Understanding Animals: A Comprehensive Introduction to the Animal Kingdom

Animal

Introduction to Animal

An animal is a member of the kingdom Animalia, one of the great kingdoms of living things. Animals are found in all parts of the world—in tropical and polar regions, in deserts and in bodies of water, underground, on mountains, and in the air. Some animals live inside plants, and some live in the bodies of other animals. There are animals so small that they can barely be seen with the unaided eye; at the other extreme are such gigantic creatures as elephants and whales. Many animals possess what can be called intelligence, but none can compare with humans in the quality and degree of such thought processes as insight, reasoning, and imagination.

The study of animals is a branch of biology called zoology.

More than one million living species of animals have been identified, and more are constantly being discovered. Fossils (impressions or other traces, usually found in rock, of organisms that lived in past ages) show that additional millions of species of animals once lived that are now extinct.

The terms "higher" and "lower" are sometimes used in describing animals. These terms refer to the relative position of various animals on the time scale of biological evolution. Biologists believe that, in general, forms of life have evolved from the simpler to the more complex. Animals such as sponges are believed to be similar to the earliest forms of life and therefore lower on the evolutionary scale than animals more complicated in structure, such as the frog. However, there are exceptions to this rule, and some simple animals have evolved from complex ones. To avoid confusion, the terms "higher" and "lower" have been avoided in this article.

For an explanation of the classification system of the animal kingdom, and cross references,

The variety of animal life found in the world today is the result of evolutionary changes that have taken place over millions of years. Fossil evidence indicates that simple animal life appeared about 700 million years ago. By the end of the Cambrian Period, about 500 million years ago, there were marine animals belonging to most of the major groups, or phyla, of animals known today. Among these animals were sponges, corals, jellyfish, worms, mollusks, and arthropods. The appearance of these animals, which occurred over several million years, is called the "Cambrian explosion."

During the next 260 million years, many kinds of fish developed, both in the sea and in freshwater. Also during this time, insects developed; they and other arthropods were the first animals to live on land. Amphibians appeared, becoming the first vertebrates (animals with backbones) to spend part of their lives on land. Reptiles, the first vertebrates capable of living entirely on land, also appeared.

During the following 175 million years, known as the Mesozoic Era, many species of reptiles developed. Among these reptiles were the dinosaurs, the largest land animals that ever existed. Also during this era, birds and mammals came into existence.

As the Cenozoic Era dawned—it began about 65 million years ago and is still continuing—mammals and birds were developing into many different species. Eventually, the primates, including humans, appeared.

Telling Animals From Other Organisms

Most biologists classify organisms into five kingdoms: monera, protists, fungi, plants, and animals. It is not always obvious to which kingdom an organism belongs. For example, some protists (such as slime molds) can move from place to place under their own power—a trait normally associated with animals. However, not all such protists are multicelled (composed of more than one cell), whereas all animals are. Another example is the sponge, which is immobile as an adult and may seem to be a plant. However, biologists place the sponge in the animal kingdom because it is mobile while immature and has a cell structure like an animal. In deciding whether an organism is an animal, biologists consider a number of characteristics, including the following:

• All animals are multicelled, distinguishing them from monera and most protists.
• The cells of animals lack a cell wall, a rigid structure that encloses plant and fungus cells. Animal cells are enclosed in a pliable structure, the cell membrane.
• Most animals can move about under their own power throughout life, and all animals are mobile during at least one phase of their life. Plants and fungi are incapable of moving from one spot to another under their own power.
• Animals obtain food by eating other organisms. Plants manufacture their food from inorganic substances by a process called photosynthesis. Fungi absorb nutrients from decaying organic matter.
• Animals have a greater degree of irritability, or responsiveness, than do other organisms. Most kinds of animals have nervous systems, which enable them to respond rapidly to stimulation.

Animal Bodies

Basic Structure

The fundamental structural unit of all living things is the cell, consisting of a fluid material called cytoplasm, surrounded by a membrane. The cells of an animal are organized into groups called tissues. All the cells of any one kind of tissue are similar in structure, and are specialized for one or more particular functions. Several kinds of tissues may form an organ, such as the eye, heart, or stomach.

Organ Systems and Their Work

The various organs of animals are usually associated in organ systems. Most invertebrates (animals without backbones) and all vertebrates (animals with backbones) have the following organ systems:

Integumentary System

This system consists of the skin and other body covering. The covering protects inner tissues from injury and disease. It usually consists of several layers. Feathers, scales, and hair are part of the body coverings of various animals.

Skeletal System

The skeleton supports the body, and in many animals works with the muscular system during movement. Some animals—such as insects, lobsters, shrimp, clams, and snails—have exoskeletons (external skeletons). Exoskeletons are composed of layers of proteins and of a non-protein material, called chitin, containing some calcium salts. In clams, snails, and most other mollusks, the exoskeleton is a hard shell having a higher proportion of calcium salts. Vertebrates have endoskeletons (internal skeletons) composed of cartilage or cartilage and bone.

Muscular System

The movements of an animal's body, as a whole or in parts, are achieved by the contraction and relaxation of its muscles.

Digestive System

Food provides animals with materials necessary for energy, growth, and manufacture of new cells. It is broken down by a process called digestion into chemical substances that the body can use. These substances are then assimilated, or absorbed, into the body cells. The digestive system of complex animals includes the mouth, esophagus, stomach, intestines, and glands.

Respiratory System

Oxygen is taken into animal cells, and carbon dioxide is expelled. The exchange of the two gases is called respiration. Most species of animals have a system of respiratory organs, which include air holes such as nostrils or spiracles, tubes for conducting air, and organs—such as gills or lungs—in which the actual exchange of gases takes place.

Circulatory System

Food, oxygen, and certain gland secretions are moved through the body to all cells, and waste materials are carried away from the cells. In all animals except the more simple ones, food is carried to the cells and waste carried away through the circulatory system, which includes the heart and blood vessels.

Immune System

Through the immune system, most animals have the capacity to resist the development of diseases. In vertebrates, the immune system is also called the lymphatic system; it includes the lymph nodes, lymphatic vessels, spleen, and thymus gland.

Excretory System

Animals excrete (eliminate) waste materials. Carbon dioxide is excreted through the respiratory system. Excess water and other materials may be excreted directly through the body covering in the simpler animals. Complex animals have a system of excretory organs, including the kidneys, the ureter, and the bladder.

Endocrine System

The glands of internal secretion, or endocrine glands, help regulate body functions. They secrete substances called hormones, some of which have stimulating, others inhibiting, effects on body processes.

Nervous System

Most animals have a system of especially responsive organs—including the brain, nerves, and sense organs—that (together with the endocrine system) regulate body functions.

Reproductive System

All living things are capable of reproducing their own kind. Some invertebrates reproduce asexually by various methods, such as division of the parent cell into separate individuals. All vertebrates and many invertebrates reproduce by the joining of specialized cells called gametes, one from a male and one from a female parent. This process is called sexual reproduction.

Symmetry of Animal Bodies

The bodies of most kinds of animals are symmetrical in the arrangement of their parts. Radially symmetrical animals are those whose body parts radiate from a central point. Echinoderms (such as starfish and sea urchins) and cnidarians (such as hydra and corals) are radially symmetrical. Most animals, including humans, are bilaterally symmetrical; that is, a plane passing from top to bottom and back to front through the center would divide the body into two halves that, except for certain internal organs, are approximate mirror images of each other.

Adaptations

The various characteristics of animals that enable them to obtain food and shelter, to reproduce, and to protect themselves from predators are called adaptations. In a broad sense, all the characteristics of an organism are adaptations that have evolved through many generations to adapt, or fit, the organism for life in its normal environment.

Body AdaptationsBirds are specially adapted for flying.

Primates (such animals as humans, apes, monkeys, and lemurs) are among the many kinds of animals whose bodies are adapted to climbing trees. All primates have hands that are prehensile (fitted for grasping), and some have prehensile tails as well.

Animals' bodies are also adapted to obtaining and eating certain kinds of food. Carnivorous (flesh-eating) animals that catch large prey have strong, sharp teeth for tearing flesh. Some herbivorous (plant-eating) animals that feed on tough, bulky vegetation have broad-surfaced molars fitted for grinding. Their intestines are longer than those of carnivorous animals, and are adapted to breaking down plant fibers during digestion. A few animals, including humans and bears, are omnivorous, having bodies fitted for eating both animal and plant materials.

The bodies of animals are also adapted to protect them from their natural predators. Deer and gazelles have long legs and strong leg muscles to help them run away. Bears and cats have teeth and claws suited for defense as well as for attack. Skunks have glands that emit foul-smelling liquids to repel attackers. The hard shells of turtles and spiny coverings of porcupines are protective. The colors, markings, and shapes of many animals are helpful in concealing them from their predators.

Behavioral Adaptations

The behavior of animals is also adaptive. The walking stick, an insect shaped like a stick or twig, positions its body in such a way on trees and shrubs that it resembles a twig even more, making it more difficult for predators to see it. A tree finch of the Galapagos Islands breaks off a cactus spine and holds it in its beak to dig out insects from tree bark. Similarly, chimpanzees use twigs to probe holes in wood or termite nests to get the insects out.

Where Animals Live

Generally speaking, each animal is adapted for existence in a certain environment and may be unable to survive or reproduce in other environments. Environment includes such factors as temperature; light; moisture; atmospheric and water pressure; and gas and mineral content of air, water, and soil. It also includes other animals and plants. For example, a fish cannot live on land. It also cannot live in water that has the wrong temperature or lacks the kind of food the fish eats, and it cannot live at a depth where its body cannot withstand the water pressure.

The various factors of the environment in which any particular animal lives may not remain constant at all times. Most animals are adapted to withstand certain environmental variations. For example, birds and mammals maintain a fairly constant internal temperature in spite of limited external temperature changes. Such animals are popularly termed warm-blooded. However, some warm-blooded animals go into an inactive state called hibernation during cold seasons. Invertebrates, fish, amphibians, and reptiles have no internal means of regulating their temperatures, and cannot survive wide temperature variations. They are popularly called cold-blooded.

Amphibians are cold-blooded animals.Migration

Many birds, and some insects, fish, and mammals, travel regularly from one location to another to accommodate themselves to environmental variations, or to their needs for certain changes in the environment. These periodic movements, which are often seasonal, are called migrations. Some animals apparently migrate for feeding purposes. As winter comes on, for example, many plant-eating or insect-eating animals of the Northern Hemisphere go south, where growing plants and insects are plentiful. Deer, elk, and some birds that spend summers high in the mountains spend winters at much lower elevations.

Some animals migrate to find conditions that are suitable for breeding. These conditions may include availability of nesting sites and materials and proper temperatures for breeding and caring for their young. Several species of whales migrate as far as 11,000 miles (18,000 km) from cold waters to warmer ones at breeding time. Some species of salmon migrate only twice. Young salmon leave the freshwater streams where they were spawned and travel to the sea, where they grow to maturity. They then migrate from the sea to lay eggs in the waters where they were spawned.

Cover, Shelter, and Nests

The environments of many animals include cover and shelter. Tall grasses give cover for many land animals, including antelopes, lions, snakes, and birds. Fish take cover among the stalks of water plants, and even the huge hippopotamus can hide under water-lily pads by submerging up to its nostrils. In addition, an animal may take shelter from predators by climbing a tree, or by dodging into a hole in the ground or a hollow log. Gorillas and chimpanzees take shelter in trees at night, building temporary platforms of boughs and leaves in which to sleep. Bears and bats are among animals that are attracted to caves for sleeping and raising their young.

Some animals build special places called nests in which they produce their young and care for them. Birds build their nests in a variety of places, including rocky ledges, trees, shrubs, or clumps of grass. Some fishes hollow out nests in the mud or gravel of pond bottoms or stream beds. Termites build large and elaborate "hills" of earth in which to breed, take shelter, and hoard food.

Habitat and Distribution

The natural environment of an organism is called its habitat. The term is applied to the three large life zones: (1) salt water (divided further into depth zones), (2) fresh water (running water and standing water), and (3) land (divided into climatic or vegetation zones). The term is also used more specifically. For example, the habitat of certain land-dwelling beetles is the tropical forest zone, and more specifically the underside of rotting logs.

No particular animal is found in all the places in which it could survive. It is found only in a certain range, or area of distribution. Distribution is limited by geographic and climatic barriers such as water (for land animals), land (for water animals), high mountains, and temperature extremes, and by biological barriers such as the absence of food or the presence of predators. Population increase, climatic changes, and destruction of shelter are among other influences on distribution, forcing animals to expand their range or shift from one range to another. Ranges are sometimes extended when changing conditions have made new areas suitable.

When the ranges of the various undomesticated animals, especially mammals, are marked on a world map, several distinct regions, each with a unique composition of wildlife, are evident. These regions are called zoogeographic regions. (Similar maps showing the ranges of undomesticated plant species can be drawn; these regions are called phytogeographic regions. When the ranges of both undomesticated animals and plants are marked together on a map, these regions are called biogeographic regions.) Although there are many animals, such as bats and mice, that have a worldwide distribution, and many animals that overlap between two or more regions, the combination of animals found in each region is distinct. In addition, each region has certain animals that are not found in other regions.

There is no complete agreement among biologists on how many of these regions there are or what they should be called, but most accept the following six:

The Palearctic region includes most of Europe, central and northern Asia, Africa north of the Sahara, and most of China. Animals unique to this region include the mole rat and hedgehog.

The Nearctic takes in the United States, Canada, Greenland, Iceland, and the northern section of Mexico. Animals found exclusively in the Nearctic include the Rocky Mountain goat, musk ox, and American bison.

The Ethiopian region covers most of Africa and the Arabian peninsula. Here the zebra, African elephant, and gorilla are unique.

The Oriental region consists of the tropical areas of Asia, including Borneo, the Philippines, and most of India. Animals native only to this region are the gibbon, tree shrew, and tarsier.

The Neotropical region includes southern Mexico, Central America, and South America. Exclusive to this region are the sloth, New World monkeys, and the giant ant-eater.

The Australian region covers Australia, New Guinea, and a few of the smaller islands of the Malay Archipelago. Here the wombat, kangaroo, and koala are unique.

Many different animals share the same habitat.

Animal Relationships

Between Plants and Animals

Many animals eat only plant materials. The energy derived from plants goes into building the bodies of these animals, and is transferred to the bodies of animals that prey on them. There are some plants that capture and digest insects. Some animals—such as aphids and certain roundworms---live in or on living plant tissues.

In the food-making process, plants release free (uncombined) oxygen, without which animals could not live. In the process of respiration, animals in turn release carbon dioxide, which plants use in making food. When plants and animals die, the various chemical compounds of which their bodies are composed become part of the soil that nourishes new plants and, eventually, other animals. Many animals can live in otherwise unfavorable areas because of the moisture and shade provided by trees and other plants. Numerous insects breed under the bark of trees and shrubs.

Some animals help plants to reproduce. Certain insects, birds, and mammals (bees, hummingbirds, and some bats, for example) that visit plants for food or shelter carry pollen from one flower to another, bringing about fertilization. Seeds of plants are often widely distributed by being carried in or on the bodies of animals.

Between Animals

Animals not only depend upon other animals for food, but may also compete with them for food. For example, plant-eating insects such as locusts may compete with rabbits, rodents, and cattle. Lions, leopards, and cheetahs may compete with one another for the flesh of antelopes and zebras.

Animals of the same species compete with each other primarily for food, but may also compete for shelter or mates. Animals of the same species also often cooperate with each other. In some species, individuals assemble into groups for various purposes. Birds, for example, form groups when migrating. Some animals live permanently in groups, often as a protective measure.

When animals live in groups, there is usually a leader and a social order in which some individuals dominate others. Social insects—which include the termites and certain species of bees, ants, and wasps—have a highly developed social organization. Separate "castes" perform such tasks as laying eggs, caring for the young, and guarding the nest and the group. The members of each caste differ in body structure from members of the other castes, making the division of labor permanent. Among certain other animals with a social order the same individual does not necessarily perform the same task all its life.

Symbiosis

Sometimes animals of different species, or an animal and a plant, live together in a special relationship. Symbiosis is the general term for relationships of this type. If both partners benefit from the association, it is called mutualism. An example is the relationship between the clown fish and the sea anemone. The bright colors of the clown fish attract prey to the stinging tentacles of the sea anemone. In turn, the clown fish, which is immune to the anemone's deadly sting, receives shelter from predators.

If one partner only benefits, it is called commensalism. Barnacles, for example, live on the skin of whales. The whales are not harmed, but the barnacles are benefited by being carried to fresh feeding grounds. If the relationship benefits one and harms the other, it is called parasitism. Tapeworms are parasites that live in the intestinal tract of humans and other animals.

Animal Behavior

The term animal behavior is used to collectively designate how an animal responds to the environment that surrounds it and to conditions within its body. Environment includes plants and other animals as well as such nonliving constituents as light and sound. Knowledge about animal behavior comes primarily from observations of the animal's activities both in controlled laboratory experiments and in the wild. The study of animal behavior is called ethology.

All kinds of behavior, both simple and complex, result when an animal responds to a stimulus. A stimulus is a change in some aspect of an animal's environment or a change that occurs inside the body. The behavior of an animal is limited by its physical structure, especially the level of development of its nervous system and sense organs. Each successful animal species has through time evolved the physical characters and the appropriate behavior to allow it to survive and reproduce.

Depending on what sense organs an animal has, it may react to temperature, light, sound, touch, taste, and smell. In addition, an animal responds to various stimuli from inside its body. Mating behavior, for example, depends almost entirely on the presence of various hormones in the body. Drives for food, water, and oxygen are triggered largely by events inside the body. (A satiated animal will seldom eat even the most tempting of foods placed before it.)

When behavior is neither learned nor modified by experience it is said to be instinctive. Instinctive behavior is said to be innate; that is, its pattern is inborn and determined by the animal's heredity. For example, a newly hatched pipefish instinctively knows how to hunt for food, and it will hunt as well as the adult members of its species.

A special type of innate response found in animals with a central nervous system is the reflex action. The knee-jerk reflex in humans is an example. A similar action occurs in frogs: if a frog's toe is pinched, the leg is lifted. This reaction occurs even if the frog's brain has been removed.

A reflex usually involves a single muscle or single set of muscles, but instinctive behavior involves the animal as a whole. Furthermore, a reflex will occur only as long as the stimulus is present, but instinctive actions continue even after the stimulus that started the activity has disappeared.

Many animals, from earthworms to apes, are capable of learned behavior; that is, they can alter their instinctive behavior by drawing on past experience. Learned behavior includes habituation—in which an animal learns not to respond to certain stimuli. A dog, for example, learns not to react to certain noises that its past experiences have shown to have no harmful effects. Most animal behavior is a combination of instinct tempered with learning.

Intelligent behavior, in which an animal behaves with insight and reason, is found in varying degrees in many animals. It is often difficult, however, to distinguish between learned behavior and intelligent behavior. There is evidence, however, that such animals as dogs, rats, bears, apes, and certain species of birds are capable of some intelligent actions.

Social Behavior

Although relatively few species of animals display the complex social activities of social insects, very few animals are not at one time or another social. The interactions between parent and offspring are social activities; so are courtship and mating between the sexes. Social behavior, like other behavior, contributes to the success of the species, and it depends on communication.

Human beings are the only animals to possess language in the true sense of the word. Other animals communicate mainly through comparatively simple signals. Signaling behavior is largely instinctive, but responses to signals (though often instinctive) are in many animals improved by learning. Scent, vision, and hearing are all used for communication.

Communication between different kinds of animals is basically limited to a warning to stay out of private territory. Among their own kind, animals give signals to warn of impending danger, to protect their territory and young, to attract mates, and to tell the location of food sources.

To communicate with other geese, the graylag goose depends largely on changing its posture. Certain fish change not only the position of their fins but also their color to communicate fear, aggression, or readiness to spawn. Frogs, some reptiles, and almost all mammals and birds communicate to a large degree by sound. Dolphins and whales communicate through a wide variety of underwater sounds.

Bees can communicate to other bees the location of food by performing a specialized dance. Various animals give off a scent that can be readily identified by others. Such scents are used to set off territories, attract mates, and mark the way to food supply.

Animals and Man

Interest in animals and curiosity about them has inspired art, literature, and religion since before the beginning of history. Thousands of years ago primitive peoples painted pictures of animals on rocky hillsides and on the walls of caves. Legends about animals have been handed down for generations. Many peoples have believed in a spiritual relationship between themselves and certain animals, which they revered as ancestors and guardians. In ancient times, many peoples worshiped gods in animal form, and believed that certain animals were sacred to the gods.

Use of Animals. People have made use of other animals and their products for thousands of years. The flesh, blood, and milk of various animals have provided food. Clothing has been made from hides and fur, and by weaving wool and silk into fabrics. Hides have also served for making tents and other shelters and—processed as leather—countless other useful articles. People use animals for transportation and other work, and as pets. Vaccines, antitoxins, hormones, and various drugs are derived from animal tissues. To better meet human needs, the characteristics of domesticated animals are often modified through controlled breeding.

Certain animals are used in laboratories for scientific research, such as the testing of new drugs. This research, sometimes called vivisection, has brought about many advances in medicine, psychology, and other fields, but has also aroused much opposition on the grounds that it is cruel and unnecessary. In the United States, the Animal Welfare Act (passed in 1966 and amended in 1970 and 1976) and certain Public Health Service guidelines establish standards for the treatment of laboratory animals.

Harmful Animals

Wild animals—such as lions and leopards in Africa and tigers in Asia—occasionally kill and eat humans. Weasels, wolves, rats, and bears sometimes prey on domestic animals. Plant-eating animals such as rabbits, gophers, and numerous insects destroy crops. Poisonous snakes, spiders, and scorpions, and certain biting and stinging insects, are a source of danger or annoyance in some localities.

The animals most harmful to humans are those that carry or cause disease. Certain mosquitoes spread malaria; in parts of Africa, the tsetse fly spreads trypanosomiasis (sleeping sickness in humans and nagana in domestic animals). Parasitic worms such as hookworms, tapeworms, trichina worms, and various flukes cause serious diseases in many parts of the world.

How Humans Affect Other Animals

The domestication and controlled breeding of animals have changed the body structure and habits of many animals. The stocky build of beef cattle and the long, slender legs of race horses are examples of traits developed by controlled animal breeding.

Humans have transported animals from their natural habitats into new localities, often with unexpected results. Mongooses introduced into the West Indies to destroy rats also destroyed many harmless and desirable animals, and themselves became pests. Rabbits and deer introduced into New Zealand by Europeans became pests to agriculture and forestry, since there were no predators to control them.

In addition, humans have destroyed habitats and reduced the range of many animal species by the development of agriculture and industries. In many cases, this destruction has been beneficial to humans. In others, interference with natural conditions has resulted in harm to humans themselves. Recognition of this fact has led to the study of methods of wise conservation of animals and their natural environment.

In spite of various conservation efforts, however, dozens of species of wildlife have vanished from the earth during the past 200 years, mostly as the direct result of human alteration of their habitat or excessive hunting. Examples are the dodo, passenger pigeon, and Steller's sea cow. Hundreds of other animal species are in danger of extinction, and worldwide efforts to save them are underway. Endangered animals include the tiger, Brazilian tapir, giant otter, California condor, whooping crane, and numerous species of whales.