thumb|Brachiating primates such as this [[siamang (Symphalangus syndactylus) have long forelimbs and curved fingers]]
Brachiation (from "brachium", Latin for "arm"), or arm swinging, is a form of arboreal locomotion in which primates swing from tree limb to tree limb using only their arms. During brachiation, the body is alternately supported under each forelimb. This form of locomotion is the primary means of locomotion for the small gibbons and siamangs of southeast Asia. Gibbons in particular use brachiation for as much as 80% of their locomotor activities. Some New World monkeys, such as spider monkeys and muriquis, were initially classified as semibrachiators and move through the trees with a combination of leaping and brachiation. Some New World species also practice suspensory behaviors by using their prehensile tail, which acts as a fifth grasping hand. Evidence has shown that the extinct ape Proconsul from the Miocene of East Africa developed an early form of suspensory behaviour, and was therefore referred to as a probrachiator.
Upon further observations and more in depth understandings of the anatomy and behaviour of primates, the terms semibrachiator and probrachiator have largely fallen out of favour within the scientific community. Some children's parks include monkey bars which children play on by brachiating.
As well as shaping the evolution of gibbon body structure, brachiation has influenced the style and order of their behaviour. For example, unlike other primates who carry infants on their backs, gibbons will carry young ventrally. It also affects their play activities, copulation, and fighting. It is thought that gibbons gain evolutionary advantages through brachiation and being suspended by both hands (bimanual suspension) when feeding. While smaller primates cannot hold themselves by both hands for long periods, and larger primates are too heavy to exploit food resources on the ends of branches, gibbons can remain suspended for a significant period and use their long arms to reach food in terminal branches more easily. Another theory postulates that brachiation is a quieter and less obvious mode of locomotion than quadrupedal jumping and climbing, thereby more successfully avoiding predators.
Types of brachiation
thumb|Although humans primarily move through [[bipedal locomotion, they retain several adaptations that allow them to use brachiation, as demonstrated here with a group of U.S. Marines using monkey bars.]]
Continuous contact
This form of brachiation occurs when the primate is moving at slower speeds and is characterized by the animal maintaining constant contact with a handhold, such as a tree branch. This gait type utilizes the passive exchange between two types of energy, gravitational potential and translational kinetic, to propel the animal forward at a low mechanical cost.
Ricochetal
This type of brachiation is used by primates to move at faster speeds and is characterized by a flight phase between each contact with a handhold. Ricochetal brachiation uses an exchange of translational and rotational kinetic energy to move forward, and is compared to a "whip-like" motion. The use of gravitational acceleration to effect movement can be found in both the brachiating primate and the moving ball in a pendulum model. This would explain why living apes and humans share many unusual morphological aspects of the upper limb and thorax. Specialized suspensory behaviour was shown to have evolved independently between hominid groups. Many climbing adaptations have been found in early hominins and some of these adaptations can still be seen in present-day humans. The distinctive body posture, limb proportions and trunk design identified in living apes are better explained by the previous adaptation of climbing behaviours.