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The animals of the Americas (реферат)

North and South America comprise the only continu­ous land mass that reaches from the north to south polar regions, a distance of more than 14,500 km (9,000 mi). The combined area of the two continents is 41.4 million sq km (16 million sq mi), in which are found all terrestrial biomes.

The two continents have been joined for the past two or three million years. Earlier South America was an island, set apart from the northern land mass for at least 60 million years. This gave time for animal spe­cies unique to the continent to evolve. After the Isth­mus of Panama emerged, there was an interchange of animals between North and South America, much as that experienced by Eurasia and America during the Ice Ages. One of the animals found in both Eurasia and America is the polar bear. Its habitat is along the entire Arctic coast. It has even been sighted hunting seals on ice floes hundreds of miles at sea. The polar bear's heavy coat insulates it from the icy water and air. Thick hair growing between its toes keep it from slipping on the ice. The thick, white pelt made the animal a prized trophy and reduced its population. The bear is now protected throughout its range.

The musk ox, resident of the far north, also has had to be protected from excessive hunting. At one time it came very close to extinction. A member of the cow family, the musk ox has adapted to the bitter cold by developing a heavy, shaggy coat consisting of two parts — a coarse outer covering of long guard hairs and a soft inner coat so dense that neither cold nor moisture can penetrate.

Musk oxen form a defensive ring when threatened. Adults stand along the perimeter, heads and horns pointing out, and the calves cluster together inside. This defensive posture works well against the ox's chief enemy, wolves, but is of little avail when high-powered rifles are the enemy.

Wolves prey on many species in the north — musk ox, caribou, moose, deer, hares, and even rodents. These carnivores are among the most maligned of all animals, victims of false myths and legends and syste­matic programs of extermination. They are accused of attacking humans and destroying entire herds of domestic animals. But their depredations of livestock are less severe than often claimed. And unprovoked at­tacks by healthy wolves in North America on humans are unknown. Those recorded from Europe's Middle Ages are thought to have been made by rabid animals or hybrids.

The world will be a far lonelier place if the last wolf dies. As biologist Ernest P. Walker wrote in his book, Mammals of the World, "The howl of the wolf and coyote, which to some people is of more enduring significance than superhighways and skyscrapers, should always remain a part of our heritage."

Some Arctic wolves remain snow white year round, an adoption to their environment. Three other predators of the far north— the snowy owl, Arctic fox, and weasel— are white at least part of the year.

The life cycle of the snowy owl demonstrates the close relationship which can exist between predator and prey. This owl hunts hares and lemmings. When these mammals are plentiful, female owls lay clutches of seven to ten eggs. When the food supply drops, only one to three eggs are laid.

Lemmings are among the most plentiful animals of the far north. These tiny rodents, found throughout the Arctic, are characterized by wide fluctuations in population. When vegetation is plentiful, the lem­mings' numbers skyrocket. This population density seems to trigger a drive to migrate. Hordes of lem­mings move out. Nothing deters them — swamps, forests, lakes, rivers. Eventually some reach the sea, which seems just one more obstacle. They plunge in, swim out, and drown.

Each summer the far north comes alive with the millions of birds which have migrated from the south to mate, build nests and raise their young. Waterfowl make up the majority of these migrants. Shore birds, pelagic birds, geese and ducks abound in the short Arctic summer. Some have come thousands of miles. The champion migrant is the Arctic tern, which flies • 16,000 km (10,000 mi) from the Antarctic, and in au­tumn flies back again.

When the birds leave the Arctic at the end of sum­mer, they follow ancient flyways south. One of the flyways follows the Pacific coastline from Alaska to California. Small ponds and estuaries along the coast resound to the gabbling of hundreds of ducks.

The southern edge of North America's tundra bor­ders on the taiga. Here wildlife tends to stay on the forest's edge, in meadows, along streams, on lakes and in old burns. Grass, sedges, and willows grow most profusely in these openings.

The lakes of Wood Buffalo Park in Canada's taiga are the summer nesting sites of the whooping crane, the rarest of all cranes and the object of a decades-long conservation effort. In 1949 there were only 21 left out of a population which once ranged from the East Coast to the Rocky Mountains. With complete protection, the population rose to 109 birds by 1979. Eighty-three lived in the wilderness; the others were captives.

Twice a year the wild birds migrate a hazardous 4,000 km (2,500 mi) from their nesting grounds in Wood Buffalo Park to the Aransas Wildlife Refuge on the Texas coast. The possibility of a major storm or devas­tating disease striking this flock is a threat which makes biologists shudder. One of the basic rules in the management of an endangered species is to spread the risk. A daring experiment was undertaken with the whooping cranes. Eggs were removed from nests in Wood Buffalo Park for artificial incubation and place­ment under setting sandhill cranes, a related, more plentiful species. The artificially incubated eggs are hatching and producing birds that are raised in captiv­ity. Several whooping cranes have been hatched and are being raised by their foster parent sandhills in Idaho. If the experiment succeeds, a new flock of whooping cranes will have been produced, one which migrates a much smaller distance, over a different route, than the original group. A fringe benefit of taking eggs is that it stimulates the female bird to continue laying, thus generating more than the usual number of clutches per year. The most common grazing animal of the American coniferous and deciduous forests is the white-tailed deer. In the far West, it is replaced by the mule deer. There are actually more deer now in North America than when Europeans first arrived, because of the clearing of forest land, plus game management.

Bears once occurred throughout the forests of America north of Mexico. The world's largest is a brown bear, the Alaskan or Kodiak. The grizzly, also a brown bear, has been known to launch unprovoked attacks against humans.

American black bears are quite common in much of their range — practically all the wooded areas of North America north of central Mexico. They usually occur in their familiar black color phase, but also have been known to be a cinnamon color, brown, and even blue. The rare blue or glacier bear occurs only in southeastern Alaska, where there are about 500 left.

South of North America's taiga is the immense grass­land known as the Great Plains. This covers most of the continent's interior and stretches 3,900 km (2,400 mi) from southern Canada deep into Mexico. It is prairie country, a seemingly flat land, devoid of trees except­ing along the river courses. Almost all of the original grasses were plowed under for the raising of crops, and of the tremendous number of wild animals which once lived there, practically nothing remains. As the naturalist Peter Farb wrote, "Not even the eastern forests have suffered the almost complete destruction that European man has brought to the grassland."

The story of the American pronghorn, the only "an­telope" native to the New World, illustrates his point. When Europeans first settled in the Western Hemi­sphere, there were an estimated 50 to 100 million pronghorn on the plains. Four centuries later by the turn of the 20th century, only 20,000 were left. Today, through strenuous conservation efforts, the prong-horn is safe, although consigned to a small fraction of its former range.

Another example of what happened to the plains' wildlife concerns a "dog." Before the Europeans came, hundreds of millions of rodents, called prairie dogs because of their dog-like call, lived in underground "towns" from southern Canada to Mexico. One such system of burrows in Texas covered more than 65,000 sq km (25,000 sq mi) and contained approximately 400 million animals. With the coming of civilization, the burrows were plowed under and the animals poi­soned. Few prairie dog towns still exist.

As the prairie dogs disappear, they are taking with them at least one of their predators, the black-footed ferret. This member of the weasel family has prairie dogs as its prime food. It has become overspecialized and is caught in an evolutionary trap.

North America's arid areas occur in the southwest­ern United States and parts of Mexico. Large grazers and browsers include bighorn sheep, mule deer and javelinas, also called peccaries. Hawks, foxes, owls, coyotes, and several species of reptiles are among the carnivores. Among them, the coyote is one of the few which has thrived in the face of human intrusion into its habitat. Not only has it maintained its former range; it has expanded it.

One of the resident birds of the North American southwest is the roadrunner, a member of the cuckoo family. Primarily a ground bird, it can run at speeds of up to 24 kmph (15 mph). Its diet consists of lizards and other reptiles which it kills by repeated blows from its heavy beak. If prey proves too large to swallow, the roadrunner ingests a bit at a time. The birds can be seen dashing along the desert with snakes or lizards hanging from their mouths.

The world's smallest owl, the 14 cm (5 1/2 in) high elf owl, also is a resident of the American desert. This tiny predator uses the hollowed-out nests of wood­peckers, located in cactuses, as its home.

The desert also has its reptiles, including many spe­cies of lizards, plus two of the four poisonous snakes of North America — the rattlesnake and coral snake.

Rattlesnakes are pit vipers, a group of reptiles which also includes the fer-de-lance, bushmaster, water moc­casin, and the copperhead The pit is an opening below the snake's eyes which contains a heat-sensing organ.

Only two of North America's lizards are poisonous — the gila monster and Mexican beaded lizard. Unlike poisonous snakes which inject their venom through hollow fangs, these lizards bite their victims, hold on, and allow poison to flow into the open wound from fangs which are grooved at the rear.

The coastlands and adjacent lands of the United States are the habitat of a wide variety of reptiles, birds and mammals. Water moccasins and copperheads are found in the warmer portions, and the largest of all North American reptiles, the alligator, lives in the riv­ers and bayous of the southeast.

Alligators can be distinguished from the closely re­lated crocodiles by their broader heads and the lower teeth which are out of sight when the mouth is closed. A crocodile's teeth are visible at all times.

There are no authenticated cases of wild alligators attacking humans. Crocodiles, on the other hand, can attack people.

Many species of shorebirds live in North America. One of them, the brown pelican, came close to extinc­tion on the continent because of DOT. The pesticide was sprayed and dusted on croplands, then percolated into the ground water and was carried to sea where it entered the ocean's food chain. The pelicans, being ultimate consumers, got heavy doses. Although the chemical didn't kill them, it did weaken the shells of their eggs. The result was few pelican hatchlings. After DDT was banned the pelican population began to grow again. In 1979, 1,200 nests were counted in California, a remarkable comeback.

Marine mammals of the U.S. Pacific coast include four species of pinnipeds — members of the seal group. They are elephant seals, harbor seals, Steller sea lions and California sea lions.

South of the United States and northern Mexico, the character of the land and its wildlife changes. Desert, chaparral, and plains give way to tropical forest. In places rainfall exceeds 500 cm (200 in) annually, and a mild average temperature of 27°C (81°F) prevails.

As in most rain forests, primates dominate. In America they consist of dozens of species of monkeys and marmosets. New World monkeys are only distantly related to those of the Old World. Many species have prehensile tails, a feaure lacking in the Old World monkeys. This "fifth hand" is especially well devel­oped in the spider monkey.

Not all of the rain forest's primates have prehensile tails. Marmosets of the forests of Panama and the Ama­zon basin lack it. And the uakari has a mere stub of a tail, making it the only short-tailed New World monkey.

South America is home to approximately 40 percent of the world's birds, and most of them live in its rain forest. Two groups of rain forest birds are among the most colorful in the world — the hummingbirds and parrots.

Known as "living jewels," hummingbirds are found only in the New World, where they live from southern Alaska to Tierra del Fuego. However, they are primar­ily tropical birds. There are 319 known species which range in size from the world's smallest bird, the 57 mm (2 1/2 in) long Cuban bee hummer, to the giant hummingbird of the high Andes, measuring 216 mm (8 1/2 in) in length.

A second group of colorful rain forest birds, the parrots, are distributed worldwide in the tropics and on all lands in the southern hemisphere excepting the southern tip of Africa and some of the more remote Pacific islands. In the New World, they reach north­ward into southern Arizona and New Mexico, where they are represented by occasional visits of the en­dangered thick-billed parrot.

The only parrot native to the United States is now extinct. In the early 19th century, the Carolina parakeet ranged from North Dakota and central New York south to eastern Texas and Florida. It was especially abun­dant in the Mississippi River bottoms and along the Atlantic seaboard The little bird was slaughtered for sport and to control its depredations on fruit crops The last one was sighted m the Florida Everglades m the early 1920 s

In addition to its wealth of birds, the South Amen can rain forest is the home of a wide variety of other animals The world s slowest mammal, the sloth which spends long periods hanging upside down from tree branches, is a forest dweller So are opossums, anteaters, poisonous frogs, jaguars, tapirs, and several snakes, among them the anaconda, the world s largest An anaconda can measure more than 9 m (30 ft) in length Its prev includes the world s largest rodent, the hog sized capybara, and the caiman, South America s counterpart of the alligator

To the west, the rain forest terminates at the Andes, the mountain ranges stretching the length of South America The highest point m the western hemi sphere, 7,000 m (22,834 ft) tall Mt Aconcagua, is m the Andes

America s smallest deer, the pudu, and one of the world s largest flying birds, the Andean condor, live in these mountains Probably the best known of Andean animals are the guanacos, vicunas, llamas, and alpacas, New World relatives of camels, which are found at high elevations. Llamas have been domesticated as beasts of burden since pre-Columbian times; vicunas and alpacas are prized for their high-quality wool.

The cold water off South America's west coast is rich with plankton, a link in a food chain which reaches up through fish and ends with the millions of sea birds living on the South American coast and nearby islands. Among them, the guanay cormorant breeds in enor­mous numbers. Cormorant rookeries are not particu­larly pleasant places for humans. They reek of drop­pings, dead birds and regurgitated food, and there are flies everywhere. The droppings, called guano, make a superb fertilizer and are harvested commercially in Peru and Chile.

South America's grassland is called the pampas. Al­though similar to the Great Plains of North America, the pampas never was home to the vast herds of wild animals which once roamed North America.

One of the world's large, nonflying birds, the com­mon rhea, lives on the pampas. It was once hunted by gauchos on horseback for its tail plumes, which were used as dusters. A second species, Darwin's rhea, roams the Andean foothills from Peru to Bolivia and south to the Straits of Magellan. It is an endangered species.

The pampas' predators include foxes, skunks, rattlesnakes, hawks, and one which is found only in South America, the rare maned wolf. This mammal looks more like a fox than like a wolf. It is solitary, nocturnal, and wide-ranging. It hunts small mammals, birds, and reptiles and also eats fruits and other plant material.






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