Bipedalism

Bipedalism refers to movement on two legs, a locomotor strategy adopted by a relatively small number of modern and extinct species. The term derives from the Latin bis (two) and pes (foot), distinguishing it from quadrupedalism, which employs four limbs. A wide range of animals can rise onto their hind legs for feeding, defence or display, but only some species use a bipedal gait as a habitual or primary mode of locomotion. Bipedal movement takes various forms, including walking, running, hopping and skipping, each associated with different anatomical specialisations and ecological functions.

Evolutionary Background

The origins of bipedalism differ across evolutionary lineages. In archosaurs during the Triassic period, several groups evolved bipedality, and many early dinosaurs were fully bipedal. Their descendants, the birds, form an exclusively bipedal clade. Among mammals, habitual bipedalism has arisen multiple times through convergent evolution, appearing in kangaroos, macropods, springhares, pangolins, hopping mice and hominins. Early fossil evidence for bipedal reptiles includes Eudibamus, a bolosaurid from approximately 290 million years ago, whose hindlimb proportions and joint morphology suggest a bipedal stance.
In dinosaur evolution, the earliest species such as Eoraptor were likely small bipedal predators, and later lineages, including some iguanodontians and pseudosuchians, also evolved bipedal forms independently. Birds retain this ancestral condition, although some juvenile birds retain vestigial climbing adaptations such as wing claws, as seen in hoatzin chicks.

Mammalian Evolutionary Pathways

Though quadrupedalism dominates among mammals, several groups demonstrate the repeated emergence of bipedal locomotion. Macropods evolved hopping locomotion by no later than the mid-Cenozoic era. Bipedality in rodents such as kangaroo rats and jerboas appears to aid predator evasion in open habitats. In primates, upright walking evolved for different biomechanical and ecological reasons, with human bipedalism being the most extensively studied example. Fossil evidence from australopithecines indicates that habitual bipedalism preceded the expansion of brain size in human evolution.

Occasional and Facultative Bipedalism

A large number of species adopt a bipedal posture intermittently. Certain lizards, including the spiny-tailed iguanas, sprint bipedally at high speeds when escaping predators. Bears and many primates occasionally stand or walk on two legs to survey surroundings, interact socially or manipulate objects. Tree kangaroos combine arboreal quadrupedal manoeuvring with terrestrial hopping. Primates such as gibbons and members of the family Indriidae walk bipedally during brief ground travel, often using their arms for balance.
Although zoologists sometimes differentiate between facultative bipedalism (used optionally) and obligate bipedalism (used by necessity), these distinctions can be ambiguous. Humans normally walk upright but can adopt quadrupedal postures when required, whereas rare neurological conditions may produce habitual quadrupedalism in some individuals without altering their underlying anatomy. For this reason, descriptions of bipedalism often focus on locomotor patterns rather than strict behavioural categories.

Forms of Bipedal Movement

Bipedal motion comprises a range of distinct movement types:

• Standing: maintaining an upright posture on two legs, involving continual balancing adjustments.
• Walking: moving one foot at a time with constant ground contact.
• Running: a faster mode with aerial phases when neither foot touches the ground.
• Hopping or jumping: moving by simultaneous propulsion from both legs.
• Skipping: alternating stepping patterns combined with brief hops.

Species employ these movements depending on ecological pressures, anatomical constraints and behavioural needs.

Advantages of Bipedalism

Bipedalism can confer several functional advantages:

• Expanded field of vision: positioning the head high off the ground aids detection of predators, prey and distant resources.
• Access to resources: upright posture allows animals to reach higher vegetation or deeper water.
• Freeing of forelimbs: non-locomotory limbs can evolve for manipulation, defence, digging or flight. Birds’ wings, the digging claws of pangolins and the manipulative hands of primates illustrate such specialisations.
• Thermoregulation: reduced surface area exposure to solar radiation may provide cooling benefits in open habitats.
• Endurance: although peak bipedal speeds are generally lower than those of quadrupedal sprinters, long-distance efficiency in humans is high enough to enable endurance running, supporting hypotheses regarding persistence hunting.

In other cases, such as kangaroo rats, bipedal hopping improves manoeuvrability and predator evasion.

Bipedalism Across Taxonomic Groups

Birds and Other Archosaurs

All birds are bipedal walkers or runners, inheriting this condition from their theropod dinosaur ancestors. Archaeological and fossil evidence suggests that early dinosaur lineages were predominantly bipedal, with later groups independently refining or reverting this trait. Although pterosaurs were once believed to be bipedal, trackway evidence indicates they moved quadrupedally on land.

Reptiles

Many lizards adopt bipedal running at high speeds. Fossil taxa such as Eudibamus demonstrate that reptilian bipedalism has a deep evolutionary history, predating the rise of dinosaurs.

Mammals

Modern mammals exhibiting habitual bipedalism include:

• Humans, with a spinal curvature and pelvic structure adapted to upright gait.
• Macropods, using elastic tendons for efficient hopping.
• Ground pangolins, which walk upright while balancing with their tails.
• Various rodents such as jerboas and hopping mice.

Other mammals—such as tree kangaroos and some primates—use bipedalism situationally rather than as their primary mode of travel.

Primates

Primate bipedalism differs markedly from that of most other bipeds. With upright spinal alignment and typically reduced or absent tails, primates rely heavily on balance, muscular coordination and pelvic structure for two-legged walking. Chimpanzees, bonobos, gorillas and baboons all exhibit forms of bipedal movement but employ quadrupedalism as their usual mode. Gibbons and sifakas demonstrate unique terrestrial bipedal styles linked to their arboreal lifestyles. Humans, uniquely among primates, employ bipedal locomotion as their normal and most efficient movement pattern.

Ecological and Behavioural Contexts

Bipedalism serves numerous ecological and behavioural roles. Many species rear on hind legs for display or defence. Others use brief upright postures when accessing food or scanning for predators. The persistence of bipedal traits across diverse lineages highlights the versatility of this locomotor strategy, despite its relative rarity among vertebrates.
Across geological time, bipedalism has arisen repeatedly in response to environmental pressures, anatomical innovations and behavioural opportunities. Its evolutionary significance is seen not only in iconic clades such as dinosaurs and hominins but also in a wide array of modern animals that employ two-legged movement to navigate and survive within their respective habitats.