A form of arboreal locomotion among primates in which the animal progresses using the forelimbs only. The animal swings below branches using alternate left and right handholds while the body undergoes 180° rotation to the opposite side. This type of locomotion is observed to varying degrees among hominoid primates but is especially characteristic of the gibbons and siamangs (the hylobatids). Adaptations for brachiation include forelimbs that are longer than the hindlimbs; a broad and short thorax; a shoulder blade that is positioned dorsally to allow the forelimb to be raised above the head; highly mobile shoulder, elbow, and wrist joints; and long, hooklike hands. The hylobatids are extremely proficient brachiators, and they are able to cross gaps in the tree canopy by propelling the body through a free-flight phase between successive handholds. This type of agile, acrobatic brachiation is referred to as ricochetal brachiation.
There has been considerable confusion over the use of the term brachiation in the literature. It was originally applied to the specialized form of arm swinging seen in the hylobatids, but it has since been increasingly used to describe the more deliberate forms of arm swinging and forelimb suspensory behaviors of the great apes. In the 1960s, brachiation was further extended to include similar, but less specialized, behaviors in New World monkeys and colobines and was termed “semibrachiation.” Similarly, evidence of incipient forelimb suspensory behaviors in the fossil ape Proconsul from the Miocene of East Africa led to it being described as a “pro-brachiator.” However, these latter terms have largely fallen into disuse. With further observations on the anatomy and behavior of living apes and monkeys, most researchers prefer to restrict the term brachiation to the locomotor behaviors of hylobatids (true brachiation) and the great apes (modified brachiation), while the less specialized behaviors observed in other primates are referred to as forelimb suspensory postures and locomotion.
Classification of the locomotion of primates using such terms as brachiation is rather restrictive and clearly does not encompass the full range of behaviors that characterizes the locomotion of individual primate species. For this reason, researchers prefer to document all of the different types of postural and locomotor behaviors observed for each species and then identify the most frequently used according to the time dedicated to them. In this way, it can be shown that gibbons use brachiation for more than 80% of their locomotor bouts, while the African apes (i.e., gorillas and chimpanzees) use it for less than 5%.
- Fleagle, J. G. (1999). Primate adaptation and evolution. San Diego, CA: Academic Press.
- Martin, R. D. (1990). Primate origins and evolution: A phylogenetic reconstruction. Princeton, NJ: Princeton University Press.
- Napier, J. R., & Napier, P. H. (1985). The natural history of the primates. Cambridge, MA: MIT Press.