Brachiation

Brachiation is a form of locomotion in which primates move by swinging from one branch to another using the arms. This suspensory mode is most fully developed in the lesser apes, especially gibbons and siamangs (family Hylobatidae), though other arboreal primates may employ arm-swinging to a lesser extent.

In true brachiation, the organism propels itself with the forelimbs while the body remains relatively elongated and suspended beneath the shoulders. Movement is typically continuous and pendular, achieved by grasping successive branches in a hand-over-hand or alternating-arm sequence. Grasping is accomplished with a firm hook-like grip; the frequency and reach of swings enable rapid travel through the canopy.

Anatomical adaptations supporting brachiation include long forelimbs in relation to body size, a highly mobile shoulder girdle, strong rotator cuff muscles, and curved fingers for secure grasping. In many gibbons the thumbs are relatively reduced, facilitating efficient hook-like grabbing of branches. These features reduce drag and energy cost during rapid swinging.

Ecology and behavior: Brachiation enables fast travel across gaps in tall tropical forests and facilitates patrols, foraging, and territorial displays. Gibbons and siamangs use brachiation as their primary locomotor mode, often combined with loud vocalizations to maintain territory and spacing. In other primates, arm swinging is less central to locomotion and is typically supplemented or replaced by climbing and leaping.

Evolution and variation: Brachiation is a hallmark of hominoid evolution, with gibbons representing an extreme specialization. Other apes retain suspensory abilities but rely more on climbing or knuckle-walking, and no other living primates match gibbons in the speed and efficiency of arm-swinging.