Vertebrates have an internal skeleton mad~ of bone, a type of tissue. Bones grow as an animal grows. Backbones, also called vertebral columns or spines, can support large and heavy bodies, such as those of elephants, giraffes, and whales. Yet they can also bend to allow motion. Skeletal joints, like hinge joints and ball-and-socket joints, allow bone movement.

Most vertebrates have similar body organization. A head equipped with sensory organs sits at the top of the vertebral column. There may also be a tail at the base. Vertebrates have two pairs of limbs that may be specialized as legs, arms, wings, fins, or flippers. Systems of internal organs tend to be organized in similar ways and positions.

Vertebrates respond, or react, to the world around them in many ways. Some behaviors, such as reflexes and instincts, are inherited from the parents. A reflex is a rapid, automatic response that usually protects an animal from harm. One example is the rapid closing of an eyelid when an object moves too closely toward an animal’s eye.

An instinct is an automatic behavior that is usually more complicated than a reflex. The way a bird builds a nest or follows its mother, for example, is an instinct. A mother acting to protect her young is another instinct. So are certain reactions to danger or threat, such as fleeing, fighting, or hiding. In each of these examples, an animal acts without having been taught why or how to act.

Most vertebrates can also learn new behaviors. If you repeatedly place food in one spot of an aquarium, a fish will look there for its dinner. A parrot learns to imitate certain words. A human learns to speak, read, and write.

Intelligent behavior, such as problem solving and decision-making, is more complex. Among vertebrates, birds and mammals are more likely to show intelligent behavior.

Cold-blooded Animals

The term cold-blooded does not mean that an animal has cold blood. Instead, it means that the animal’s body temperature changes with its surroundings. In a cold environment, the animal’s temperature cools. In a warm environment, the temperature rises.



There are three main groups of fish. All have an internal skeleton and an outer covering of scales.

Some fish, like sharks and rays, have internal skeletons made of cartilage, a tough, elastic tissue. These are cartilaginous fish. Most fish, however, have bony skeletons. Trout, tuna, and salmon are examples of bony fish. Jaw-less fish like lampreys and eels form a third group of fish.

All fish live in water. They have fins and a tail for swimming. Gills help them take in oxygen from the water.

Fish reproduce by laying eggs. The males spread sperm over the eggs. Most fish leave their eggs, and the young hatch on their own. But some fish guard the eggs and even the young offspring.

A variety of fish can be found in almost every water habitat. Some fish can live only in salt water, while others can live only in freshwater. Large fish have long been an important food source for humans. Now, over-fishing and pollution is causing the numbers of these fish to dwindle.


Amphibians include frogs, toads, and salamanders. Like insects, amphibians undergo metamorphosis as they grow and develop. A frog’s life cycle begins when it hatches from an egg in water. The young frog is called a tadpole, and it looks and lives much like a small fish. Like fish, young frogs use gills to breathe.

During metamorphosis, a tadpole develops lungs for breathing air and legs for moving on land. As adults, frogs live on land. When they are ready to reproduce, they return to the water to lay their eggs.


Because they are cold-blooded, the activity level of some amphibians decreases when the temperature drops. In cold areas, they may bury themselves in mud and hibernate. When amphibians are inactive, they live off body fat until they can feed again in the spring. Some amphibians also bury themselves during very hot, dry weather and become inactive.

In addition to breathing through their lungs, adult amphibians may also breathe through their skin. The skin must be thin and supple to let gases, such as oxygen and carbon dioxide, pass through it. This means that many pollutants can pass through the skin as well. Often the pollutants are in the water. Because pollutants can accumulate in the bodies of frogs and other amphibians, these species are useful monitors of water quality and overall environmental health.


Familiar reptiles include snakes, turtles, alligators, and lizards. Dinosaurs were reptiles too, but they have been extinct for a long time. No one knows for sure why the dinosaurs became extinct, but many scientists think that the process began when a huge asteroid hit Earth.

The bodies of modern reptiles are dry and covered with scales. The scales are made of a hard material that is similar to human fingernails. The scales help to protect the animal from enemies and from drying out. Reptiles have well-developed lungs for breathing. Their skin is not used for respiration and does not need to remain moist. As a result, they can live their entire lives on land.


Unlike amphibians, reptiles do not go through metamorphosis. They never live in water like tadpoles. Instead, young reptiles are small versions of the adults. Reptiles do not even need water to lay their eggs. Instead, the eggs have a shell that keeps the moisture inside the egg. The male sperm must reach the egg before the shell forms, so reptile fertilization is internal. A few reptiles give birth to live young.

The American alligator is a reptile that was once very close to becoming extinct. Its habitat, the swamps and wetlands of the southeastern United States, was disappearing rapidly. There was a big demand for products made of alligator skin, such as belts, pocketbooks, and shoes. To keep up with this demand, hunters killed many alligators.

Today, the American alligator is one of the greatest conservation success stories. Populations of American alligators are now thriving, thanks to laws that preserve their habitats and protect them from being hunted.

Warm-blooded Animals

Birds and mammals are warm-blooded. This means that their internal temperature does not depend on the weather outside. They use the energy released from food to keep their bodies warm. Feathers and fur trap air, which provides insulation from the cold. As a result, the body temperature of birds and mammals remains nearly constant.



Birds are more similar to other animals than they may appear to be. Wings are specialized front limbs. Feathers are modified versions of the reptile’s scales. Bird’s bone~ are light and hollow, which helps many birds fly. Other birds, such as the ostrich, only walk and run. Still other birds are excellent swimmers.


All birds are warm-blooded and have feathers, a beak or bill, two legs, and two forearms that serve as wings. They lay eggs on land, and parents typically care for eggs and young chicks. Beyond these common features however, birds are quite varied. A hummingbird weighs less than an ounce, whereas a tall ostrich weighs more than 300 pounds.

Earth is home to about 9,000 species of birds. Tropical lands are home to about half of these species, among them parrots, toucans, and parakeets. Other species live in forests, grasslands, and wetlands. Unfortunately, many birds are now endangered. Lands where they once lived have been cleared for cities, farms, and highways.

Birds fill many roles in the environment. Some eat insects, others are scavengers that clean up the decaying remains of dead organisms. A few birds also pollinate flowers, which helps plants reproduce.



The fossil record shows that the first mammal appeared about two million years ago. A mammal is a vertebrate that has hair and that makes milk for its young.

Early mammals were small shrew-like creatures. They probably ate insects and parts of plants. They were warm-blooded and had large eyes, which suggests they were active in the cool of the night when the great dinosaurs were resting. After dinosaurs died out, the mammals spread throughout the world. Some mammals, such as bats, fly. Some, such as moles and shrews, live underground. Whales and dolphins live in the ocean. Hundreds of species of mammals now share the land.

Although some mammals fay eggs (the platypus), most mammals are born alive. Some mammals, such as kangaroos and opossums, nourish their newborn young in pouches. These animals are called marsupials. Some mammals get together only for mating. Others live in pairs or family groups. Still others live in large herds or colonies. Mammals are further divided into groups according to their foot and tooth structures.

As a group, mammals are the most intelligent creatures on Earth. Their ability to adapt to new situations, to learn from past experience, and to communicate with one another has enabled them to thrive in many habitats. Many mammals use tools and build simple shelters. They live in complex social groups that require cooperation and communication.