Tvåbent

Tvåbent, literally meaning two-legged, is a term used in biology to describe organisms whose locomotion is primarily on two legs, i.e., bipeds. The term can describe habitual bipeds, which regularly move on two legs, and facultative bipeds, which can switch between two and four legs depending on circumstances. It is commonly used in contrast to quadruped locomotion, where four limbs bear most of the body weight.

Examples of bipeds include humans, many birds, and several extinct and extant mammals such as kangaroos. In birds, the hind limbs are the main support for standing and walking, with the forelimbs modified as wings. In humans and other primates, the pelvis, spine, knees, and feet are adapted to upright bipedal locomotion, including an S-shaped spine and arches in the foot that help absorb shock. Some animals are capable of bipedal locomotion temporarily, such as bears and lizards when standing to inspect surroundings. Kangaroos move by hopping, which is a distinctive form of bipedal locomotion.

Evolutionarily, bipedalism arose independently in several lineages. In hominins, habitual bipedalism emerged several million years ago and is linked to changes in pelvis shape, limb proportion, and foot mechanics, possibly driven by energy efficiency, the ability to use hands for tool-related tasks, and environmental factors. In birds, bipedalism reflects the ancestry of theropod dinosaurs and is associated with weight-bearing adaptations and tail balance. The study of bipeds encompasses biomechanics, evolution, and functional morphology, as well as behavioral ecology.