Zoos are places where animals are kept so that you can learn about them. Zoos are also places where scientists study animals. Some zoos help preserve endangered animals.
WHAT DOES A ZOO LOOK LIKE?
Zoos once kept animals in cages. Today, they try to keep animals in larger, open places. The places look like the home of each animal that lives there. They have rocks and trees, bushes, and other plants that the animal likes. Some places might look like deserts or prairies or rain forests. Polar bears have ponds of ice-cold water to play in. Monkeys have trees they can climb. Pandas have lots of bamboo to eat.
The places where the animals live have fences and other borders. But the fences are hidden and hard to see. Part of the San Diego Wild Animal Park in California looks like a plain in Africa. It seems like the animals are free to go anywhere.
Many zoos have places with closed-in tops where hundreds of birds can fly around. Some zoos have places you can walk through to see butterflies sipping nectar (juice) from flowers.
WHO WORKS AT A ZOO?
A zoo needs many people to care for the animals. Zoologists are scientists who study animals. They learn about how the animals live. They learn what makes the animals happy.
Veterinarians are animal doctors. They take care of any sick zoo animals. They try to keep all the animals healthy.
Zookeepers take care of everything the animals need. They make sure the animals have plenty of food and water. They watch for any problems with the animals.
Zoos have other workers, too. They have guides who give tours and talk about the animals. They have cooks who work in zoo kitchens to make food for the animals. They even have people who build the homes for zoo animals to live in.
WHERE DO ZOOS GET ANIMALS?
In the past, most animals in zoos were captured wild animals. Today, zoos want their animals to breed (mate and have babies).
It is easy to get some animals to breed at a zoo. Lions living at zoos have many cubs. It is hard to get other animals to breed at a zoo. Cheetahs and giant pandas rarely have babies when they live in a zoo.
PROTECTING ENDANGERED ANIMALS
Zoologists try to save some endangered animals by breeding them at a zoo. Sometimes, they put animals born in a zoo back into the wild. They put the endangered Père David’s deer into an animal preserve in China. They put red wolves in places in North Carolina and Tennessee. These zoo-born animals now live completely in the wild.
Do spiders date? Do sponges have parents? Are there boy plants and girl plants? All these questions have to do with reproduction. Every species (kind) of living thing reproduces. Reproduction is how a species survives.
Sponges reproduce in simple ways. Spiders, plants, and people reproduce in more complicated ways. But there are just two basic ways of reproducing: by asexual or sexual reproduction.
WHAT IS ASEXUAL REPRODUCTION?
Asexual reproduction means that there are no male and female parents. There is only one parent. All the offspring (new organisms) that develop are just like the parent. They all have the same genes as the parent. Genes are found in the cells of all living organisms and are passed on to the organism’s offspring. They determine the traits of offspring, such as blue or brown eyes and straight or curly hair in humans.
Sponges reproduce by budding. Bumps called buds form on the body of a sponge. The buds break off and become new sponges. Budding is a form of asexual reproduction.
WHAT IS SEXUAL REPRODUCTION?
Sexual reproduction means that there are two parents. Males have sex cells called sperm. Females have sex cells called eggs. When animals mate, the male sperm fertilize, or join with, the female eggs to form new cells.
Each parent has different genes. The offspring (children) get half of their genes from each parent. Two sets of genes make the offspring different from their parents.
WHAT IS COURTSHIP?
Most animals have a time of courtship before they mate. Human males and females date.
Animals court by instinct. Male birds show off colorful feathers. Frogs croak to get the attention of females. Male and female fireflies flash their lights.
Some male spiders bring a dead fly to a female. The male spider is not really dating the female. He brings a “gift” so the female will not mistake him for dinner and eat him.
HOW DOES A SPERM FERTILIZE AN EGG?
In some animals, the egg gets fertilized outside the female’s body. Frogs, toads, and most female fish lay eggs in the water. Males put their sperm near the floating eggs. The sperm find and fertilize the eggs.
In many other animals, the male deposits its sperm inside the female’s body and the egg gets fertilized there. The female eggs of sharks, wolves, polar bears, and human beings are fertilized inside the female’s body.
Some female animals store sperm in their bodies until they are ready to lay eggs. Spiders and queen bees are animals that store sperm.
HOW DOES A FERTILIZED EGG GROW?
A fertilized egg divides again and again to become an embryo. Embryos grow inside the bodies of female mammals and some other animals. Humans develop as embryos. When an embryo is fully formed, the mother gives birth.
Other animals lay eggs outside their bodies. Alligators, crocodiles, frogs, turtles, fish, and birds are animals that lay eggs. The eggs are protected by a shell. The shell holds food for the growing embryo. Once grown, the babies must break out of their shells.
HOW DO PLANTS REPRODUCE?
Plants can reproduce asexually or sexually. Plants reproduce asexually by growing new parts. You can watch a potato reproduce asexually. Cut a potato into several pieces. Put each piece in a cup. Set the cup in sunlight and give it water. Soon you will see new potato plants growing.
Plants that grow from seeds reproduce sexually. The male and female parts are in the flowers of some plants. The male part is called a stamen. The sperm is called pollen. The female part is called a pistil. The egg grows inside the pistil. Bees, birds, and even the wind carry the pollen to the pistils.
Some plant species have different male and female plants. But in many species, the male and female parts are on the same plant.
They may be cuddly, playful, and fun. They can delight us with their grace and beauty. They’re part of our families. Sometimes, they’re our best friends. It’s easy to see why people love their pets!
THE MOST POPULAR PETS
More than half of the homes in the United States have a pet. Cats and dogs are the most popular pets. One out of three households has a cat or dog. The most popular purebred dog is the Labrador retriever. However, cats outnumber dogs in American homes.
Birds rank next in popularity, but they’re far behind cats and dogs. The most popular pet bird is the parakeet. Many people keep small animals like rabbits, hamsters, and gerbils as pets. Fish also are popular. Turtles are the reptiles most often kept as pets.
PETS NEED LOVE AND CARE
We expect our pets to love us. It’s important that we give them the love and care they need in return. Dogs especially want close contact with their owners. Our pets depend on us for food, grooming, attention, affection, and medical care. Before you get a pet, it’s important to know what its needs are.
A pet is a responsibility as well as a companion. Keeping a pet means keeping a promise to love and care for it. If you can do that, you can have wonderful experiences with pets.
CHOOSING A PET
Dogs generally offer total and unquestioning love. Most dogs are loyal friends and great playmates. Dogs also require a lot of attention. They need to be walked and played with. They need to be housebroken and trained to obey. Different breeds of dogs have different personalities and requirements, so don’t choose a dog just by its looks!
Many people who don’t have enough time to give a dog the attention it needs choose a cat instead. Cats can live indoors, and they don’t need training. Cats like to play, and they make affectionate companions. They’re endlessly curious, and they entertain us with their pranks.
Pets in cages may be best for people without a lot of time or space. Rabbits, hamsters, and gerbils are active, friendly, and easy to care for. Parakeets enjoy talking and playing with mirrors and toys. They learn to perch on your finger. Watching fish in a home aquarium is also relaxing and fun. Tropical fish can be very beautiful.
PRACTICAL PETS
Pets are fun, but they also do work. Animals can be useful to have around. That’s why wild animals got tamed as pets.
Dogs, especially, can do lots of things. They can stand guard in the night. Hunters use dogs for fetching. Ranchers use them to help herd sheep or cattle. The police use dogs to sniff for bombs. Some dogs are trained to help blind people and deaf people.
Cats catch mice. The ancient Egyptians worshiped cats for keeping their fields free of pests. Pets can even be actors! Trained dogs sometimes appear in movies or television shows.
Having a pet can be good for your health. Pets offer companionship and make people feel less lonely. Petting an animal also helps people relax.
PROBLEMS WITH PETS
Pets can cause problems, too. Dogs dig in the garden. They chew up household objects and can even bite people. Cats like to scratch furniture. Both cats and dogs can make a mess on the rug. Some people have allergies to animals.
But the biggest problem is that there are more pets than there are homes for them. Cats and dogs give birth to more cats and dogs. The number of unwanted pets is enormous. That’s why it’s important to consider having your pet neutered or spayed. Otherwise, you could wind up with lots of extra animals.
You can see most of these tiny things only if you put them under a microscope. These tiny life forms are called microscopic life or microorganisms. They are also called microbes. Most microbes are made of just one cell. Your body, by contrast, has billions of cells.
MANY KINDS OF MICROBES
There are thousands of different kinds of microbes. Microbes called bacteria are the smallest living things. Bacteria live almost everywhere: on land, in water, and even inside you.
Some microscopic life forms are like plants. Tiny diatoms are microbes that live in lakes and the ocean. Diatoms soak up sunlight and use its energy to make food, just like plants.
Some microscopic life forms are more like animals. Protozoa such as the amoeba and paramecium are animal-like microorganisms. An amoeba can move by changing the shape of its body. It sticks out a pseudopod, or false foot. A paramecium is covered with tiny hairs. It waves its hairs rapidly to swim.
Viruses are another kind of microbe. Viruses sometimes act like living things and other times act like nonliving things. Viruses sometimes live in the cells of plants and animals. Inside a cell, a virus reproduces like a living thing. The virus uses the cell to make many copies of itself. Viruses are too small to see with an ordinary microscope. You need a powerful electron microscope to see a virus. Outside of a plant or animal cell, a virus is like a nonliving thing.
ARE MICROBES HARMFUL?
Some microbes are harmful. Some microbes are helpful. Protozoa called plasmodium cause a serious disease called malaria. Some bacteria and viruses can make you sick. We sometimes call these microbes germs.
Harmful bacteria can make food spoil. You can get sick to your stomach if you eat the food. Some harmful bacteria can cause an infection in a cut or sore. Others cause pneumonia, strep throat, tuberculosis, typhoid fever, and other diseases.
Viruses cause chicken pox, measles, flu, and common colds. They also cause very serious diseases such as rabies, polio, and acquired immunodeficiency syndrome (AIDS).
WHAT DO HELPFUL MICROBES DO?
Harmless bacteria in your body compete with harmful germs. Harmless bacteria compete for food and keep the harmful germs under control. Some bacteria in your body help you digest your food.
Some bacteria in the soil help plants grow. The bacteria break down dead plants and animals into chemicals that plants can use. Some bacteria take nitrogen gas out of air and water. They change the nitrogen so that plants can use it to help make food.
Diatoms are important food for fish. Diatoms are part of the plankton that floats on the ocean. Plankton is made up mostly of microscopic life forms. Many kinds of animals that live in the ocean eat plankton.
WHO DISCOVERED MICROORGANISMS?
A Dutch scientist named Antoni van Leeuwenhoek was the first person to see microorganisms. He invented a microscope more powerful than any that had previously been made. He saw microbes under his microscope in 1676. He called the microscopic organisms “animalcules.”
You might think that the Arctic would be a terrible place to live. For most of the year, there is nothing but ice and cold weather. A polar bear thinks the Arctic is the perfect place to live. That’s because the Arctic is the polar bear’s natural habitat.
You might think that a tropical island is a wonderful place to be. However, a polar bear would die on a tropical island. The bear would not be able to find its favorite food—seals—in the warm waters. The bear’s layer of fat and thick, white fur would make the bear too hot. A tropical island is the natural habitat for palm trees and parrots, but not polar bears.
WHAT IS A HABITAT?
A habitat is a place where a plant or animal normally lives. Fish live in water. Perch are fish that live in freshwater habitats. Sharks are fish that live in saltwater habitats.
Other animals live on land. Cattle, antelopes, and buffaloes are animals that live on grasslands. Camels and kangaroo rats like the blazing hot temperature of deserts. Bats, gorillas, and elephants thrive in lush, moist rain forests.
Pine trees can live in cold, snowy habitats. Cactuses can live in hot, dry desert habitats. Water lilies live in freshwater habitats.
There are many kinds of habitats. Some habitats are along seashores. Some are high up in mountains or on flat prairies. Others are in dense forests, in sandy deserts, or in the deep sea. Different kinds of plants and animals live in different kinds of habitats.
HABITAT DESTRUCTION
Habitats are being destroyed around the world. As humans move into new areas, they clear the land to build on. They cut down trees for building materials and fuel. They dry out swamps for farmland. They dam rivers for electricity. The animals in the habitat must adapt to the changes or move elsewhere. If they cannot, they die. Many plants die, too.
Later, a slightly larger fish eats the little fish. Then, the slightly larger fish gets eaten by a really big fish. You are in a boat on the lake fishing. You catch the big fish, cook it, and eat it for dinner.
This is a food chain in action.
WHAT IS A FOOD CHAIN?
A food chain is the way energy goes from one living thing to another through food. Plants are the first step in most food chains.
Plants use the energy in sunlight to make their own food. Plants store the energy in their leaves and stems. Plants are called primary producers in food chains.
Animals eat the plants that use the Sun’s energy to grow. Animals are called consumers. Animals that eat plants are primary consumers. Animals that eat other animals are secondary consumers. Animals store the energy in their bodies.
Energy flows from plants to bigger and bigger animals through the steps of eating and being eaten. Each part of the food chain is directly connected to the other, just like the links in a chain.
WHAT IS A FOOD WEB?
A food web is made of many food chains in a community of plants and animals. There are many tiny animals near the beginning of a food web. There are fewer but larger animals higher up in a food web. There are many more insects than fish in a food web. There are also more small fish than big fish. Human beings are at the very top of the food web. No animals are higher up to make us their steady diet.
Decomposers play important roles in some food webs. Bacteria and fungi are decomposers. Decomposers eat dead plants and animals and cause them to rot and decay. They also eat animal wastes. They break things down into chemical parts called nutrients. The nutrients go back into the soil. Plants take up water and nutrients to make food. Nutrients move through food webs over and over again.
HOW THE SUN’S ENERGY GETS INTO FOOD
The Sun provides the energy that starts the whole process. A plant uses sunlight to make its own food. The energy is stored in the plant. An insect eats the plant. The insect stores the energy in its body. Then the energy is transferred to a fish that eats the insect. When you eat the fish, your body gets the energy stored in that fish.
When you eat salad, fruits, and potatoes, you take in the energy stored in these plants. When you eat fish, beef, or chicken, you take in the energy stored in these animals. When you eat your dinner, your body is getting energy that first came from the Sun. You use this energy to do your schoolwork. You use this energy to run and play.
You have never watched a dodo bird scurry along the ground. You have never been frightened by a saber-toothed cat crouched in a tree. You have never seen the huge tusks of a mastodon. These animals once lived on Earth, but they all died out. They went extinct.
You can see still see blue whales, giant pandas, and tigers. But these animals are in danger of becoming extinct. There aren’t many of them left. Blue whales, giant pandas, and tigers are some of the endangered animals on Earth. There are thousands of others.
WHY DO ANIMALS GO EXTINCT?
Different kinds of animals have appeared and disappeared throughout Earth’s history. Some animals go extinct because the climate where they live changes. The climate may become wetter or drier. It may become warmer or cooler. If the animals cannot change, or adapt, to the new climate, they die.
Some animals go extinct because they cannot compete with other animals for food. Some animals go extinct because they are killed by enemies. New kinds of animals are always evolving. Evolving means that the animals are changing slowly from generation to generation. Small differences between parents, children, and grandchildren slowly add up over many, many generations. Eventually, a different kind of animal evolves.
Sometimes many of the animals on Earth go extinct at the same time. Scientists call this a mass extinction. Scientists think there have been at least five mass extinctions in Earth’s history. The last mass extinction happened about 65 million years ago. This mass extinction killed off the dinosaurs.
WHAT KILLED THE DINOSAURS?
Scientists are not sure what caused the extinction of the dinosaurs. Many think an asteroid or comet may have crashed into Earth. The crash would have made a huge dust cloud. The cloud could have blocked out the Sun’s light for years. Without light, plants, dinosaurs, and other animals would die.
Many new animals, including large mammals, evolved after the dinosaurs died off. Scientists think that mass extinctions make way for new kinds of animals to develop.
HOW DO WE KNOW ABOUT EXTINCT ANIMALS?
Scientists study fossils to learn about extinct animals. Fossils are the remains of animals that have died. Fossils can be imprints of the animals left in rock. They can be bones that have turned to stone.
Scientists study fossils to learn what the animals looked like. They study other rocks around the fossils to learn when the animals lived. Some fossils are millions of years old.
ENDANGERED ANIMALS
Many animals are currently in danger of going extinct. Human beings are causing thousands of kinds of animals to die off. Chemicals that pollute the environment can endanger animals. Hunters kill some animals. Hunters kill rhinoceroses for their horns. They kill tigers for their skins. They kill elephants for their ivory tusks.
Most endangered animals today are in trouble because people are destroying their habitats. A habitat is a place where an animal normally lives. Forests, deserts, and wetlands are kinds of habitats. Many endangered animals live in tropical rain forests. People cut down rain forests to build farms, roads, and towns. They drain water from wetlands. Gorillas and koala bears are endangered because of destruction of the forests in which they live.
CAN WE SAVE ENDANGERED ANIMALS?
Governments can pass laws that make it illegal to hunt or sell endangered animals. They can pass laws against using chemicals that harm certain animals. DDT was a bug killer that once endangered the bald eagle and the peregrine falcon. DDT was banned, and the populations of these birds increased.
Sometimes people try to save one kind of endangered animal. They take some of the animals to zoos or laboratories. They breed the animals there. When there are enough animals, they set them free in the wild. Biologists are trying to save giant pandas and the California condor in this way.
Some biologists believe the best way to save endangered animals is to save their habitats. They work to set up wilderness areas such as parks and national forests where the animals can live. They keep the animals safe from hunters and other dangers.
WHAT IS AN ECOSYSTEM?
An ecosystem is all the living and nonliving things in a certain area. All the plants and animals, even the microorganisms that live in the soil, are living parts of an ecosystem. Air, water, and rocks are nonliving parts of an ecosystem.
Ecosystems are smaller parts of all the living environments on Earth. Earth’s entire living environment is called the biosphere. The biosphere is made up of large areas called biomes. Land biomes include grasslands, deserts, coniferous forests (forests of cone-bearing trees), deciduous forests (forests of trees that shed their leaves), and tropical rain forests. There are also biomes in bodies of water, such as the ocean.
The biomes, in turn, are made up of many ecosystems. The desert biome, for example, covers all the deserts of the world. Each individual desert is an ecosystem. The Mojave Desert in California is a desert ecosystem.
WHY DOES AN ECOSYSTEM NEED PARTS?
Every living thing in an ecosystem depends in some way upon other living and nonliving things in the ecosystem. All the parts of an ecosystem work together.
The living things in an ecosystem are either producers or consumers. Producers do not eat other living things. Producers make food. Trees, grasses, and other green plants make food. Green plants are called primary producers. Plants use nonliving nutrients—nourishing substances, such as the chemicals in soil and water—to help them make food. They use energy in sunlight to make food. They also use carbon dioxide gas in air to make food.
Consumers are animals that eat other living things. Animals that only eat plants are called herbivores. Herbivores are primary consumers. Rabbits, mice, and plant-eating insects are primary consumers.
Animals that eat other animals are called carnivores. Carnivores are secondary consumers. Bears and hawks are secondary consumers.
Ecosystems also have decomposers. Decomposers break down dead plants and animals. They break down animal wastes. Fungi, such as mushrooms and mold, and bacteria are decomposers. They turn dead material and waste into chemical nutrients. Plants take up the nutrients with their roots. They use the nutrients to make more food.
HOW BIG IS AN ECOSYSTEM?
Some ecosystems are huge, and some are small. A tropical rain forest ecosystem might cover hundreds of square miles. A mangrove swamp ecosystem might stretch only a few miles along the shore of an island.
A place can have more than one ecosystem. A rain forest and a mangrove swamp could be on the same island. A coral reef ecosystem might be in the water around the island.
HOW DOES AN ECOSYSTEM WORK?
All things in an ecosystem are connected with one another. These connections come through food and energy. The energy comes from the Sun. Plants use the energy in sunlight to make food. Animals eat the plants. Other animals eat the plant-eating animals. The way energy flows in food from plants to animals is called a food chain. Food chains that overlap are called food webs.
Let’s look at an ecosystem in a forest. Water flowing in a river makes the riverbanks wet. Plants that need lots of water grow along the riverbanks. Insects feed on plants in or along the river. A salmon swimming by eats the insects that fall in the water. A brown bear that lives in the forest wades into the river and swipes its paw in the water. The bear catches and eats the salmon.
The bear tosses the salmon bones and some meat onto the riverbank. Bacteria and fungi now go to work. The tiny bacteria and fungi feed upon the remains of the salmon. They break down the salmon into chemical nutrients. Nutrients from the salmon go into the soil.
The roots of plants along the riverbank take up the nutrients. They use the nutrients to make food. In this way, nutrients get recycled back through the ecosystem.
WHAT CAN HARM AN ECOSYSTEM?
Any change in one living or nonliving part of an ecosystem can cause changes in other parts. Droughts, storms, and fires can change ecosystems. Some changes harm ecosystems. If there is too little rainfall, plants will not have enough water to live. If a kind of plant dies off, the animals that fed on it may also die or move away.
Some changes are good for ecosystems. Some pine forests need fires for the pine trees to reproduce. The seeds are sealed inside pinecones. Heat from a forest fire melts the seal and lets the seeds out.
Polluting the air, soil, and water can harm ecosystems. Building dams on rivers for electric power and irrigation can harm ecosystems around the rivers. Bulldozing wetlands and cutting down forests destroys ecosystems.
Ecologists are working with companies and governments to find better ways of catching fish, cutting down trees, and building dams. They are looking for ways to get food, lumber, and other products for people without causing harm to ecosystems.
A sea turtle glides over your head. Tiny blue and yellow fish flash by. A big shark swims right at you. You are walking around on the bottom of the sea. Yet you are perfectly safe and dry. You are visiting an aquarium.
WHAT IS AN AQUARIUM?
An aquarium is a water-filled tank in which fish swim about. You can keep a small aquarium at home. You can visit a large aquarium in most cities. This kind of aquarium is a building with fish and other water animals in large tanks. A large aquarium may have otters, turtles, dolphins, and other sea animals. Most aquarium tanks also have plants.
VISITING AN AQUARIUM
Many of the newest aquariums are located next to an ocean, bay, or river. They have huge water tanks. Some tanks are big enough to hold sharks and other large fish. Some aquariums have a passageway at the bottom of a huge tank. The passageway is made of thick, clear plastic. When you walk through the tunnel, you see fish swimming on both sides of you and even above you. What a cool way to watch fish!
Sea life in an aquarium looks very real. Many tropical fish live on colorful coral reefs. The reefs are made of tiny animals called corals. A tropical fish tank may look just like a coral reef. The tropical fish feel right at home in this tank.
You can do many things at a big aquarium. You can watch and learn how sea animals live. You can learn about dangers they might face. You can learn about kinds of pollution that could harm sea animals. You can even watch dolphins put on shows at some aquariums.
Scientists work at many big aquariums. The scientists study fish. They want to know how to protect life in the sea.
CREATING A HOME AQUARIUM
Many people have small aquariums at home. A small aquarium can be a bowl. It can be a square tank. Some home aquariums are made of glass. Some are made of a strong plastic called Plexiglas. Some people keep freshwater fish in their aquariums. Some people keep saltwater fish.
Suppose you want to make a home aquarium. There are many things you have to pick out. What should your aquarium tank be made of? You can see fish better in a Plexiglas tank. But Plexiglas scratches more easily. Plexiglas also costs more than glass.
Which type of fish do you want to keep in your tank: freshwater fish or saltwater fish? Goldfish and tetras live in fresh water. Angelfish and butterfly fish live in salt water. Most people with aquariums have freshwater fish. Freshwater fish are easier to take care of. Freshwater fish cost less than saltwater fish.
Do you want to keep fish that live in cold water or fish that live in warm water? Tropical fish live in warm water. Tropical fish come in reds, yellows, blues, purples, and many other colors. Goldfish live in cold water. Goldfish are mostly golden red.
Pick out the kind of tank and fish you like best. You can decorate your tank with plants, colorful stones, and objects that the fish can swim through and hide under. Find out what to feed your fish and how to keep them healthy. Then have fun watching the fish in your home aquarium.
Animals live everywhere. They roam the land. They burrow in the ground. They swim in the sea. They fly through the air. They creep, they leap, they soar, and they dive. A very few—including corals and barnacles—stay in one place.
Animals come in all sizes. The biggest animals are whales, which can be 100 feet (30 meters) long. The smallest animals can only be seen through a microscope.
Zoologists (scientists who study animals) have found more than 2 million species (kinds) of animals. They think they have discovered only a small portion of all animals on Earth.
Several things make animals different from other living things. Unlike plants, animals cannot make their own food. Animals eat other living things—plants and other animals—to get energy. Animal bodies are made up of more than one cell, unlike bacteria and other life forms with only one cell. Cells are the building blocks of living things. Animals also have senses, such as eyes or ears, that tell them what is going on around them.
WHAT KINDS OF ANIMALS ARE THERE?
Zoologists divide animals into about 30 groups. First, they divide them by whether they have a backbone. Animals that have a backbone are called vertebrates. Animals that do not have a backbone are called invertebrates.
The biggest and best-known animals are vertebrates. Mammals, birds, reptiles, amphibians, and fish are vertebrates. You are a vertebrate. Your backbone is also called your spine. There are about 40,000 species of vertebrates.
There are far more species of invertebrates. Even though you can probably think of many vertebrates, the many kinds of invertebrates greatly outnumber vertebrates. Almost all invertebrates are small animals. Insects, spiders, mollusks, and worms are all invertebrates. The biggest invertebrate is the giant squid. It can be up to 60 feet (18 meters) long.
COLD-BLOODED AND WARM-BLOODED ANIMALS
Zoologists divide vertebrates into two types, cold-blooded and warm-blooded animals. The body of a cold-blooded animal is the same temperature as the air or water around it. A cold-blooded animal has to stay in the sun to get warm. It must find shade to cool off. Reptiles, amphibians, and fish are cold-blooded.
The body of a warm-blooded animal stays about the same temperature all the time. Warm-blooded animals use food energy to stay warm. Some warm-blooded animals can sweat to cool off. Birds and mammals are warm-blooded.
You are warm-blooded. The normal temperature of your body stays at about 98.6° Fahrenheit (37.0° Celsius). When it is hot outside, you feel hot. You might sweat or look for an air-conditioned place. But your body temperature does not change. When it is cold outside, you feel cold. You might put on a coat or go indoors. But your body temperature does not change much.
PLANT EATERS AND MEAT EATERS
All animals eat plants, other animals, or the remains of dead animals. Animals that only eat plants are called herbivores. They eat seeds, nuts, grasses, stems, or flowers. Some mammals, such as cows, are plant eaters. Some insects, such as termites, only eat plants. Bees, moths, and butterflies suck nectar from flowers.
Animals that only eat meat are called carnivores. The meat can be from other living animals or animals that have died. Many meat eaters hunt the animals that they eat. Sharks go after smaller fish. Lions and wolves hunt deer and other mammals. Owls swoop down on rabbits, squirrels, rats, and mice. Some frogs and lizards zap insects with their sticky tongues. Hyenas and vultures mainly eat animals that are already dead.
Animals that commonly eat both animals and plants are called omnivores. Bears and opossums are omnivores. Humans are omnivores. They eat fruit, vegetables, fish, chicken, and steak.
HOW ANIMALS BREATHE
All animals must breathe oxygen to stay alive. They must breathe out a waste gas called carbon dioxide.
Some animals breathe through lungs. Lungs take oxygen out of air. Cattle, dogs, cats, whales, people, and other mammals breathe through lungs. Birds and reptiles also breathe air through lungs.
Lungs cannot take air from water. Seals, whales, dolphins, and other mammals that live in water breathe through lungs. They can stay underwater a long time because they can hold their breath for a long time.
Sharks and other fish breathe through gills. Gills take oxygen out of water. Snails, slugs, clams, squids, octopuses, and other mollusks breathe through gills. Crabs, crayfishes, lobsters, and shrimp have gills. Gills cannot take oxygen out of air.
Some animals breathe through their skin. Insects have small holes in their bodies called spiracles. Air comes in through the holes. Oxygen from the air goes through tubes to all parts of an insect’s body.
Amphibians, animals that live on land and in water, can also breathe through their skin. Amphibians also may have lungs or gills or both.
