From Here to There
Adult humans, alone among mammals, walk on two limbs all the time. This style of locomotion is considered a symbol of evolutionary superiority, since it frees a pair of limbs for other uses. But other mammals employ a wide array of efficient methods of locomotion to perform feats well beyond the capabilities of humanity.
Most mammals are tetrapods — that is, four-footed. This makes evolutionary sense, given that the ancestors of all terrestrial vertebrates are descended from fishes, four fins of which eventually evolved into four limbs. Even cetaceans and sirenians were once terrestrial tetrapods before they took to the sea.
Being four-legged means that the most characteristic movements of mammals are walking and running. Humans — like bears, among others — plant their feet flat on the ground, a stable configuration for walking. But dogs and cats move on their toes, hocks eternally raised in a running position. Horses, deer and antelope walk on the toenails of one digit and so take tiptoeing to a pointed extreme. They also have limiting joints that hinge their feet, and they can't flop their wrists or ankles. Instead, their limbs are braced for the efficiency of back and forth flexing, essential for forward propulsion. Furthermore, a large portion of their leg-muscle mass is positioned relatively high on the limb, meaning that the animal swings a lighter load and thus moves fast and stops quickly.
Speedy mammals also often have long limbs hinged on a fulcrumlike joint, which yields maximum quickness when the muscles pull. Tendons store and release energy with each step, and shoulders or hips rock back and forth to extend the stride. One feature that gives horses, elephants and other large mammals great endurance is a stiff back and relatively straight legs. This served them well when habitat changes starting millions of years ago meant they had to cover great distances in search of grass to graze on.
The cheetah has a supple back that arches as the legs are pulled underneath, then releases and stretches as the cat lunges ahead. A champion sprinter, it is the fastest mammal — streaking along at top speeds of almost 70 miles (110 km) per hour. In 15-second bursts, the cheetah can catch a Thomson's gazelle, an animal that can reach 50 miles (80 km) per hour, but usually travels more slowly since it is often weighed down with semi-digested plant food.
Getting around on all fours may be the most common form of locomotion, but when up on two feet, a red kangaroo (Macropus rufus) can hop its way to Olympic gold at about 40 miles (64 km) per hour. The kangaroo also has a vertical leap of more than 10 feet (3 m) — about triple that of Michael Jordan — and at top speed, it may bound almost 56 feet (14 m) per hop. When a kangaroo walks, it gets down on all fives: It balances on its heavy tail and forelimbs while swinging its back feet forward to step ahead.
A variety of mammals that have abandoned ground in favor of an arboreal life use a tail as a grasping limb. Woolly monkeys are particularly adept climbers with their tail. The gibbon does well enough without, leaping more than 20 feet (6 m) between handholds in trees. Its small thumbs slip out of the way as it swings from grip to grip.
Aquatic mammals use a wide range of motions. A sea lion paddles with its forelimbs and steers with its hind limbs, while a walrus propels itself with all four flippers. A whale's hind limbs have been modified into paddlelike flukes that give enough propulsive force to move it at more than 30 miles (50 km) per hour.
