Vertebrates: Mammals, Birds, Reptiles, Amphibians, and Fish

Vertebrates are animals with a backbone, or vertebral column, and they are grouped into five classes: mammals, birds, reptiles, amphibians and fish. In brief: mammals have hair and feed their young milk, birds have feathers and lay hard-shelled eggs, reptiles have dry scaly skin, amphibians have moist skin and usually start life in water, and fish have gills and fins. A frog (amphibian), a lizard (reptile) and a salmon (fish) are all vertebrates.

Vertebrates make up only a small part of animal species, but they include the largest and best-known animals. This guide describes what all vertebrates share, compares the five classes in a table, shows how to identify the class of an animal with a worked example and explains the common mistakes in classifying them.

What are vertebrates?

Vertebrates belong to the phylum Chordata. At some stage of development, chordates have a stiffening rod called the notochord. In vertebrates, this rod is replaced during development by a column of bones or cartilage called the backbone, which protects the spinal cord.

Other shared features include a skull that protects the brain, a closed circulatory system with a heart, and an internal skeleton that grows with the animal. Vertebrates have bilateral symmetry and, in most species, two pairs of limbs or fins.

A common way to organize the groups is by whether they control their body temperature using their own metabolism. Birds and mammals are endothermic, which means they produce most of their body heat internally. Fish, amphibians and reptiles are ectothermic and rely mainly on the environment to warm up.

What are the characteristics of fish?

Fish live in water and breathe with gills. Their bodies are usually streamlined, with fins for steering and swimming, and many have scales covered with mucus. Most fish have a two-chambered heart and fertilize eggs in water.

There are three main groups: bony fish such as tuna and trout, cartilaginous fish such as sharks and rays, and jawless fish such as lampreys. Bony fish usually have a swim bladder, an air-filled organ that helps them control their buoyancy.

What are the characteristics of amphibians?

Amphibians, including frogs, toads, newts and salamanders, live part of their lives in water and part on land. Their skin is thin, moist and without scales, and it allows gases to pass through. Many species can absorb oxygen through the skin in addition to using lungs.

Most amphibians lay soft, jelly-covered eggs in water. The young, called larvae or tadpoles, have gills and later undergo metamorphosis into adults with lungs and legs. Because of their permeable skin and need for water, amphibians are sensitive to pollution and are used as indicators of environmental health.

What are the characteristics of reptiles?

Reptiles include snakes, lizards, turtles and crocodiles. They have dry skin covered with scales, which reduces water loss, and they breathe air with lungs for their whole lives. Most reptiles lay eggs with leathery or hard shells on land, though some give birth to live young.

As ectotherms, reptiles bask in the sun to warm up and move into the shade to cool down. This low use of energy means they can live on much less food than a mammal of the same size.

What are the characteristics of birds?

Birds are the only living animals with feathers. They have wings that are modified forelimbs, a beak without teeth and lightweight, hollow bones. Most birds can fly, but ostriches, penguins and kiwis are flightless.

Birds are endothermic, have a four-chambered heart and lay hard-shelled eggs that they usually incubate. Feathers insulate the body, enable flight and sometimes serve for display. Most birds care for their young after hatching.

What are the characteristics of mammals?

Mammals have hair or fur, at least at some stage of life, and females produce milk in mammary glands. They are endothermic, have a four-chambered heart and a muscular diaphragm that helps breathing. Mammals have three small bones in the middle ear and typically have different kinds of teeth.

Most mammals give birth to live young that develop inside the mother, nourished through a placenta. There are two exceptions: monotremes such as the platypus lay eggs, and marsupials such as kangaroos give birth to very undeveloped young that finish development in a pouch.

Vertebrate classes compared

Worked example: identifying the class of an animal

An animal has a backbone, dry skin covered with overlapping scales, breathes with lungs and lays eggs with leathery shells on land. It basks in the sun in the morning. Use the table above as a key.

1. Backbone: It has one, so it is a vertebrate.
2. Body covering: Dry scales. This excludes birds and mammals, and moist-skinned amphibians. Fish also have scales, so more clues are needed.
3. Breathing: Lungs. Fish breathe with gills, so fish are excluded.
4. Temperature and eggs: Basking suggests ectothermy, and leathery land eggs fit the reptile pattern.

The animal is a reptile, and a lizard or snake is a likely example. Using several features is important. The covering alone does not separate fish from reptiles, but the covering together with the breathing organ does.

How did the vertebrate groups evolve?

The fossil record shows that fish appeared first, more than 500 million years ago. Some lobe-finned fish later developed limbs, and the first amphibian-like vertebrates moved onto land around 370 million years ago. Reptiles evolved shell-covered eggs, which freed them from the need for water to reproduce.

Birds evolved from a group of feathered dinosaurs, so in an evolutionary sense they are living members of the reptile lineage. Mammals arose from a different reptile-like ancestor group in the Triassic period. Biologists therefore sometimes place birds with reptiles in more modern classifications, although school courses still keep the five classes.

Which mistakes are common?

Mistake 1: Calling whales and dolphins fish

They live in water, but they breathe air with lungs, are endothermic, give birth to live young and feed them milk. They are mammals.

Mistake 2: Saying that all reptiles lay eggs

Most do, but some snakes and lizards give birth to live young. The better statement is that most reptiles lay eggs.

Mistake 3: Confusing “cold-blooded” with having cold blood

Ectotherms do not have cold blood. Their body temperature changes with the environment, and a basking lizard can be warmer than a mammal.

Mistake 4: Assuming all birds fly

Flightlessness has evolved in several birds, such as penguins, ostriches and emus. The defining feature of birds is feathers, not flight.