Structure

The forelimbs and hindlimbs each consist of a series of bones, meeting the trunk of the body at the pectoral(forelimb) or pelvic (hindlimb) girdle. The pectoral girdle of most mammals consists of a shoulder blade (scapula) and in many, a clavicle. Mammalian pectoral girdles are very much simplified compared to the pectoral regions of their ancestors, which contained a number of additional bones. These bones are either lost or incorporated into the scapula of modern mammals. Monotremes are an exception; their pectoral girdles include several of these primitive elements.

The scapula lies alongside the rib cage and spine. It is connected to those structures by muscles and ligaments rather than being immovably fused to them. The clavicle, if it is present, runs from the region of the articulation between scapula and forelimb to the anterior part of the sternum.

The pelvic girdle of mammals is made up of three bones, the ilium, ischium, and pubis. At the junction of these three bones is the socket (acetabulum) for the hind limb. Unlike the pectoral girdle, the pelvic girdle is firmly attached to the spine, by a bony fusion between the ilium and sacral vertebrae.

The forelimb itself consists of a humerus (which meets the scapula), paired radius and ulna, a set of carpalsand metacarpals, and (primitively) five digits, each made up of several phalanges. The bones of the hind limb are the femur (articulates with the acetabulum of the pelvis), the tibia and fibula, the tarsals andmetatarsals, and (primitively) five digits, each made up of 2 or more several phalanges. The first digit of the forelimb (the thumb of humans) is called the pollex; the first digit of the hindlimb is the hallux. A patella lies over the knee joint (junction of tibia and femur). Tarsal and carpal bones are referred to collectively as podials, and metacarpals and metatarsals are metapodials. Thecalcaneum is a large tarsal bone that extends behind the ankle, forming the heel. It provides an important lever arm for muscles that move the hind foot. The astragalus is another important bone of the tarsus; it lies between calcaneum and tibia. These two bones are considerably modified in some groups of mammals.

At the tip of the digits are usually found nails, claws, or hooves. These structures are derived from the skin and are composed of the protein keratin, not bone. Claws are composed of a dorsal plate, called the unguis, and a ventral subunguis. The unguis curves around the tapered terminal phalanx, surrounding the subunguis except at the very tip of the digit, where the edges of the unguis do not meet. A nail is similarly constructed except that the unguis is more or less flat, not folding over the subunguis. The subunguis is very much reduced to a thin band just under the outer edge of the nail. Hooves are also derived from claws; in a hoof, the unguis completely surrounds the subunguis, which remains exposed only at the tip of the digit.

Limbs are drastically modified to different ends in various groups of mammals. Here, we are concerned primarily with modifications that affect how an animal runs.

Running Fast

Several terms describe how and where an animal moves.Aquatic animals swim; volant animals fly. Cursorialanimals (cursors) run rapidly and for long distances.Scansorial animals are climbers; in the extreme, they arearboreal, spending most of their lives in the trees. Hoppers are termed saltatorial. If they use their hindlimbs only and in a fast succession of hops, they are said to be ricochetal. Fossorial forms are diggers, usually living in burrows. Here, we focus on the adaptations of cursors.

A full cycle of motion of a running or walking mammal is called a stride. An animal’s speed is the product of its stride length times rate. There are two ways of increasing the speed of running, increasing stride length and increasing stride rate. Some animals are clearly specialized to increase speed through increasing stride length; the giraffe is an extreme example. Others move rapidly by having a very fast stride rate; these would include, for example, shrews and voles.

Stride length

1. One way to increase stride length is to run on the tips of the toes. We recognize three basic patterns.

  • Plantigrade species are those that place the full length of their foot, including podials and metapodials, on the ground during each stride. Humans and bears are examples.
  • Digitigrade species walk with most of the length of their digits, but not the soles of their feet, in contact with the ground. Dogs and cats are examples.
  • Unguligrade species walk on their tiptoes, often on hooves. Deer and horses are examples.

2. A second way of increasing stride length, often found in unguligrade species, is to lengthen limb elements. A common way to do this, found in several groups, is elongation of the metapodials. This is often coupled with a reduction in number of these bones. Cursorial carnivores such as canids and cheetahs have metapodials that are very long compared to their digits, and the first metapodial (and digit) is reduced or lost. The extreme cases of elongation are seen in the ungulate orders, Perissodactyla and Artiodactyla. In each order, a progression of elongation can be seen among families. Among perissodactyls, rhinos and tapirs have three or four toes, but the center one (3) is enlarged and bears much of the weight (a condition termed mesaxonic). In horses, the 3rd metapodial is very long, similar in length to the other main limb elements. The other toes are lost or reduced to the point of being slivers of bones that fuse with the 3rd. Among the much more diverse artiodactyls, pigs, peccaries and hippos have moderately long metapodials, which are unfused. The third and fourth metapodials, however, are larger than the others and bear most of the weight (paraxonic). In camels, the third and fourth metapodials are very long and fused for most of their length, although the distal ends remain separate. Fusion is complete in bovids and cervids, and the resulting bone is called a cannon bone. Other digits are much reduced or lost.

Lengthening and fusion of metapodials is also associated with ricochetal locomotion. Kangaroos, for example, have a very long fourth digit, especially the metatarsal. The fifth digit is also important, but smaller than the fourth. The second and third digits are fused for most of their length (syndactylous), except the claws, which remain separate and are used for grooming. The first toe is lost. Another example is provided by some ricochetal dipodid rodents. In these species, the 3 central metatarsals are not only elongated but fused, forming a distinctive cannon bone.

3. A third and very common means of lengthening stride is to include the scapula as part of the limb, allowing it to swing forward and back with each stride. In cursorial mammals that use their scapulae in this fashion, the clavicle (which would restrict the movement of the scapula) is much reduced or lost. It is easy to watch the scapula in a cat move with each stride as the animal walks or runs.

4. Another way of lengthening the stride involves flexing the spine. This is associated with a bounding or galloping form of locomotion. When the animal pushes off with its hind feet, it extends its back. Contact with the ground by the hind legs prevents the rear part of the animal from moving backward, and the increase in body length becomes part of the forward stride. Hildebrand (1974) calculated that cheetahs are so good at this movement that they could run nearly 100 km/hr without any legs at all!

5. Yet another means of lengthening the stride is to increase the distance traveled by the animal when its feet are off of the ground. This is determined by the animal’s gait, or the sequence and manner by which it moves its feet when running. Weakly cursorial animals generally keep at least one foot on the ground most of the time; highly developed cursors have extended unsupported periods; and the feet of richochetal animals may be off the ground for more than half of the stride (85% for the spring).

6. A final means of lengthening the stride has to do with the mechanics of muscle action. A muscle can move a joint through a wider angle the closer it inserts to the joint.

Stride rate

The second general means of increasing speed is to increase stride rate. Mammals have approached this problem in a number of ways; here, we will mention only a few conspicuous ones. Hildebrand’s (1974) excellent and thorough discussion of the biomechanics of motion by mammals should be consulted for more a more detailed treatment of the subject.

1. Muscles acting on bones behave as simple force-and-lever systems. Muscles inserted close to joints not only move the joint through a wider angle, but they make the bones on which they insert move faster. In doing so, however, they must exert more force than a muscle inserted farther away from the joint. We expect the muscles of fast runners to be inserted near the joint, while the muscles of animals that require considerable power (diggers, climbers, etc.) to be inserted farther away. We might also expect that bony processes to which major limb muscles attach (such as the olecranon process of the ulna) might be longer in animals requiring more power, as this would increase the mechanical advantage of the muscle as it contracts. Because of the effort required to move their limbs, some large cursors have pairs of muscles that appear to act almost like the gearshift of a car; one muscle, inserted away from the joint, gets the limb moving; a second muscle inserted closer in, then takes over and moves the limb rapidly.

2. A bullet fired from a stationary rifle moves at the velocity imparted by the shell. A bullet fired from a speeding jet moves at the velocity imparted by the shell plus the velocity of the airplane. Similarly, the end of the femur moves at the velocity given it by the thigh muscles. The end of the tibia moves at the velocity given it by its muscles, plus the velocity of the end of the femur. One way of increasing speed is by adding parts, each moving at its own velocity. The overall velocity is then determined by the sum of the velocities of the parts. Adding motion by the scapula, and adding the metapodial and toe joints as moving parts (through adopting a digitigrade stance), accomplishes this.

3. The effort required to move a limb is a function of its mass and the distance of its center of gravity from the joint. The greater that distance, the greater the effort. The ulna of cursors typically is reduced or fused to the radius, and the fibula to the tibia. The number of digits is also reduced, concentrating the weight on one or two and decreasing the mass of the foot (this also this moves the center of gravity of the limb closer to the body). The mass of muscle at the end of the legs is also reduced by simplifying the motion of the joints so that almost all motion is forward and backward, and by strengthening the joint for motion in that plane by changes in the bony articulation of its components and their ligamentous support (rather than strengthening by adding muscle mass).

References and literature cited:

DeBlase, A. F., and R. E. Martin. 1981. A manual of mammalogy. Second Edition. Wm. C. Brown, Publishers. Dubuque, Iowa. xii 436 pp.

Hildebrand, M. 1974. Analysis of Vertebrate Structure.John Wiley & Sons, Inc, New York. xv 710 pp.

Vaughan, T. A. 1986. Mammalogy. Third Edition. Harcourt Brace Jovanovich, Publishers, Orlando Fl. vii 576 pp.

Phil Myers