Tripedalism

Tripedalism (from the Latin tri = three + ped = foot) is locomotion by the use of three limbs. It has been said that parrots (Psittaciformes) display tripedalism during climbing gaits,[2] 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[3] 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.[4]

Male cockatiel climbing from a log to a ladder using its 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]

Real-world tripedalism is rare, in contrast to the common bipedalism of two-legged animals and quadrupedalism of four-legged animals. The code for bilateral symmetry seems to have become entrenched very early in evolution, appearing even before appendages like legs, fins or flippers had evolved; with that template came a built-in bias toward even-numbered limb configurations.[5]