Knuckle-walking - Apes

Apes

Chimpanzees and gorillas engage in knuckle-walking. This form of hand-walking posture allows these tree climbers to use their hands for terrestrial locomotion while retaining long fingers for climbing. It may also allow small objects to be carried in the fingers while walking on all fours. Although this is the most common type of movement for gorillas, they also will stand upright on two legs, called bipedalism. They do this by counterbalancing their weight by swinging their arms parallel to the opposite leg like a human.

Their knuckle walking involves flexing the tips of their fingers and carrying their body weight down on the dorsal surface of their middle phalanges (middle segments of their fingers). The outer fingers are held clear on the ground. The wrist is held in a stable, locked position during the support phase of knuckle-walking by means of strongly flexed interphalangeal joints, and extended metacarpophalangeal joints. The palm as a result is positioned perpendicular to the ground and in-line with the forearm. The wrist and elbow are extended throughout the last period in which the knuckle-walker's hand carried body weight.

There are differences between chimpanzees and gorillas: juvenile chimpanzees engage in less knuckle-walking than juvenile gorillas. Another difference is that the hand bones of gorillas lack key features that were once thought to limit the extension of the wrist during knuckle-walking in chimpanzees. For example, the ridges and concavities features of the capitate and hamate bones have been interpreted to enhance stability of weight-bearing; on this basis, they have been used to identify knuckle-walking in fossils. These are found in all chimpanzees but in only two out of five gorillas. They are also less prominent when found in gorillas. They are however found in primates that do not knuckle-walk.

It has been suggested that chimpanzee knuckle walking and gorilla knuckle walking are biomechanically and posturally distinct. Gorillas use a form of knuckle-walking which is "columnar". In this forelimb posture, the hand and wrist joints are aligned in a relatively straight, neutral posture. In contrast, chimpanzees use an extended wrist posture. These differences underlie the different characteristics of their hand bones.

The difference has been attributed to the greater locomotion of chimpanzees in trees, compared to gorillas. The former frequently engage in both knuckle-walking and palm-walking branches. As a result, to preserve their balance in trees chimpanzees, like other primates in trees, often extended their wrists. This need has produced different wrist bone anatomy and, through this, a different form of knuckle-walking.

Knuckle-walking has been reported in some baboons. It has also been suggested that fossils attributed to Australopithecus anamensis and A. afarensis had specialized wrist morphology that was retained from an earlier knuckle-walking ancestor.