Many forms of behavior help animals to survive severe environmental conditions. Two examples are hibernation, which enables animals to survive cold and food shortages in winter; and estivation, which allows animals to survive drought and heat in summer. True hibernators, such as brown bears and some rodents, become completely inactive during winter, and their body temperature falls close to freezing. While in this state, they survive entirely on food reserves stored in their bodies. Estivating animals, which include land snails and some amphibians, seal themselves up when conditions become dry and only become active again when it rains. Between these two extremes, many other animals show less drastic patterns of behavior that are triggered by cold or heat. Winter wrens, for example, often crowd together for sleep when temperatures fall below freezing. On warmer nights, they sleep on their own.

Special forms of behavior also help animals to find food, to avoid being eaten, and to protect their young. One of the most advanced forms of this behavior is the use of tools. Several kinds of animals, particularly primates and birds, pick up implements such as twigs and stones and use them to get at food. More rarely, some tool-using animals seek out a particular object and then shape it so that it can be used. Woodpecker finches probe for insect grubs by making tools from cactus spines, and chimpanzees sometimes dig for termites using specially prepared twigs.

Defensive behavior is exhibited by individual animals and also by animal groups. Group defense is common in herding mammals, particularly in species such as the musk-ox, which form a protective ring around their calves when threatened by wolves. It can also be seen in swallows, starlings, and other songbirds, which instinctively mob hawks and other birds of prey. By grouping together to harass their enemies, they reduce the chances that they or their young will be singled out and attacked.

Individual defensive behavior is often based on threatening gestures that make an animal look larger or more dangerous than it actually is. Sometimes it involves some highly specialized forms of deception. One of the most remarkable is playing dead. Seen in animals such as the Virginia opossum and some snakes, this last-ditch defense is effective against predators that habitually hunt moving prey but leave dead animals alone. After the predator has inspected the "dead" animal and moved on, the prey comes back to life and makes its escape.

IX		Origins of Animals
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Most biologists agree that animals evolved from simpler single-celled organisms. Exactly how this happened is unclear, because few fossils have been left to record the sequence of events. Faced with this lack of fossil evidence, researchers have attempted to piece together animal origins by examining the single-celled organisms alive today.

Modern single-celled organisms are classified into two kingdoms: the prokaryotes and protists. Prokaryotes, which include bacteria, are very simple organisms, and lack many of the features seen in animal cells. Protists, on the other hand, are more complex, and their cells contain all the specialized structures, or organelles, found in the cells of animals. One protist group, the choanoflagellates or collar flagellates, contains organisms that bear a striking resemblance to cells that are found in sponges. Most choanoflagellates live on their own, but significantly, some form permanent groups or colonies.

This tendency to form colonies is widely believed to have been an important stepping stone on the path to animal life. The next step in evolution would have involved a transition from colonies of independent cells to colonies containing specialized cells that were dependent on each other for survival. Once this development had occurred, such colonies would have effectively become single organisms. Increasing specialization among groups of cells could then have created tissues, triggering the long and complex evolution of animal bodies.

This conjectural sequence of events probably occurred along several parallel paths. One path led to the sponges, which retain a collection of primitive features that sets them apart from all animals. Another path led to two major subdivisions of the animal kingdom: the protostomes, which include arthropods, annelid worms, mollusks, and cnidarians; and the deuterostomes, which include echinoderms and chordates. Protostomes and deuterostomes differ fundamentally in the way they develop as embryos, strongly suggesting that they split from each other a long time ago.

Animal life first appeared perhaps a billion years ago, but for a long time after this, the fossil record remains almost blank. Fossils exist that seem to show burrows and other indirect evidence for animal life, but the first direct evidence of animals themselves appears about 650 million years ago, toward the end of the Precambrian period. At this time, the animal kingdom stood on the threshold of a great explosion in diversity. By the end of the Cambrian Period, 150 million years later, all of the main types of animal life existing today had become established.

A

Moving onto Land

When the first animals evolved, dry land was probably devoid of any kind of life, except possibly bacteria. Without terrestrial plants, land-based animals would have had nothing to eat. But when plants took up life on land over 400 million years ago, that situation changed, and animals evolved that could make use of this new source of food. The first land animals included primitive wingless insects and probably a range of soft-bodied invertebrates that have not left fossil remains. The first vertebrates to move onto land were the amphibians, which appeared about 370 million years ago.

For all animals, life on land involved meeting some major challenges. Foremost among these were the need to conserve water and the need to extract oxygen from the air. Another problem concerned the effects of gravity. Water buoys up living things, but air, which is 750 times less dense than water, generates almost no buoyancy at all. To function effectively on land, animals needed support.

In soft-bodied land animals such as earthworms, this support is provided by a hydrostatic skeleton, which works by internal pressure. The animal's body fluids press out against its skin, giving the animal its shape. In insects and other arthropods, support is provided by the exoskeleton (external skeleton), while in vertebrates it is provided by bones. Exoskeletons can play a double role by helping animals to conserve water, but they have one important disadvantage: unlike an internal bony skeleton, their weight increases very rapidly as they get bigger, eventually making them too heavy to move. This explains why insects have all remained relatively small, while some vertebrates have reached very large sizes.

B

Speciation and Extinction

Like other living things, animals evolve by adapting to and exploiting their surroundings. In the billion-year history of animal life, this process has created vast numbers of new species, each capable of using resources in a slightly different way. Some of these species are alive today, but these are a minority; an even greater number are extinct, having lost the struggle for survival.

Speciation, the birth of new species, usually occurs when a group of living things becomes isolated from others of their kind (see Species and Speciation). Once this has occurred, the members of the group follow their own evolutionary path and adapt in ways that make them increasingly distinct. After a long period—typically thousands of years—their unique features mean that they can no longer breed with their former relatives. At this point, a new species comes into being.

In animals, this isolation can come about in several different ways. The simplest form, geographical isolation, occurs when members of an original species become separated by a physical barrier. One example of such a barrier is the open sea, which isolates animals that have been accidentally stranded on remote islands. As the new arrivals adapt to their adopted home, they become more and more distinct from their mainland relatives. Sometimes the result is a burst of adaptive radiation, which produces a number of different species. In the Hawaiian Islands, for example, 22 species of honeycreepers have evolved from a single pioneering species of finch-like bird.

Another type of isolation is thought to occur where there is no physical separation. In this case, differences in behavior, such as mate selection, may sometimes help to split a single species into distinct groups. If the differences persist for a long enough time, new species are created.

The fate of a new species depends very much on the environment in which it evolved. If the environment is stable and no new competitors appear on the scene, an animal species may change very little in hundreds of thousands of years. But if the environment changes rapidly and competitors arrive from outside, the struggle for survival is much more intense. In these conditions, either a species changes, or it eventually becomes extinct.

During the history of animal life, on at least five occasions, sudden environmental change has triggered simultaneous extinction on a massive scale. One of these mass extinctions occurred at the end of the Cretaceous Period, about 65 million years ago, killing all dinosaurs and perhaps two-thirds of marine species. An even greater mass extinction took place at the end of the Permian Period, about 200 million years ago. Many biologists believe that we are at present living in a sixth period of mass extinction, this time triggered by human beings.

X		Animals in the Balance of Nature
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Compared to plants, animals make up only a small part of the total mass of living matter on earth. Despite this, they play an important part in shaping and maintaining natural environments.

Many habitats are directly influenced by the way animals live. Grasslands, for example, exist partly because grasses and grazing animals have evolved a close partnership, which prevents other plants from taking hold. Tropical forests also owe their existence to animals, because most of their trees rely on animals to distribute their pollen and seeds. Soil is partly the result of animal activity, because earthworms and other invertebrates help to break down dead remains and recycle the nutrients that they contain. Without its animal life, the soil would soon become compacted and infertile.

By preying on each other, animals also help to keep their own numbers in check. This prevents abrupt population peaks and crashes and helps to give living systems a built-in stability. On a global scale, animals also influence some of the nutrient cycles on which almost all life depends. They distribute essential mineral elements in their waste, and they help to replenish the atmosphere's carbon dioxide when they breathe. This carbon dioxide is then used by plants as they grow.

A

Animals and People

Until relatively recently in human history, people existed as nomadic hunter-gatherers. They used animals primarily as a source of food and also for raw materials that could be used for making tools and clothes. By today's standards, hunter-gatherers were equipped with rudimentary weapons, but they still had a major impact on the numbers of some species. Many scientists believe, for example, that humans were involved in a cluster of extinctions that occurred about 12,000 years ago in North America. In less than a millennium, two-thirds of the continent's large mammal species disappeared.

This simple relationship between people and animals changed with domestication, which also began about 12,000 years ago. Instead of being actively hunted, domesticated animals were slowly brought under human control. Some were kept for food or for clothing, others for muscle power, and some simply for companionship.

The first animal to be domesticated was almost certainly the dog, which was bred from wolves. It was followed by species such as the cat, horse, camel, llama, and aurochs (a species of wild cattle), and also by the Asian jungle fowl, which is the ancestor of today's chickens. Through selective breeding, each of these animals has been turned into forms that are particularly suitable for human use. Today, many domesticated animals, including chickens, vastly outnumber their wild counterparts. In some cases, such as the horse, the original wild species has died out altogether.

Over the centuries, many domesticated animals have been introduced into different parts of the world only to escape and establish themselves in the wild. Together with stowaway pests such as rats, these "feral" animals have often had a highly damaging effect on native wildlife. Cats, for example, have inflicted great damage on Australia's smaller marsupials, and feral pigs and goats continue to be serious problems for the native wildlife of the Galápagos Islands.

Despite the growth of domestication, humans continue to hunt some wild animals. Some forms of hunting are carried out mainly for sport, but others provide food or animal products. Until recently, one of the most significant of these forms of hunting was whaling, which reduced many whale stocks to the brink of extinction. Today, highly efficient sea fishing threatens some species of fish with the same fate (see Fisheries).

Since the beginning of agriculture, the human population has increased by more than two thousand times. To provide the land needed for growing food and housing people, large areas of the earth's landscapes have been completely transformed. Forests have been cut down, wetlands drained, and deserts irrigated, reducing these natural habitats to a fraction of their former extent.

Some species of animals have managed to adapt to these changes. A few, such as the brown rat, raccoon, and house sparrow, have benefited by exploiting the new opportunities that have opened up and have successfully taken up life on farms, or in towns and cities. But most animals have specialized ways of life that make them dependent on a particular kind of habitat. With the destruction of their habitats, their number inevitably declines.

In the 20th century, animals have also had to face additional threats from human activities. Foremost among these are environmental pollution and also the increasing demand for resources such as timber and fresh water. For some animals, the combination of these changes has proved so damaging that their numbers are now below the level needed to guarantee survival.

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Across the world, efforts are currently underway to address this urgent problem (see Endangered Species). In the most extreme cases, gravely threatened animals can be helped by taking them into captivity and then releasing them once breeding programs have increased their number. One species that has been saved in this way is the Hawaiian mountain goose or nēnē. In 1951, its population had been reduced to just 33 birds. Captive breeding has since increased the population to over 2500, removing the immediate threat of extinction.

While captive breeding is a useful emergency measure, it cannot assure the long-term survival of a species. Today animal protection focuses primarily on the preservation of entire habitats, an approach that maintains the necessary links between the different species the habitats support. With the continued growth in the world's human population, habitat preservation will require a sustained reduction in our use of the world's resources to minimize our impact on the natural world.

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Contributed By: David Burnie, B.S. Professional writer of natural history and science books. Author of How Nature Works.

Further Reading

How to cite this article: "Animal," Microsoft® Encarta® Online Encyclopedia 2004 http://encarta.msn.com © 1997-2004 Microsoft Corporation. All Rights Reserved.

© 1993-2004 Microsoft Corporation. All Rights Reserved.