Primate locomotor habits can be divided into several major categories, each characterized by different patterns of limb use and body positions. These categories are quadrupedal running and climbing, vertical clinging and leaping, arm suspension, and bipedal walking. A primate chiefly uses one of the four types, but may use other types at least some of the time. Primate locomotor characteristics evolved in small quadrupedal mammals with a body weight of less than 100 grams. Contrary to other tree-living small mammals, such as squirrels or tree shrews, primate ancestors inhabited the small branches of the forest canopy, where the diameters of supporting branches are often smaller than the diameter of the animal’s trunk. The evolution of prehensile autopodia, combined with the use of a diagonal footfall pattern, provides stability and balance on such unstable supports.
Primates walk along branches with flexed limbs to keep the trunk close to the support. Thus, primates are able to avoid strong vertical oscillations of the trunk by accommodating to uneven substrates. Independent of their anatomical length differences, the fore- and hind limbs undergo equal angular excursions, and their proximal pivots are on the same level. The longer hind limbs are much more flexed than the shorter forelimbs; this allows the animal to accelerate immediately powered by the hind limbs without losing time in flexing the joints.
Arboreal quadrupedalism is the ancestral and most common locomotor mode among primates, and it was the basis for the evolution of specialized locomotor behaviors (for example, slow climbing, terrestrial quadrupedalism, and arm suspension).
Slow quadrupedal walking and climbing is a distinctive mode of quadrupedalism in the Loridae family (lorises and pottos). Members of this group are known for their rather stereotypical, stealthy loco-motor behavior, practiced in any direction in space, on the branch or underneath. Loridae do not leap or perform any other fast gait, but they use large step lengths produced by great limb angular excursions. They have a very powerful grasp, because their first digits are more abducted against the other digits. The high oxidative capacity of their limb muscles enables these primates to hold tightly to a branch for hours without fatigue.
Unlike arboreal quadrupeds, the locomotive apparatus of Old World cercopithecine monkeys shows functional and morphological adaptations to terrestrial running on broad, flat surfaces. These adaptations are seen even in secondary arboreal cercopithecine monkeys. Old World monkeys have a narrow, deep trunk, and parasagittally oriented, more-extended limbs. The fore- and hind-limb length of terrestrial quadruped monkeys is nearly identical.
Primates of all sizes can cross gaps in the three-dimensional meshwork of their arboreal habitat by leaping. The necessary speed for take-off is attained by a rapid extension of the hind limbs from a deep crouched posture. Larger primates gain additional momentum by swinging their long arms. At the end of the leap, primates use their strong, long hind limbs as shock absorbers during the landing.
Vertical clinging and leaping is a highly specialized type of leaping between vertical supports. Vertical clingers and leapers are members of the prosimian families Galagidae, Tarsiidae, Indriidae, and Lepile-muridae. Because they are performed backwards, leaps between vertical supports are more complex than those between horizontal supports. From a clinging-squatted position on a vertical trunk, the thrust of the feet launches the body up in the air, with a rotatory momentum that twists the trunk around its longitudinal axis. The animal rotates to face the target in a symmetrical position. Then, a rotation around the transverse axis shifts the feet to a position suitable for landing. Tarsiers and galagines use their tails to correct their rotatory momentum during the flight phase.
Many primates can hang below arboreal supports by various combinations of arm and leg support. A more specialized mode of suspensory locomotion is brachiation. Brachiation represents slow to moderate pendular arm swinging, where the trunk undergoes rotation under the supporting forelimb. It differs from ricochetal brachiation by the speed and by the absence of an aerial phase. Brachiation is used in its most stereotypical and finest style by hylobatids and atelines. Atelines frequently use their prehensile tail to maintain an additional hold when they brachiate. Brachiating primates have long forelimbs. Their trunks are relatively short and have a broad thorax with a short lumbar region. The shoulder joint has the greatest range of movement of the whole locomotor apparatus. The thorax of apes and atelines is broad, and the scapula has been repositioned dorsally changing the orientation of the glenoid fossa to a more lateral and cranial position. Redirection of the glenoid sockets increases the span of the arms and their range of circumduction. Brachiation and arm suspension in the stem lineage of the Hominoidea explain many of the morphological peculiarities of the upper limb and thorax shared by living apes and humans.
Great apes are generally capable of vertical and horizontal climbing and suspensory arm swinging as well as quadrupedal fist or knuckle walking, and bipedalism. Knuckle walking is a type of terrestrial quadrupedalism, which is unique to the African great apes pan and gorilla. The knuckles are used to carry the weight of the upper body.
Human beings are the only primate species that habitually walk bipedally. Human beings walk with a fairly extended knee and hip joint. They use their legs in a pendular-like fashion in contrast to the spring-like manner of quadruped mammals. Additionally, human bipedality differs from the facultative bipedality of apes in the systematic use of a torsional twisting of the trunk, and the contralateral symmetry of leg- and arm-swinging.
Vertical climbing is considered to be a bio-mechanical link between brachiation and bipedalism. To date, this hypothesis is the most popular and generally accepted explanation of humans’ evolution to bipedalism. However, shared features of the wrist in African apes and humans, features which are functionally related to stabilizing the wrist during knuckle-walking, support the idea of a terrestrial quadrupedal phase in hominid evolution.
References:
- Fleagle, J. G. (1999). Primate adaptation and evolution. San Diego, CA: Academic Press.
- Gebo, D. L. (1996). Climbing, brachiation, and terrestrial quadrupedalism: Historical precursors of hominid bipedalism. American Journal of Physical Anthropology, 101, 55-92.
- Jenkins, F. A., Jr. (Ed.) (1974). Primate locomotion. New York: Academic Press.
