Tripedalism

Tripedalism (from the Latin tri = three + ped = foot) is locomotion by the use of three limbs. Real-world tripedalism is rare, in contrast to the common bipedalism of two-legged animals and quadrupedalism of four-legged animals. Bilateral symmetry seems to have become entrenched very early in evolution, appearing even before appendages like legs, fins or flippers had evolved.^[2]

Male cockatiel climbing from a log to a ladder using his beak. In 2022, it was proven that parrots use their necks and heads as a third limb with propulsive and tangential forces equal to or greater than those forces generated by forelimbs in non-human primates when climbing vertical surfaces.^[1]

In nature

It has been said that parrots (birds of the order Psittaciformes) display tripedalism during climbing gaits,^[3] which was tested and proven in a 2022 paper on the subject, making parrots the only creatures to truly use tripedal forms of locomotion.^[1] Tripedal gaits were also observed by K. Hunt^[4] in primates. This is usually observed when the animal is using one limb to grasp a carried object and is thus a non-standard gait. Apart from climbing in parrots, there are no known animal behaviours where the same three extremities are routinely used to contact environmental supports, although the movement of some macropods such as kangaroos, which can alternate between resting their weight on their muscular tails and their two hind legs and hop on all three, may be an example of tripedal locomotion in animals. There are also the tripod fish. Several species of these fish rest on the ocean bottom on two rays from their two pelvic fins and one ray from their caudal fin.^[5]

Quadrupedal amputees and mutations

Four-legged animals such as dogs have been documented as capable of adapting to live with three legs, being made artificially tripedal through amputation surgery

There are some three-legged creatures in the world today, namely four-legged animals (such as pet dogs and cats) which have had one limb amputated. Animals made into tripeds through amputation may suffer from long term health consequences; these issues include osteoarthritis and spinal deformity and are largely a result of disruption to natural locomotion cycles and symmetry.^[6]

Health consequences vary depending on which limb was lost, with hind legs being less consequential of a loss than front legs in quadrupeds like dogs

There are also cases of mutations or birth abnormalities in animals (including humans) which have resulted in three legs. With humans, a third limb may arise from the surgical bisection of conjoined or parasitic twins. Such conditions will often result in a limb with reduced or limited mobility. In rarer cases such as with Frank Lentini, a third limb may be usable in activities requiring moderate motor function, such as kicking a football, but will still lack the fine motor skills of the natural limbs.^[7]

See also

• Bipedalism
• Quadrupedalism
• Terrestrial locomotion
• Tetrapod
• Uniped