Primate Locomotion

Locomotion

Almost all primates are, at least to some degree, quadrupedal. However, most primates use more than one form of locomotion, and they’re able to do this because of their generalized anatomy.

• The limbs of terrestrial quadrupeds are approximately the same length . In arboreal quadrupeds, forelimbs are somewhat shorter.
• Vertical clinging and leaping, another form of locomotion, is characteristic of some lemurs and tarsiers. As the term implies, vertical clingers and leapers support themselves vertically by grasping onto trunks of trees or other large plants while their knees and ankles are tightly flexed. By forcefully extending their long hind limbs, they can spring powerfully away either forward or backward.
• Brachiation, or arm swinging, is a suspensory form of locomotion, and the body moves by being alternatively suspended by one arm or the other. Only the small gibbons and siamangs of Southeast Asia use this form of locomotion almost exclusively. Brachiation is seen in species characterized by arms longer than legs, a short, stable lower back, long curved fingers, and shortened thumbs. .
• Some New World monkeys, such as spider monkeys, are called semibrachiators, since they practice a combination of leaping with some arm swinging. Also, some New World monkeys enhance arm swinging by using a prehensile tail, which in effect serves as an extra hand. It’s important to mention that no Old World monkeys have prehensile tails.
• Lastly, all the apes, to varying degrees, have arms that are longer than legs, and some (gorillas, bonobos, and chimpanzees) practice a special form of quadrupedalism called knuckle walking. Because their arms are so long relative to their legs, they support the weight of their upper body on the back surfaces of their bent fingers.

Napier, Walker, Washburn and Hywes have made significant studies pertaining to the evolution of primate locomotion. According to them, primates show 

4 types of locomotion:

• 1) Vertical clinging and leaping.
• 2) Quadrupedalism.
• 3) Brachiation.
• 4) Bi-pedalism.

An examination of these 4 types of locomotion will reveal the story of evolution of primate locomotion.

1) Vertical Clinging and leaping – This type of locomotion occurs among the pro-Simiians. The pro-Simiians emerged during Paleocene in the thick forest canopy around 60 million years ago. Thus, this mode of locomotion is the basic type from which all other patterns of locomotion evolved.

With regard to vertical dinging the forelimbs provide the basic support and the hindlimbs propel the body and the animal leaps on the branches. What happens is that the animal keeps its trunk erect and grasps the branch on the tree with both its limbs and finally takes a leap. The hindlimb is long and fore limb is short. Therefore, the animal can leap in trees with propulsion from hind limbs. Tarsius and Lemurs of present day are typical examples of this pattern of locomotion.

2) Quadrupedalism – According to scientists, the second stage of evolution of locomotion is Quadrupedalism. This form of locomotion is found among the tree shrew, some of the prosimiians, all monkeys, the chimpanzee and the Gorilla.

• Quadrupedalism is the four-legged walking or running and occurs both on the ground and in the trees.
• Branch running and walking: This form is simply running or walking in trees. The thumb and the big toe diverge from the rest of the toes, so both hands and legs are pre-hensiled. Tree shrews, some monkeys, and some pro-Simiians practice .
• A third type of Quadrapedal locomotion is called run and walk on the ground . It is found among the group-dwelling monkeys, such as the baboon. This form usually does not involve a pre-hensile group. They moving on the soles of the feet and the underside of the fingers of their hands. 
• A fourth Quadrupedal locomotion is called New-World Semi-brachiatron. It is practiced by the howler monkey and the various spider monkeys. In this type of arboreal locomotion the arms are used extended above the head to suspend the body or to propel it through space.
• Old World Semi-brachiation represents a fifth type of quadrupedal locomotion. It is practiced by the colobinae family of old world monkeys such as (tie Colobus and langur). It involves leaping, wherein the arms extend out ahead of the body to reach a handhold or to check momentum.
• The locomotion of African apes, the gorilla and the chimpanzee represents a sixth form of Quadrupedalism, called knuckle Walking, when apes perform Quadrupedalism, their thickly padded fingers are bent in order to make them touch the ground. To effect quadrupedalism in apes, the bones of pelvis and the hind limbs and muscles associated with these regions show the following characteristics: 

• a) The Pelvis is elongated. This is because its constituent bones namely ilium and ischium are very long. 
• b) The head of the femur does not form great obtuse angle with the shaft. Hence, movement of the foot is very much limited.
• c) The knee is habitually bent.
• d) The whole foot makes contact with the surface.

3) Brachiation – It is the chief pattern of locomotion of Asian apes though all apes are not equally efficient in brachiation. Hands are the main organs in brachiation. The forefingers, other than the thumb, form a sort of hook and the animal swings around it.

Foot, however, also has some role to play. Foot is prehensile in the ape. The big toe deviates at an angle of 45° from rest of the toes. This provides excellent grip to the foot. Further, arms are well suited for brachiation. In this regard, the clavicle and the nature of attachment of head of humerons facilitate free rotation of the arm in the overhead position. Arms are thus capable of all round movement. In monkeys it is not so because in them the arms can perform only forward and backward movement but not lateral (side) movements.

Gibbons and chimpanzees are particularly efficient in brachiation because of their small body sizes. They may be found upto 100 feet high on the trees. On the other hand, gorillas take to the trees for food and sleeping and that to in the lower strata of forest canopy, seldom exceeding 10 feet of height. This is because of their large bodies.

4) Bi-pedalism – According to Buettner Janusch and Napier, bi-pedaiism or bi-pedal standing or striding is characteristic of humans, For true bi-pedalism, an erect posture is a must; and only humans have complete erect posture. According to Napier, human bi-pedalism developed from vertical dinging and leaping and leaping through quadrupedalism. Apes brachiation and knuckle walking is a highly specialized locomotion from which human bi-pedalism cannot be desired.

Washburn, points out that bi-pedal gait demands lesser energy in covering greater distances in comparison to other gaits. Therefore, human ancestors at the dose of Miocene had no option but to venture out for hunting to longer distances. 

Gordon Hewes, on the contrary proposes that bi-pedal gait was acquired form of locomotion and used the same in feeding from bushes and grasses, carrying food for provisioning the family or for future use, use of weapons and tools.

According to Napier, human bi-pedalism has evolved due to the following anatomical changes, 

• a) Shifting of Foramen Magnum of skull forward.
• b) Formation of 4 spinal curves namely cervical, thoracic, lumbar and sacral.Here the thoracic and sacral curves are congenital but cervical and lumbar curves develop during childhood.
• c) Shortening and broadening of the pelvis. The short, broad pelvis has formed into a basin-like structure to support the viscera from below against gravitational pull. Further, the ischium of the pelvis have flattened allowing humans to sit comfortably.
• d) Elongation of hind limbs with respect to the fore limbs. The bones of the leg, femur, tibia, fibula are elongated especially femur shows some positive changes for bi-pedal gait. The head of femur has undergone shifting and forms 120° angle with the femoral shaft. The head of the femur after its attachment with the pelvis is capable of all round development. Linea aspera developed in humans so as to keep hind limbs straight during bi-pedal walking.
• e) Re-shaping of the foot. The axis of the human foot lies between the big toe and the second toe. For this reason, the big toe has become more opposable In addition foot has to bear maximum load transmitted to the foot developed two arches namely dorso planter and medis lateral arches. The two arches support the weight of the body, when standing the load is distributed equally on each side of the axis of the two arches; while in motion the ioad is dynamically distributed from the heel through the fifth metatarsal to the big toe that provides propulsive threat.