Animal. Animals come in many shapes and sizes. They live throughout the world. Animals walk or crawl on land and dig through the soil. They swim in the water and fly through the air. They even live inside the bodies of other animals. Bats, dogs, horses, kangaroos, and moles are all animals. So are butterflies, frogs, jellyfish, pigeons, sharks, snakes, and worms.
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Most kinds of animals are less than 1 inch (2.5 centimeters) long. Many are so tiny that they can be seen only with a microscope. The largest animal is the blue whale. It is about as long as five elephants in a row.
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Animals are not the only kind of living things. Scientists divide living things into six main kingdoms (groups)—animals, plants, fungi, protists, archaea, and bacteria. Fungi include molds, mushrooms, and yeasts. Protists, such as amebas, cannot be seen without a microscope. Archaea and bacteria, collectively known as prokaryotes, rank as some of the smallest, simplest forms of life.
Animals are different from other living things in many ways. For example, the bodies of animals are made up of many cells. But the bodies of prokaryotes and most protists have only one cell. Like animals, plants and fungi also are made up of many cells. However, animals can move around. Most plants and fungi are held to one place in the soil by roots or rootlike structures. For a more complete discussion of the differences between the members of the six kingdoms, see the article on Kingdom.
No one knows exactly how many species (kinds) of animals there are. So far, scientists have classified (grouped) and named more than 11/2 million kinds of animals. Over half of these are types of insects. Many new species are discovered each year. Scientists believe there may be from 2 million to as many as 50 million kinds of animals alive today. Many other kinds of animals used to live on Earth but have died out. They include dinosaurs and dodos.
This article provides general information on animals other than human beings. It includes a classification table and pictures of many animals. Separate World Book articles give details about hundreds of animals. For information on human beings, see Human being.
Importance of animals
Animals and the web of life.
Living things depend on one another. They are connected in what is sometimes called the web of life. Plants capture the energy from sunlight and use it to make roots, stems, leaves, flowers, and fruits. Animals eat the plants, or they eat other animals that feed on the plants. When animals die, their bodies decay and release materials that help fertilize the soil for plants.
Animals and plants are also connected in other ways. When animals breathe, they take in oxygen from the air and give off carbon dioxide. Green plants take in carbon dioxide and give off oxygen in a food-making process called photosynthesis. Many plants with flowers need insects or birds to carry their pollen from plant to plant. Without this transfer of pollen, these plants are not able to reproduce (create new individuals of their own kind). Some seeds are prickly and cling to the fur or feathers of animals. When the animals move from place to place, they take the seeds with them. In this way, the seeds get dropped in new areas where they can grow into plants.
The web of life relies on balance among its parts. A change in one part may mean disaster for others. For example, if all the trees in an area are cut down, then many animals that depend on them will die. For more information on how living things are linked, see Ecology.
Animals and people.
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Dog behavior
Animals have provided people with food and clothing since prehistoric times. Without animals, people would not have such things as meat, honey, eggs, wool, leather, or silk.
At least 10,000 years ago, people began domesticating (taming) animals. Some of these animals provide food and clothing. For example, cattle supply meat, milk, and leather. Chickens lay eggs. Sheep provide wool and meat.
Some domesticated animals help people work. Water buffaloes pull plows in Asian rice fields. Horses and camels carry people from one place to another. At first, people kept cats in their houses to catch rats and mice. They raised dogs to help them hunt. Today, cats and dogs are kept largely as pets.
Certain insects are useful to people. Bees make honey, which people harvest for food. Bees also pollinate many food crops, including fruits and vegetables. Silk comes from fiber made by silkworms.
Some animals harm people. On rare occasions, crocodiles, lions, and tigers attack and kill people. So do grizzly bears and polar bears. Sharks sometimes kill and eat human beings. Bites from such poisonous snakes as rattlesnakes and cobras can cause death. The black widow spiders have a poison that makes people extremely sick.
Some animals pass diseases along from person to person. Certain mosquitoes transmit malaria and yellow fever. Some ticks carry the bacteria that cause Lyme disease and Rocky Mountain spotted fever. Some animals cause disease themselves. Worms called flukes, which live in human organs, can cause schistosomiasis. This disease infects millions of people in many African, Asian, and Latin American countries.
Kinds of animals
People often divide animals into various groups based on certain similarities the animals share. For example, some animals can be kept as pets, but others are wild. Arranging animals according to their similarities is a handy way of remembering and understanding them.
Some common ways of grouping animals.
Animals can be grouped in many ways. They can be arranged according to whether they live on land or in water. Animals that live on land are known as terrestrial animals. They include cats, dogs, lizards, mice, and worms. Animals that live in water are called aquatic animals. They include eels, fish, lobsters, octopuses, and whales.
Animals can be arranged by the number of legs they have. Dogs, frogs, and lizards have four legs. Bats and birds have two legs. Insects have six legs, and spiders have eight. Snakes and worms have no legs.
Another way to group animals is according to how they move. Bats, most birds, and many insects fly. Whales, fish, and squid swim. Snakes and worms crawl. Antelope and cheetahs run. Frogs, kangaroos, and rabbits hop.
Some animals are cold-blooded, and others are warm-blooded. The bodies of cold-blooded animals are warm when their surroundings are warm and cool when their surroundings are cool. Warm-blooded animals, on the other hand, almost always have the same body temperature, regardless of the warmth of their surroundings. Birds, mammals (animals whose babies drink the mother’s milk), and a few species of fish and insects are warm-blooded. All other kinds of animals are cold-blooded.
Animals are also commonly divided into groups according to whether they have backbones. Invertebrates do not have backbones, but vertebrates do. The vast majority of animals are invertebrates. They include clams, insects, jellyfish, sea urchins, snails, spiders, sponges, and worms. Birds, fish, mammals, and reptiles are vertebrates. So are amphibians—frogs, salamanders, and other animals that spend part of their lives in water and part on land.
The scientific classification of animals
involves grouping animals according to the biological relationships among them. This orderly arrangement of animals depends in part on the features the animals share. In general, the more features they share, the more closely they are related. However, the scientific classification of animals is based mainly on the belief that certain animals share a common ancestor. Animals with a more recent common ancestor are more closely related than those who share an ancestor further back in time. In a somewhat similar way, brothers and sisters are more closely related than are cousins. Brothers and sisters share parents. First cousins share grandparents.
In classifying animals, zoologists (scientists who study animals) divide them into ever-smaller groups that have more and more features in common. All animals belong to one large group, the kingdom Animalia. This kingdom consists of a number of smaller groups called phyla. Each phylum is divided into groups called classes. The classes are broken down into orders, and the orders into families. The families are split into genera, and the genera into species. The singular form of genera is genus, but the word species may be either singular or plural.
Scientists have classified more than 11/2 million kinds of animals. They continue to discover and classify new species. Scientists believe that they have classified most of the members of some groups. For example, scientists have classified about 4,500 species of mammals, and it is unlikely that a large number of mammals remain undiscovered. For other groups, scientists believe that they have classified only a fraction of the actual number of species. For instance, scientists have classified more than 1 million species of insects. However, most scientists believe that another 5 to 30 million insects have not yet been classified. As these numbers illustrate, there are great differences between the numbers of species in each group of animals.
Each species of animal belongs to one phylum, one class, one order, one family, and one genus. For example, tigers belong to the kingdom Animalia, the phylum Chordata, the class Mammalia, the order Carnivora, the family Felidae, and the genus Panthera. They are members of the species Panthera tigris. Lions are related to tigers. They belong to the same kingdom (Animalia), phylum (Chordata), class (Mammalia), order (Carnivora), family (Felidae), and genus (Panthera) as tigers. But lions are classified in a different species—Panthera leo, also written simply as P. leo.
A table of animal classification, showing some of the major groups of animals, appears at the end of this article. See also Classification, Scientific.
Where animals live
Animals live in many kinds of places. The kind of place where an animal lives is called its habitat. Each type of habitat presents a special challenge to animals. For example, animals that live in polar regions must withstand bitter cold. Those that inhabit the tropics face extreme heat. In spite of these challenges, animals can be found everywhere on Earth. They live on the highest mountains and in the deepest oceans. They roam the driest deserts and the wettest rain forests. They swim in fresh water and salt water.
Each habitat supports many kinds of animals. In most cases, the animals are the same kinds that have lived in those surroundings for thousands of years. As a result, the animals have developed bodies and ways of life that suit them to that particular habitat. No single species of animal can survive everywhere. For example, tropical fish from the Amazon River thrive in warm water but cannot withstand the cold streams of the Andes Mountains. On the other hand, many kinds of fish that live in the Arctic Ocean would die if they were exposed to the warm waters of the Caribbean Sea. However, some animals may travel between habitats from time to time. For example, African elephants eat both grass and tree parts and so move between grassland and forest. But these animals would not be able to withstand the freezing temperatures of the polar regions.
Some habitats, including many forests and grasslands, are being destroyed by human beings. The destruction of these habitats usually causes the death of many animals. When people convert grassland to farmland, for example, they destroy the homes and source of food of many species. Without these necessities, some animals will die immediately. Others may try moving to another grassland. But the new area may not have enough food and shelter to support the additional wildlife. As a result, many more animals will die.
This section tells about some of the major animals, grouped according to seven types of habitats: (1) mountains, (2) grasslands, (3) temperate forests, (4) tropical forests, (5) deserts, (6) polar regions, and (7) oceans. For descriptions and pictures of animals grouped according to the continent on which they live, see the articles on Africa; Antarctica; Asia; Australia; Europe; North America; South America.
Animals of the mountains
Mountains support a variety of animal life. The numbers and kinds of animals found on mountains vary with altitude. More animals and more kinds of animals live at lower altitudes than at higher ones, largely because of the differences in climate between elevations. Generally, mountain climates become colder, wetter, and windier with increasing altitude. The air also gets thinner and has less oxygen. In addition, fewer plants are found at higher elevations, and therefore less food is available for animals.
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Bears, deer, elk, and mink make their homes on the forested lower slopes and in the wooded or grassy valleys of mountains. Rainbow trout and graylings swim in mountain streams. Many mountains have meadows of grasses and herbs. These meadows are home to chinchillas, ibexes, llamas, vicunas, and yaks. Butterflies, grasshoppers, and spiders also live there.
Above the timber line—that is, the line beyond which trees will not grow because of the cold—stand rocky cliffs and peaks dotted with shrubs, mosses, and other plants. Small meadows are also found there. Sure-footed bighorn sheep and mountain goats dwell among the windswept rocks, as do furry marmots and pikas. High on the snow-capped peaks, only a few insects, spiders, and ice worms can survive. Golden eagles and some other birds fly above the mountains. A large African vulture, Ruppell’s griffon, has been known to soar as high as 36,600 feet (11,150 meters).
Animals of the grasslands
Grasslands include the prairies of North America, the Pampas of South America, the plains of Europe, and the steppes of Asia. The savannas of east Africa have more grassland animals than any other area.
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Rainfall in grasslands is seasonal, and animals sometimes travel great distances to find green grass. Gazelles, wildebeests, and zebras migrate by the thousands through the African savannas. Smaller groups of elephants and rhinoceroses also feed on the grasses there. Such meat-eating mammals as cheetahs, hyenas, and lions roam the savannas in search of prey. The savannas are also home to giraffes, jackals, ostriches, secretary birds, and termites. In addition, hippopotamuses live in and near bodies of water in African grasslands. Animals of other grasslands include the kangaroos and wombats of Australia, the cavies and rheas of South America, and the coyotes and prairie dogs of North America.
Many animals of the grasslands have become endangered due to loss of their habitat and to overhunting. The rich soils of grasslands are ideal for farming, and people have converted many such areas to farmland. Many of the large grassland animals are favorite big game for hunters. For example, the once-plentiful pampas deer of South America have become extremely rare. As the Pampas are converted to farmland, the tall grass that grows there disappears. Without this grass, the pampas deer have no shelter and become easy prey. Bison once grazed in huge herds in the Great Plains of North America. But so many of these animals were killed by hunters or died as their grassland habitat was converted to farmland that they were nearly wiped out.
Animals of the temperate forests
Temperate forests consist largely of deciduous trees and evergreen trees. Deciduous trees shed their leaves in the fall and grow new ones in the spring. Evergreen trees have leaves that live two or more years. Some evergreens have needle-shaped leaves. Most temperate forests are in Asia, Europe, and North America. Australia also has some temperate forests.
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Many forest animals have small bodies that allow them to move easily through the underbrush. Forest mammals include chipmunks, mice, opossums, porcupines, raccoons, skunks, and squirrels. Bears, deer, and wild boars also live in temperate forests. Bobcats and wolves were once common in woodland areas. However, so many of these predators (hunting animals) have been hunted and trapped through the years that they have become rare.
Salamanders are often plentiful in temperate forests. They hide in the leaf litter or under rocks, where they feed on insects and other small organisms. In wet forests, slugs and other snails are common. Beavers, fish, frogs, muskrats, otters, salamanders, and turtles live in or near woodland streams, ponds, and lakes. Great numbers of birds nest in the trees and shrubs.
Many temperate forests have been cleared for farms and cities, and many others have been cut down for fuel and lumber. This deforestation (destruction of forests) places woodland animals in danger. Extensive logging in the Pacific Northwest of the United States, for example, has destroyed much of the habitat of the spotted owl, threatening the existence of that species.
Animals of the tropical forests
Tropical forests stay warm all year and receive plentiful rainfall. These forests are found in Africa, Asia, Australia, Central and South America, and the Pacific Islands. More kinds of animals live in tropical forests than in any other habitat. Scientists estimate that perhaps as many as 30 million species of tropical animals have not even been discovered yet.
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Insects make up the largest single group of animals that live in tropical forests. They include brightly colored butterflies, huge colonies of ants, mosquitoes, and camouflaged stick insects. Spiders are also plentiful. Many tropical birds, such as quetzals and parrots, are spectacularly colored.
The broad leaves of trees in tropical forests form a thick overhead covering called a canopy that blocks nearly all sunlight from reaching the forest floor. Many kinds of animals live in the canopies of tropical forests. They include harpy eagles and toucans; tree frogs; flying dragons; spider monkeys and howlers; gibbons and orangutans; sloths; slow lorises; tree boa constrictors; bats; and wasps, beetles, and leaf-cutting ants.
Jaguars, tapirs, and bushmaster snakes live on the ground in tropical forests. Chimpanzees and lowland gorillas alternate between the ground and the trees. Crocodiles, fish, and turtles inhabit rivers and ponds.
People are rapidly destroying tropical forests for wood and for farming. The continuing destruction of this habitat means that many animals will disappear forever. Scientists believe countless species have already been wiped out.
Animals of the deserts
Most deserts lie near the edges of the tropics. Food and water are often scarce in deserts, and temperatures in the summer can be scorching. Despite these conditions, many kinds of animals live there. They include geckos, iguanas, and skinks; bees, butterflies, and moths; spiders; elf owls and roadrunners; sidewinders; dorcas gazelles and mule deer; and bobcats, coyotes, and dingoes.
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Animals of the deserts have developed special bodies and ways of life that enable them to survive the extreme heat. Centipedes, kangaroo rats, rattlesnakes, and scorpions spend the day in burrows. They come out to search for food only when temperatures drop at night. Many insects, lizards, and tortoises can tolerate high desert temperatures and are active in the daytime. But even they must retreat underground or find the shade of a tree during the hottest part of the day. Some snails, insects, frogs, lizards, mice, and ground squirrels estivate (sleep through the summer).
Many desert dwellers have light-colored skin, which helps keep them cool by reflecting sunlight. Desert foxes and hares have long ears. When overheated, these animals move to a cool cave or burrow where they can get rid of excess body heat through their ears. The Cape ground squirrel makes its own shade by using its fluffy tail like a parasol. Fairy shrimp and spadefoot toads may spend months or years underground waiting for rain to create ponds. Then they quickly feed and reproduce before the ponds dry again.
Animals of the polar regions
Animals that live in polar regions must withstand extremely cold temperatures. No land animals except ice worms and a few species of insects live in polar regions that have ice and snow the year around. But the seas of the Arctic and Antarctic have large numbers of wildlife, including fish, giant sponges, whales, and tiny shrimplike creatures called krill. In addition, polar bears, sea lions, and walruses spend much of their time on floating sheets of ice in the Arctic. Penguins and seals live on the Antarctic coast.
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Many animals inhabit the vast Arctic tundras (cold treeless plains) of northern Asia, North America, and Europe. They include caribou, ermine, musk oxen, reindeer, lemmings, snowy owls, and wolves. Shallow ponds in the region provide a place for mosquitoes and many other insects to lay their eggs. These insects serve as food for the birds that migrate to the tundra each summer to nest.
Animals that live in polar regions have developed bodies and ways of life that enable them to deal with the frigid winter weather. Caribou, musk oxen, and polar bears have thick fur, which helps them stay warm. The Arctic fox and Arctic hare have short ears and tails that keep them from losing much body heat. Arctic ground squirrels hibernate (sleep through the winter). They curl up in a burrow, and their body temperature drops, saving energy during the long winter. They also do not eat in the winter. They live off fat stored in their bodies.
Animals of the oceans
Animals of many kinds are found everywhere in the vast oceans. Some of the smallest animals live in the sea, as does the world’s largest, the blue whale. Cod, halibut, seals, and whales swim the frigid waters of the polar regions. Lobsters, sea urchins, and many types of brightly colored fish inhabit coral reefs in warm tropical seas. Some ocean animals live near the shore—in shallow water, in tide pools, and on rocks. They include anemones, barnacles, mussels, octopuses, and starfish. Other marine animals—mostly such tiny shrimplike creatures as krill and copepods—are found in the open sea. Krill and some species of copepods form part of the group of organisms called plankton. Many fish and some whales feed on plankton.
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The great depths of the ocean are completely dark, and the water there is bitterly cold. Even so, anglerfish, clams, and certain other creatures live there. On the other hand, flyingfish, manta rays, marlins, and porpoises generally swim near the ocean surface. Albatrosses, gulls, and petrels fly above the sea.
Oceans provide people with such foods as crab, fish, lobster, and shrimp. However, the demand for seafood has led to the overfishing of halibut, herring, and some other marine animals. Millions of dolphins, which are mammals, have drowned in fishing nets that were intended to catch cod, tuna, and other fish. In addition, spills of toxic materials and other forms of pollution have reduced the numbers of some ocean species.
The bodies of animals
Animals have special body features that enable them to survive in their environment. These special features, called adaptations, result from the ability animal species have to adapt (adjust) over time to changes in their surroundings. Adaptations for survival enable animals to move about, to eat, to breathe, and to sense their environment. Legs, wings, and fins help animals move. Teeth and jaws help them eat. Lungs and gills help them obtain oxygen. Eyes and ears help them find food and detect predators.
Animals live in many kinds of environments. The body features of an animal that work well in one type of environment may not work in others. For example, the adaptations that enable fish to breathe in water do not let them breathe on land. Even in the same environment, animals may have different adaptations for survival. Shrimp, fish, and sea turtles can all swim in the ocean, but they have different body features for doing so. Shrimp have tiny swimming legs, fish have fins and muscular tails, and turtles have flippers. Because animals adapt to their surroundings in many ways, there is a wide diversity of animals in any environment.
This section describes some of the ways animal bodies are adapted for moving, eating, breathing, and sensing the environment. For information on the basic process of adaptation, see Adaptation.
Adaptations for moving about
Legs and feet.
Mammals, birds, insects, and many reptiles and amphibians have legs with feet that enable them to walk or run on land. Most amphibians, mammals, and reptiles walk on four legs. Birds and people walk on two. Insects have six legs, and spiders have eight. Millipedes may have up to 200 legs.
Animals can crawl without legs and feet. Such tiny creatures as planarians and other flatworms slide by moving many small hairlike structures, called cilia, back and forth like miniature oars. Snails move by coating the ground with a sticky fluid from their bodies. They then crawl through the fluid using a muscular organ called a foot. Most snakes slide along the ground by bending their bodies from side to side. An earthworm crawls through the soil by alternately lengthening and shortening parts of its body.
Many walking and crawling organisms live in water. Crabs and lobsters have legs that enable them to walk across the bottom of a body of water.
Wings.
Three groups of animals have the ability to fly under their own power: (1) insects, (2) bats, and (3) birds. Most insects have two pairs of wings. Muscles inside the thorax (middle section of an insect’s body) move the wings up and down.
Bats are the only mammals with wings. Batwings are made up mostly of skin stretched over long finger bones. Muscles in the wings raise and lower them.
Birds have powerful muscles attached to their wings and breastbone. Bird wings are covered with feathers, which also aid in flight.
Some animals, including flying squirrels and flying lemurs, can glide but not fly. Such animals jump from trees or mountains. They have big feet or folds of skin that spread out to serve as “wings” for gliding.
Fins, tails, and flippers.
Many types of animals swim in fresh or salt water. Fish have well-developed tails and fins. Most fish swim by bending their powerful, muscular tail from side to side. Fins on the top, bottom, and sides of fish are used to maintain balance and to maneuver in tight areas. Dolphins, porpoises, and whales swim by moving their massive tails up and down rather than side to side. Turtles swim by paddling with their webbed feet or their flippers.
Jellyfish and squids swim by jet propulsion. When a jellyfish pushes water out from under its body, it is thrust in the opposite direction. A squid takes water into its body cavity and then squirts the water out through a small opening called a funnel. This action repeated many times pushes the squid forward.
A number of species of birds can swim. Some ducks and gulls paddle on the surface of the water using their webbed feet as oars. Torrent ducks and loons dive underwater, where they swim by kicking their feet. Penguins use their feet and their wings to swim.
Adaptations for eating
All animals need food to survive. Animals eat plants, other animals, or both plants and other animals. Animals that eat plants are called herbivores. Zebras, cows, and moose are herbivores. Animals that eat other animals are called carnivores or meat-eaters. Dogs, lions, and sharks are carnivores. Animals that eat both plants and animals are known as omnivores. Bears are omnivores.
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Biologists describe the relationships between animals in a habitat and the foods they eat as a food chain. Technically, a food chain involves the flow of energy from the sun to green plants to animal consumers. For example, a simple food chain in a meadow links the grasses, the deer that eat the grasses, and the wolves that eat the deer. Sometimes, many kinds of animals and plants are involved in complex networks of food chains. Such networks are called food webs.
Most animals eat a variety of foods. For example, pigeons eat fruits, grains, and nuts, and they sometimes feed on insects, snails, and worms.
Some animals eat only a few foods. A snail called a cone shell preys only on a single species of marine worm. Several kinds of snakes eat only slugs or other snails. Hummingbirds and honey possums live on the nectar of flowers. A sapsucker drills holes in trees and eats the sap that flows from the holes. The koala of Australia dines on the leaves of eucalyptus trees.
Filtering mechanisms.
Huge numbers of tiny organisms called plankton float or swim slowly near the surface of oceans, lakes, and other bodies of water. Plankton make up a part of an important food chain in the ocean. Plankton are too small to be captured individually by animals that feed on them. Some animals, such as barnacles, sweep water past themselves while straining out the tiny plankton, which are thereby captured. This process is called filter feeding.
Baleen whales are probably the best-known filter feeders. These animals, which do not have teeth, feed by gulping huge mouthfuls of water containing plankton, small fish, and other marine organisms. They then force the water out of their mouths through a series of strainers called baleen. The food is captured on the baleen and then swallowed. A baleen whale can consume as much as 4 short tons (3.6 metric tons) of food a day.
Teeth and jaws.
Many animals eat food that they need to tear into pieces small enough to be swallowed and digested easily. Teeth and jaws are adaptations for tearing food. Teeth may also be used to kill prey.
Teeth are adapted for the particular type of food an animal eats. Deer, giraffes, and other herbivores have teeth with broad surfaces for grinding grasses and plants into small bits. The powerful front teeth of beavers enable these animals to cut down trees for food and shelter. Lions have razor-sharp canine (pointed) teeth for killing and then tearing prey.
Birds have bills that are adapted for certain types of feeding. A hawk has a sharp, hooked beak for tearing prey. A woodpecker uses its long, pointed bill to drill into the bark of trees to find insects.
Insects have jaws and movable mouthparts that act like teeth. The jaws of grasshoppers are adapted for cutting and chewing plants. Mosquitoes have needle-shaped mouthparts for piercing skin and sucking blood.
Adaptations for breathing
Most animals need a continuous supply of oxygen to survive. The entire process of obtaining and using oxygen is called respiration. That part of the process that involves how an animal takes oxygen from its environment and gives off carbon dioxide is known as breathing. This section focuses on breathing. For a complete description of how oxygen flows to various cells of an animal’s body and how it is used by those cells, see Respiration.
The way that animals breathe depends on where they live. Land animals get oxygen from the air. Aquatic animals obtain oxygen from water.
Many land animals have lungs for breathing. As blood flows through the lungs, it picks up oxygen from the air and releases carbon dioxide. The blood then carries oxygen to the rest of the body. See Lung.
Many aquatic organisms, such as fish and tadpoles, use gills to obtain oxygen that is dissolved in water. Some animals pump water across their gills to increase the efficiency of breathing. Sharks do this by swimming continuously.
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Tiny tubes called tracheae allow insects to breathe in air. Tracheae branch throughout an insect’s body. They open to the outside air through holes called spiracles. When air enters the tracheae, oxygen is carried to every cell in the body.
Some animals that live in damp environments have unusual ways of breathing. For example, many salamanders have no lungs or gills. They breathe through their moist skin.
Adaptations for sensing the environment
Most kinds of animals have special body parts that respond to changes in the animal’s environment. Such a stimulus (change) might come from an odor, a sight, a sound, a taste, or a touch. The simplest kinds of animals, such as sponges, have no special body parts and react to stimuli with their body cells. Animals with more complex physical structures, especially vertebrates, have highly developed organs for reacting to stimuli. These organs are described in the articles on Brain and Nervous system.
Some simple animals, such as hydras, react to stimuli with special cells. These sensory cells are scattered among the outermost cells of the body. The reactions of most other kinds of animals depend largely on one or more of the major senses. These senses are sight, hearing, smell, taste, and touch. See Senses.
Some senses are more important to one kind of animal than to another. Most birds cannot find food if they cannot see it. Hearing is vital to bats. If the ears of a bat are covered, the animal will crash into objects when it tries to fly. A keen sense of smell enables dogs and wolves to find food, follow trails, and recognize danger. Taste is highly important to many insects. Some butterflies can taste the sweetness of flowers with their feet. A cat’s long whiskers serve as touch organs. They enable the cat to feel its way through underbrush and avoid bumping into objects.
A number of animals have special senses. A rattlesnake has pit organs on the side of its face that sense heat. These organs enable the snake to tell if a mouse or some other warm-blooded prey is nearby, even in total darkness. Many scientists believe that some birds and insects can detect the direction of Earth’s magnetic field. This ability may help these animals navigate.
How animals protect themselves
The world of an animal is filled with danger from predators. This section describes some of the many ways animals protect themselves from such danger.
Hiding in a safe place.
The best protection against a predator is to avoid being seen by it. Many animals rest or sleep in a safe hiding place. Some desert toads crawl down a crack in the mud. A cricket hides under a large rock or under the loose bark of a tree. Worms and moles dig underground tunnels.
Many species, such as rabbits, leave their nests mainly or only at night, when they are harder for predators to spot. Other species become active for only short periods so they are not exposed to predators for long.
Camouflage.
Many animals are difficult for predators to see because they resemble their surroundings. The various ways animals blend with their surroundings are called camouflage. For camouflage to be effective, the animals must remain motionless or nearly so.
Protective coloration
is coloring that helps animals to hide. A dark moth lying against the brown or black bark of a tree is hard to see. However, that same moth would be clearly visible if it sat on a green leaf.
A number of animals can change their colors and thus remain camouflaged even when moving among backgrounds that have different colors. The chameleon, a type of lizard, is green when surrounded by leaves but turns brown when moving slowly on bark or on the ground. The ptarmigan, an Arctic bird, is brown in summer but becomes white in winter, when snow covers the ground.
Mimicry
helps many animals avoid predators. Some animals mimic (resemble) other objects in their environment. For example, many green insects are shaped like leaves. Some caterpillars look like lizards or bird droppings. Walkingstick insects are shaped and colored like twigs. Anglerfish resemble rocks on the ocean floor.
Batesian mimicry is a form of mimicry in which an otherwise harmless animal strongly mimics an offensive animal. This type of mimicry was named after the English naturalist Henry W. Bates, who studied it in the 1800’s. Bates observed that some harmless species have coloring and behavior that make them look like a dangerous or bad-tasting animal. A predator spotting such a species may mistake it for the undesirable animal and leave it alone. For example, some species of harmless king snakes feature coloration that helps them to resemble poisonous coral snakes. For this reason, birds and other predators may avoid the harmless snakes. See Protective coloration.
Escaping by flight.
Many animals run away from an attacker. Antelope sprint away at high speed when charged by a lion or a cheetah. Many animals stay near safe places, such as burrows, and run to them if attacked. The octopus squirts a black inky fluid to conceal itself and then quickly swims for safety.
Armor.
Some species have a hard shell or covering that is used as armor against predators. Clams pull back into their shells when a predator approaches. Many turtles can pull in their head, legs, and tail when attacked. Armadillos and pangolins are covered by hard, bony plates. When frightened, these animals roll into a tight ball that is difficult for attackers to penetrate.
Playing dead.
A few species sometimes fool predators by lying motionless and appearing to be dead. If the predator does not deliver a killing blow or bite, then the animal may have a chance to escape. A threatened opossum goes limp. The hognose snake flips onto its back when a predator approaches.
Giving up a body part.
Many animals break off a nonessential part of their body when attacked. The glass lizard breaks off its tail, which flops about and attracts the attention of the predator. While the attacker struggles with the tail, the lizard escapes. In most cases, the lost body part grows back quickly.
Fighting.
Many animals have special weapons for fighting predators. The sharp hooves of a moose or the claws of an ostrich can rip open an attacker. Porcupines have long, sharp quills on their back, sides, and tail. These animals strike attackers with their quilled tails. The quills come out easily and stick in the attackers. Bees and wasps sting animals that approach their nests.
Chemical defenses.
A number of animals use special chemicals for defense. Hagfish and one kind of starfish give off huge quantities of slime when disturbed. The bombardier beetle squirts irritating chemicals at an attacker. Some cobras spit blinding venom at the eyes of attackers. Skunks spray foul-smelling chemicals. Birds from New Guinea called hooded pitohuis have poisonous feathers and skin.
How animals reproduce
All types of animals reproduce. Many animals have special organs that are used in reproduction. These organs are called gonads. Some simple animals do not have gonads, but they are still able to reproduce. The various methods used by living things to reproduce are described in detail in the Reproduction article.
There are two general forms of animal reproduction: (1) asexual reproduction and (2) sexual reproduction. In asexual reproduction, only one parent produces the offspring. In sexual reproduction, two parents of opposite sexes are needed to produce the offspring. Many of the simplest animals, including sponges, sea anemones, and some flatworms, reproduce asexually most of the time. Sometimes, they reproduce sexually as well. Most other kinds of animals reproduce only sexually.
Asexual reproduction.
Planarians and some other flatworms can reproduce by fragmentation, the division of the body into two or more pieces. When a planarian reproduces asexually, it typically divides into two sections, one with the head and the other with the tail. Each section then grows the parts that are missing and becomes a complete new individual.
Hydras and some sea anemones reproduce by budding. The animal produces small projections, called buds, from its side. These buds develop into miniature copies of the parent. The buds eventually detach from the parent, and the individuals produced by budding grow to be as large as their parents. Then they can put forth buds to create their own offspring.
Sexual reproduction.
Most animals that reproduce only sexually do so with special sex cells known as gametes. Female sex cells are called eggs and are produced in the female gonads, the ovaries. The male sex cells are known as sperm and are made in the male gonads, the testes. Sperm are much smaller than eggs and have a tail that enables them to swim toward eggs. When a sperm cell unites with an egg cell, a new animal starts to form. The process in which the sperm unites with the egg is called fertilization.
External fertilization
occurs outside an animal’s body. Many aquatic animals reproduce sexually without ever meeting. Female sea urchins release millions of egg cells directly into the water. About the same time, the males release their sperm. The sperm swim through the water, and some unite with eggs, leading to fertilization. The fertilized eggs develop into swimming offspring, which are called larvae. The larvae grow and eventually sink to the bottom of the sea, where they become small sea urchins with bodies similar to those of their parents.
Internal fertilization
occurs within an animal’s body. If gametes are released on land, they dry up and die. Consequently, land-dwelling animals that reproduce sexually have developed ways for fertilization to take place inside their bodies.
Animals mate in many ways. Males of such species as snakes, lizards, birds, and mammals mate by releasing sperm directly into an opening in the female’s body. Fertilization occurs in the female’s reproductive organs.
Male salamanders do not release sperm directly into the female’s body. Instead, they deposit a packet of sperm at the bottom of a stream or pond. When the female passes over the sperm, she draws them into an opening in her body that leads to her reproductive organs. Several other animals, including mites and scorpions, mate in a manner similar to that of salamanders. Males deposit packets of sperm on the ground, which are then picked up by females.
In almost all mammals and some reptiles, the embryo (undeveloped animal) grows inside the female’s body after fertilization. However, in birds and some reptiles, the embryo develops outside the body. The female lays an egg in which the embryo develops. See Fertilization.
Courtship behavior
consists of actions that help animals find and choose suitable mates. This behavior tends to follow a specific pattern according to species. As a result, courtship behavior helps ensure that animals mate with members of their own species. If two different species mate, they may not produce young, or their offspring may be unhealthy or unable to reproduce. Such courtship behaviors as singing and displaying colors help animals recognize their own species.
Animal mates find each other in a number of ways. Female birds are attracted to the beautiful songs and bright feathers of males. Female grasshoppers, cicadas, bullfrogs, and toads also are attracted to the calls made by males of their species. Female silkworm moths release into the air a perfumelike chemical called a pheromone to attract males from as far away as several miles or kilometers. At certain times of the year, female dogs give off a pheromone that attracts male dogs. Female fireflies watch for male fireflies that flash their lights in a certain rhythmic pattern. Male fence lizards bob their heads rhythmically when a female approaches. Siamese fightingfish perform a complicated courtship dance, followed by the release of eggs and sperm into the water.
Some animals choose particular mates. The female anole lizard typically prefers to mate with the largest male. The peacock spreads his fantastic tail feathers, hoping to coax a peahen into becoming his mate. Peahens choose males with many spots on their tail feathers. Male birds of paradise gather in a tree. When a female appears, the brilliantly colored males strut and dance to show off their bright feathers. If a female chooses to watch this display, she will usually mate with the male that has the brightest colors.
Male bowerbirds build chambers or runways, called bowers, made of sticks or other material. They decorate these structures with brightly colored stones, bones, or other objects. The male dances and bows in front of his bower, hoping that a passing female will accept him as a mate. If one does, she enters the bower with him, and they mate there.
Some male animals give food to possible mates. A male tern catches a fish and places it into the mouth of the female he wants for his mate. A male dance fly brings a dead insect to a female. She eats the insect while mating with the male. A male that does not bring a dead insect risks being eaten by the female.
Mating is dangerous for some male spiders and insects. In certain species of black widow spiders, females sometimes eat the males after mating. A female praying mantis may pounce unexpectedly on a male in her vicinity. Sometimes, she mates with a male and then eats him.
Regeneration.
Some kinds of animals, mostly simple animals, can replace lost body parts by regeneration. If a sponge is broken into small pieces, some of the fragments will grow into new sponges. Earthworms and their marine relatives can regenerate their heads or tails if those parts are broken off. Crabs and lobsters can grow new claws. Sea cucumbers sometimes throw out their intestines and other internal body parts to distract attackers. New parts grow back quickly.
Even some vertebrates can regenerate parts of their bodies. A salamander that loses a leg will grow a new one. Many salamanders can break off their tails to escape the grip of a predator. These animals soon grow new tails. Mammals can regenerate hair, nails, and some other body tissues.
How animals raise their young
The newborn young of many species need no care from their parents. Even from birth, they can move about and find food on their own. The young of other species need parental care for some time after birth. One or both parents provide them with food and protection until they are old enough to manage for themselves.
Most kinds of animals never see their parents. For example, clams and many other invertebrates release their eggs and sperm into the water, where fertilization takes place. Carried around by ocean currents, the young of these animals may travel far from where their parents live. The female leatherback turtle swims thousands of miles or kilometers in the ocean to tropical beaches. She then digs a hole on the beach and lays her eggs. The eggs hatch in the warm sand after the female has returned to the sea.
Providing food
is one of the main ways animals care for their young. Even females who never see their offspring provide them with food. The female’s eggs contain yolk and other nourishing substances that serve as food for the developing embryos. Female sea urchins and herring produce vast numbers of small eggs, each of which has little yolk. Offspring from these eggs are extremely tiny when they hatch and must find their own food to grow. Their chance for survival is relatively small. Female birds, on the other hand, lay only a few eggs, each with large amounts of yolk. Offspring from these eggs are relatively large and have a higher chance of survival.
Some animals that do not see their offspring provide their young with food in addition to that in the egg. Many flies lay their eggs on rotting fruits, which supply the young flies with food. The female digger wasp lays her egg on a grasshopper that she has stung, paralyzed, and buried. After hatching, her offspring feeds on the grasshopper. The female dung beetle finds fresh dung (manure), rolls a piece into a ball, and then buries it. She lays her egg on the dung ball. After hatching, the young beetle feeds on the dung.
Mammals nurse their babies—that is, they feed them on the mother’s milk. The nursing period lasts only a few weeks in mice, hares, and many other species. But among some larger mammals, such as elephants and rhinoceroses, the young may nurse several years before they are weaned—that is, taken off the mother’s milk.
Incubation.
In many species, the mother and sometimes the father remain with their eggs and young. Birds incubate their eggs by sitting on them in a nest. Incubation keeps the eggs warm and helps the embryo inside to develop quickly into a young bird. After the eggs hatch, the parents may make many hunting trips each day, trying to catch enough insects to feed the hungry nestlings (young birds). When the young are old enough to hunt, they leave the nest and fly away.
Among many species of birds, including pigeons and starlings, the parents take turns incubating the eggs. Among ducks, geese, and some other birds, the females are the only incubators. In most species of hornbills, the female even imprisons herself inside a walled-up nest chamber to incubate eggs. The male passes food to the female through a tiny slit in the wall. In a few species of birds, the male does all the incubating. For example, a female emperor penguin lays a single egg, which the male then incubates on top of his toes. He tucks his toes and the egg under the fluffy feathers of his belly. When the egg hatches, the little penguin stays warm and grows in this cozy “nest.”
Female pythons also incubate their eggs. They produce the heat to warm their eggs by twitching their muscles, much as people do when shivering. After the baby pythons hatch, they must find food and shelter on their own.
Providing shelter.
Some species provide shelter for their young. A female lizard may lay her eggs in an underground nest, where they are hidden from predators. The huge nests of sociable weavers, a type of African bird, protect the baby birds from bad weather and predators. Some frogs and fish build nests for their eggs and young. A few tropical frogs carry their tadpoles around on their backs until they find a safe pool of water for the young frogs.
Parents sometimes provide shelter for their offspring within their own bodies. The male seahorse carries the female’s eggs in a pouch. When the young seahorses hatch, the male releases them from the pouch. Female kangaroos, koalas, opossums, wallabies, and other marsupials give birth to tiny, poorly developed offspring. The babies mature in a pouch on the mother’s abdomen. There, they nurse and are protected by the mother. One kind of Australian frog swallows her eggs into her stomach, where they develop. After the eggs hatch, the female opens her mouth, and tadpoles and small froglets come out.
Providing protection.
Parents often protect their young from predators. A male stickleback fish will attack any predatory fish or insect that approaches its young. A female scorpion carries her babies on her back and defends them with the poisonous sting on the tip of her long tail. Female crocodiles guard their nests and will fight any predator that comes near. As young crocodiles begin to hatch, they cry out, and the female helps them dig out of the nest. She then gently picks them up in her jaws and carries them to a nearby pond. A female bear will sometimes attack hikers who venture too close to her cubs. A female pet dog may attack even her owner if she fears that her puppies are threatened.
Group care.
Some animals live together in groups of several families. As many as a hundred pairs of sociable weavers raise their chicks together in a large nest. Several female lions may care for their young cubs together. Naked mole-rats live in underground colonies. Only one female produces offspring. The other females help tend the young. Many monkeys and baboons live in small groups. All the adults in a group will work together to defend their young from an attacking leopard. When attacked by a wolf, a herd of musk oxen will protect their calves by placing them between adults.
Learning and play.
Young animals may learn many things about the world from their parents. By watching what foods its parents eat and reject, a young animal can learn to recognize the kinds of foods that are safe. If young animals see their mother show fear of another type of animal or of certain locations, they learn to avoid those animals and places. Thus, they learn which types of animals, foods, and environments are safe and which are dangerous.
Many animals play while they are young. Lion cubs may try to pounce on the twitching tail of an adult lion. They also play with one another as though they were fighting. Such games help young animals develop coordination and strength. Play also helps them learn how to defend themselves and to fight effectively. In addition, it enables some animals to learn how to stalk and capture prey.
Animal homes and communities
Animals’ homes provide shelter from harsh weather or protection against predators. Some animals have shelters that they use only once. Others make homes where they live for many years. However, a number of animals, such as fish that live in the ocean, spend their whole lives moving about. They never have homes.
A number of animals use caves, cracks in the ground, logs, plants, or rocks as temporary shelter. Garter snakes and many insects spend the night under rocks but leave this shelter the next day to hunt for food.
Some animals build their homes. Field mice collect dried grass and then construct a small nest under a protective log. Many birds and squirrels collect grass and twigs to build nests in the trees or on the ground. Gophers and moles dig burrows in the soil.
Home ranges.
Most animals live within certain areas that form their home range. An animal’s home range includes all the resources an animal needs to survive. By living within a specific area, an animal can learn where best to find food or shelter there.
The size of an animal’s home range depends typically on the animal’s size. Crickets and sea urchins have small home ranges. But elephants and lions may have home ranges that cover vast distances. Big animals require extensive home ranges to obtain the large amounts of food they need to survive.
Some animals defend their home ranges from other animals. A defended home range is called a territory. The song of a warbler, the hoot of an owl, or the roar of a lion warns other animals of their kind to stay away. Some animals use chemical warnings rather than sounds to ward off invaders from their own species. Intruders can easily smell the urine of wolves and the scent marks of cats and hyenas and know that a territory is already occupied. Often the intruder leaves without a fight. Sometimes, however, fights break out over territory, resulting in injury or death.
Group living.
Many animals live in groups. Some groups, such as herds of elephants, remain together for many years. Others are small families that come together only during the breeding season. A mother and father bird may cooperate in raising their nestlings but may separate when the young leave the nest.
Wolf packs and some other animal groups have a social order called a dominance hierarchy. In such groups, every member has a certain rank in the hierarchy. High-ranking members are called dominant individuals, and low-ranking ones are known as subordinate individuals. The dominant individuals have first choice of such resources as food and water. They also have their pick of mates. See Dominance.
Some groups are large and complex. Ants, bees, and termites live in huge colonies that consist of many thousands or even millions of individuals. The individuals in these colonies often have specific tasks. With honey bees, the queen bee is responsible for producing eggs. Workers search for pollen, make honey, and feed and care for the queen and her offspring. Drones do little but fertilize the queen’s eggs.
Fish may form large schools in the open ocean. Herring schools may consist of hundreds of millions of fish.
Animal migration
Why animals migrate.
The environment of some animals becomes extremely harsh at certain times of the year. In winter, for example, high mountains become bitterly cold. Snow and ice cover the peaks, and food becomes hard for animals to find. Some animals survive by hibernating. Others travel to places where the weather is milder and more food is available. The next spring, these animals return home. This type of regular round-trip journey is called a migration. Many animals that migrate live in the mountains or far from the equator. Migrating animals usually travel in large groups.
Animals migrate for other reasons than to escape cold weather. Some travel to favorite feeding areas or to special places to produce their young.
Animal travelers.
Many birds make seasonal migrations. Some simply move short distances from the mountains to the valleys below. Others make remarkable long-distance journeys. In the fall, huge flocks of ducks and geese fly south for the winter. European white storks spend the spring and summer in northern Europe, where they breed and raise their young. They fly as far south as southern Africa for the winter.
The Arctic tern is the champion long-distance traveler. Terns breed on islands in the Arctic Ocean. In late summer, they begin a long journey and fly all the way to Antarctica. They feed on the fish that are plentiful there before flying north to the Arctic to breed the following summer. A tern making this round trip may fly as many as 22,000 miles (35,400 kilometers).
Humpback whales and blue whales also make long migrations. They spend the summer in polar oceans, which have plentiful food. In the autumn, they swim toward the equator until they reach the warm tropical seas. There, the females that are pregnant give birth. Others mate and then give birth the next year. The warm waters provide a comfortable environment for the babies. The whales spend the winter in the tropics before returning to the polar feeding area in the spring.
Monarch butterflies and many other insects also migrate. When winter approaches, swarms of monarch butterflies travel from Canada and the northern United States to California and Florida. Some even fly as far south as southern Mexico. The butterflies begin the return trip in the spring, but few of the adults that flew south live long enough to complete it. Female monarchs lay eggs along the way back. The offspring, after maturing, continue the northward journey.
Some animals travel long distances to find a breeding site. The green sea turtle feeds along the east coast of South America. It then swims 1,200 miles (1,900 kilometers) of open ocean to breed on Ascension Island, a small island in the middle of the Atlantic Ocean. When the baby turtles hatch, they swim to South America, where they may remain for many years. When they are mature, they swim back to Ascension Island and breed.
Most salmon live for years in ocean waters. When the time comes for them to spawn (lay their eggs), they travel thousands of miles or kilometers. The salmon swim to inland waters, where they produce their young. The adult salmon die before the young hatch.
Dangers of migration.
Migrating animals may face a number of dangers, including new predators, during their long journeys. Some dangers come from human beings. For example, the fences that farmers use to corral their livestock prevent antelope from making their seasonal migrations. Farmers often shoot migrating animals that stop to feed on their crops. The draining of wetlands makes it harder for ducks and geese to find a safe place to rest and feed during migration. Some winter feeding areas are also being destroyed.
The origin and development of animals
Most scientists believe that all plant and animal species developed from a single form of life that arose about 31/2 billion years ago. The basic life form gradually changed so that through the centuries, millions of species have come into being. Some kinds are still alive. Others are extinct (no longer living). All species, whether living or extinct, are related to one another.
This set of ideas about how species change over time is called the theory of evolution. The theory is supported by a vast amount of evidence from many fields, and nearly all scientists consider the occurrence of evolution to be a scientific fact. However, some people reject the concept of evolution because it conflicts with their religious beliefs. The Biblical account of the Creation, for example, says that God took only a few days to create all living things essentially as they exist today.
This section uses evolutionary theory as the basis of a discussion of when some animals originated and how species change. For a discussion of religion and evolution and for more information on evolution, see Evolution.
When animals appeared on Earth.
Scientists believe that Earth formed as a planet at least 41/2 billion years ago. The first life forms were simple, single-celled organisms that appeared about 1 billion years later. Animals and plants did not appear until billions of years later. The first animals appeared more than 650 million years ago. The earliest animals were invertebrates. The first vertebrates—fish—developed about 500 million years ago, and the first mammals appeared more than 200 million years ago.
Another way of looking at these times is to imagine the history of life on Earth in terms of a single year. Start with the formation of Earth on New Year’s Day, January 1. Simple single-celled organisms were the first types of living things. They would not appear until March 22. Animals would not show up until November 9. Fish would evolve from their invertebrate ancestors about November 20. Mammals would appear on December 16. Monkeys and apes would not be found until December 28. Human beings would appear only a few minutes before the end of the year, on December 31.
For more information on when various types of animals appeared on Earth, see Earth (History of Earth).
How new species are formed.
Scientists consider groups of animals to represent distinct species when they become so different that they cannot produce fertile offspring together. Imagine a group of birds that lived only on one island. Then imagine that a few individuals got lost in a storm and landed on a different island. The two groups, now separated from each other, may gradually develop different traits as they adapt to different environments. If they become dissimilar enough, they cannot produce fertile offspring if they mate. They are then two separate species. This process can repeat itself many times over many millions of years, resulting in great numbers of species.
How species change.
The individuals of any given species are not the same. Some individuals are larger, some are darker, some tolerate heat better, and some are stronger. Some individuals have traits that make them better able than others of their species to survive and reproduce in their environment. Over long periods, those animals will produce more young that survive than will individuals with less desirable traits. The offspring of the better-suited species will probably share some of the desirable traits of their parents. For example, dark moths will be well hidden in a shady forest. More of their offspring will probably survive than will those of lighter moths, which may be easily seen and eaten by hungry predators. In the next generation, more moths in the forest will be dark. This process, which causes the traits of animal groups in nature to change through time, is called natural selection. See Natural selection.
Why species become extinct.
Scientists estimate that, left to natural processes, most species of animals live 1 million to 10 million years before becoming extinct. Natural causes that lead to the extinction of animals include drastic changes in climate and failure of a species to compete with other animals for food. For example, the dinosaurs died out rather suddenly about 65 million years ago. Many scientists believe that dinosaurs became extinct because of a rapid change in climate caused by the impact of a large asteroid. The dinosaurs were unable to survive in these new conditions.
Some human activities also cause animals to become extinct. Such activities are discussed in the next section of this article. For more information on why animals become extinct, see Extinction.
The future of animals
Most scientists believe that we are living in a period of mass extinction. Hundreds of species of vertebrate animals have become extinct during the last 200 years. Most of these species became extinct as a result of human activities. Thousands of other species have become endangered (in danger of going extinct). Today, however, more and more people around the world are working to preserve the variety of animal life for future generations.
How human beings endanger animals
Destruction of habitat.
When people build cities or cut down forests to obtain wood or to clear land for farming or grazing, they destroy the habitats that animals need to survive. For example, grizzly bears and mountain lions once roamed freely where the city of San Francisco now stands. But a wild grizzly bear or mountain lion could not survive in San Francisco today.
The habitats of animals in tropical forests are particularly threatened today. People are rapidly cutting down these forests to obtain such valuable hardwoods as mahogany and teak. They also are clearing the land to plant crops or to create rangeland. However, soils in such areas are not very fertile, and farms there produce crops for only a few years. To continue farming in such areas, people have to keep cutting down more of the forests to create new farmland. By the early 2000’s, much of the world’s tropical forests had already been destroyed.
Many scientists and other people are especially concerned about the destruction of tropical forests. They point out that these forests have more biodiversity—that is, a greater variety of plant and animal species—than any other place. One square mile (2.6 square kilometers) of forest in South America may have more species of birds and insects than many countries do. In fact, biologists discovered a single tree in a tropical forest in Brazil that supported 82 species of ants. That is about twice as many ant species as live in the entire United Kingdom.
Even though many types of plant and animal life can be found in one place in the tropics, the total range of some tropical species is extremely small. As a result, when a large area of forest is cleared, all the members of some species are killed.
Pollution
can also destroy animals and their habitats. Agricultural chemicals and industrial wastes sometimes drain into ponds and streams and kill the plants and animals there. Factories, power plants, and vehicles create air pollution by burning such fossil fuels as coal and oil. Such pollution has seriously damaged forests and wildlife. Acid rain—rainfall with a high concentration of sulfuric and nitric acids due to air pollution—kills fish and other animals.
An increase in carbon dioxide in the atmosphere presents a long-term threat to animals and habitats. Many factories, power plants, and vehicles release carbon dioxide into the air. Forest trees and plants help absorb this gas, but as more of them are cut down, carbon dioxide levels rise. Most scientists believe that higher amounts of carbon dioxide in the atmosphere cause global warming through a phenomenon called the greenhouse effect. Rapid global warming could destroy many kinds of plants and animals and flood large areas. See Global warming.
Introduction of new species
into an area can have harmful consequences. Such species may spread rapidly in their new environment and become invasive. In the mid-1800’s, for example, people introduced rabbits into the wild in Australia for hunting. But the rabbits had no natural predators there, and their population grew quickly, spreading over most of the continent. Partly because of the rapid spread of rabbits, rabbit-eared bandicoots, which are native to Australia, disappeared from many areas of the continent. These bandicoots had to compete with rabbits for burrow space. The traps and poisons people set out for rabbits also killed the bandicoots.
Human beings may unintentionally introduce invasive species to an area. Zebra mussels are shellfish native to the Caspian Sea region between Europe and Asia. During the late 1900’s, zebra mussel larvae were unintentionally released into the North American Great Lakes in ballast water, the water kept in a ship’s hold to keep the vessel stable. Today, the mussels are a major pest in North America. The explosive growth of zebra mussels may threaten the food supply of many fish and shellfish species native to the Great Lakes. See Zebra mussel.
Hunting and fishing.
In many parts of the world, people depend on native animals for food or medicine. However, overhunting and overfishing has led to the decline and even extinction of various animal species. Such activities have proven especially destructive during the past 200 years. Overhunting contributed to the extinction of such animals as the great auk, the passenger pigeon, and the Steller’s sea cow. Formerly plentiful fish species, such as the Atlantic cod, have become threatened partly due to overfishing.
Human population growth.
The human population is growing rapidly. Such growth may place additional pressure on natural habitats and their species. People will need more land for food and housing. Industrial activities will probably increase to process the food and manufacture the goods needed by the growing population. Many such activities cause pollution that can damage or destroy habitats. Also, increased global trade and travel will spread more and more invasive species.
For more information on why animals become endangered, see Endangered species.
How human beings protect animals
Since the late 1800’s, people have become increasingly concerned about vanishing wildlife. Such concerns have resulted in part from a growing awareness of the interconnectedness of species—the web of life. More people now recognize that the disappearance of large numbers of species threatens the survival of other living things, including human beings. People who help protect habitats and animals are called conservationists.
Protected areas.
Many countries have created national parks, game reserves, and wildlife refuges. In these areas, habitats are protected from development and hunting is banned. Many conservationists believe such areas may represent the last hope for saving some threatened species in the wild. Yellowstone National Park in the United States provides a home for rare grizzly bears, bison, bald eagles, and trumpeter swans. The African elephant and black rhinoceros are protected in parks and reserves in African savannas. Unfortunately, however, many protected areas are too small to ensure the survival of certain large species within their borders. Large animals often need lots of room to roam.
Laws
protect wildlife in many countries. Under the U.S. Endangered Species Act of 1973, officials keep an up-to-date list of species in danger of extinction. The act prohibits federal projects that would destroy the habitat of an endangered species. This act was later amended to require anyone who wants to develop or change a habitat occupied by an endangered species to show that the planned changes will not harm that species.
Government agencies also determine the number of certain game animals that can be hunted and fished each season. If an animal starts to become rare, the agencies can reduce the number of that species that can be taken legally. The population of the species then has an opportunity to recover.
Habitat restoration.
Humans destroy habitats, but they also try to restore them. Trees cleared by logging can be replanted, re-creating habitat for forest creatures. People can remove dams from rivers and restore wetlands. Parks in cities can provide homes for wildlife.
Breeding in captivity.
Some species have become so rare that scientists believe the only hope of saving them is to breed them in captivity. For example, many of the surviving California condors live in zoos in the United States. A condor chick raised in captivity has a better chance of survival than one in nature does. As the number of condors has grown, biologists have reintroduced some of the birds back into the wild. Other endangered species being bred in captivity include the Arabian oryx and the whooping crane.
Despite conservation efforts, the future of wildlife remains uncertain. The human population continues to grow. Forests and grasslands are still being destroyed. People continue to hunt African elephants, snow leopards, and other vanishing species. Air pollution, acid rain, water pollution, and climate change also still threaten wild animals.
A classification of the animal kingdom
Scientists classify animals chiefly according to the animals’ ancestry. Those with a common ancestor nearer in time are more closely related than those who share an ancestor further back in time. Closely related animals share certain unique features, some of which occur only during early stages in their life cycle. Scientists arrange animals into major groups called phyla (singular phylum). The table A classification of the animal kingdom with this article lists some of the phyla and certain important features of their members.
