg to the other. Their anatomically restricted hip joints appear to prevent the full range of motion needed for humanlike gait, suggesting that the skeletal reshaping of the hip was a prerequisite for efficient bipedalism, not just a result of it.23PubMed. Ground-reaction-force profiles of bipedal walking in bipedally trained Japanese monkeys
The Obstetric Dilemma
Walking upright reshaped the human pelvis into a bowl-like structure that supports the organs above it and allows efficient leg swing. But this same reshaping narrowed the birth canal, creating a tight fit for a large-brained infant’s head. This tension between the demands of bipedal walking and the demands of childbirth is known as the obstetric dilemma.24Evolutionary Anthropology: Issues, News, and Reviews. Bipedalism and human birth: The obstetrical dilemma revisited Humans give birth to relatively underdeveloped infants compared to other primates, which may partly reflect a compromise: delivering the baby earlier, while the head is still small enough to pass through the pelvis.25PubMed. Mechanical Constraints on the Hominin Pelvis and the “Obstetrical Dilemma”
The classic framing of this problem assumes that women cannot simply evolve wider pelvises because doing so would wreck their walking efficiency. More recent work has complicated that picture. An analysis of pelvic shape, stature, and head size in a large human sample found that women who have larger heads (and therefore tend to give birth to larger-headed babies) also tend to have birth canals shaped to better accommodate those babies. In other words, evolution has produced a correlation between head size and pelvis shape that partially eases the mismatch, even though it does not eliminate it.26PubMed Central. Covariation between human pelvis shape, stature, and head size alleviates the obstetric dilemma The dilemma is real, but the body’s solution is subtler than simply making everyone’s pelvis the same size.
What Fossil Feet Can Tell Us
Much of what we know about when and how bipedalism evolved in our own lineage comes from fossil feet, ankles, and footprints. The 3.2-million-year-old skeleton of Australopithecus afarensis (“Lucy”) has been central to these debates. One long-running question is whether Lucy had arched feet like modern humans or flat feet more like an ape’s. The angle of the ankle bones has been used as evidence for a flat-footed gait, but a study of modern human ankle X-rays found that about 8% of living people have the same backward-tilted ankle angle seen in Lucy’s skeleton, including one individual whose angle was just as extreme as Lucy’s. People with this ankle configuration tended to have lower arches in the back of the foot, but the finding shows that Lucy’s ankle angle does not by itself prove flat-footedness. It falls within the range of variation seen in modern humans who walk perfectly well.27PLOS ONE. Lucy’s Flat Feet: The Relationship between the Ankle and Rearfoot Arching in Early Hominins
Bipedal Robots and the Engineering Challenge
If you want to appreciate how difficult bipedalism actually is, look at the decades of effort engineers have poured into making robots walk on two legs. The core problem is balance. A biped on two feet has a tiny support base compared to a four-legged machine, and walking requires constantly tipping forward and catching yourself, step after step. One common engineering approach, called Zero Moment Point control, keeps the robot’s center of mass carefully aligned above its feet at all times.28IFAC-PapersOnLine. Investigating Balancing Control of a Standing Bipedal Robot With Point Foot Contact This works but produces the stiff, shuffling gait familiar from early humanoid robots, because the machine never allows itself to fall even slightly off-balance.
More recent control strategies try to mimic how humans actually walk, embracing the controlled fall. A 2025 study demonstrated a control system based on ground reaction forces that produced stable three-dimensional walking across a wide range of speeds, with hip torque profiles closely matching human data.29PubMed. Unified three-dimensional bipedal locomotion control via ground reaction force-based joint compliance modulation Modeling work on the transition from multi-legged to two-legged systems has found that something unique happens when you go from four legs to two. Only in the quadruped-to-biped transition does reducing forces under one pair of legs actually increase the reliability of the body’s vertical oscillations, suggesting that bipedal locomotion occupies a distinct mechanical niche that does not simply scale down from more-legged systems.30PubMed Central. The Smooth Transition From Many-Legged to Bipedal Locomotion-Gradual Leg Force Reduction and its Impact on Total Ground Reaction Forces, Body Dynamics and Gait Transitions
Balance Without Sight
Staying upright on two legs is a sensory feat as much as a mechanical one. Humans rely on vision, the vestibular system in the inner ear, and touch and pressure signals from the feet and joints to maintain balance. Lose one of these inputs and the others have to compensate. Research on people who are congenitally blind has found that they develop enhanced reliance on somatosensory and vestibular inputs to maintain bipedal standing balance, and that auditory cues positioned behind them can further improve their postural control.31Gait & Posture. Auditory cues behind congenitally blind subjects improve their balance control in bipedal upright posture The brain, in other words, is flexible enough to reroute the information it needs to keep a biped standing, even when a major sensory channel has been absent from birth. Bipedalism is not just a skeletal arrangement or a set of muscle patterns. It is a whole-body, whole-brain project that integrates structure, energy management, and sensory processing into something that looks, from the outside, as simple as putting one foot in front of the other.