Brachiation

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Brachiation (from "brachium", Latin for "arm") is a form of arboreal locomotion in which primates swing from tree limb to tree limb using only their arms.

Brachiators
The only true brachiators are the lesser apes (gibbons and siamangs). A gibbon can brachiate at speeds as high as 35 mph (55 km/h) and can travel as far as 20 feet (6 m) with each swing. Spider monkeys and orangutans are considered semibrachiators.

Traits that aid brachiation
Some traits that allow primates to brachiate include short fingernails (instead of claws), inward-closing hook-like fingers, opposable thumbs, long forelimbs, and freely rotating wrists.

Brachiation and great apes
Modern humans retain many physical characteristics that suggest a brachiator ancestor, including flexible shoulder joints and fingers well-suited for grasping. In lesser apes, these characteristics were adaptations for brachiation. Although great apes do not normally brachiate (with the exception of orangutans), our human anatomy suggests that brachiation may be a preadaptation to bipedalism, and healthy modern humans are still capable of brachiating. Some children's parks include monkey bars which children play on by brachiating.