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
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,
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
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
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
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
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
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
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
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
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
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
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
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 AmericaThe highest point m the western hemi sphere,
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
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
Реферат на тему: The animals of the Americas (реферат)