Animal Groups: Mammals, Birds, Reptiles and More

Sorting the animal kingdom

There are millions of different animals on Earth, from tiny ants to enormous whales. To make sense of so many, scientists sort them into groups. Animals in the same group share important features, like building blocks that fit together.

This sorting is called classification. It is a bit like tidying a huge box of mixed toys: you might put all the cars together, all the building bricks together, all the soft toys together. Scientists do the same with living things, looking carefully at how each animal's body is built.

The big split: backbones

The very first question scientists ask is simple: Does the animal have a backbone?

Run your fingers down the middle of your back. You can feel a row of bumpy bones — that is your backbone, or spine. Animals that have one are called vertebrates. That includes you, your pet dog, a fish and an eagle.

Animals with no backbone are called invertebrates. Think of worms, snails, spiders, crabs and insects. Their bodies are held up in other ways — some have a hard shell on the outside, like a beetle, and some are soft, like a slug. Surprisingly, most animals on Earth are invertebrates!

In this lesson we will focus on the five great groups of vertebrates.

The five vertebrate groups

Mammals

Mammals have hair or fur on their bodies, even if it is only a little. They are warm-blooded, which means their bodies stay at a steady warm temperature. Most importantly, mother mammals feed their babies on milk from their own bodies.

You are a mammal. So are dogs, cats, elephants, bats and dolphins. A bat can fly and a dolphin lives in the sea, but both have hair, breathe air, and feed their young milk — so both are mammals. Even baby animals look a lot like their parents; you can explore this in Baby Animals and Their Parents.

Birds

Birds are the only animals with feathers. They have a beak instead of teeth, two wings, and two legs. Birds are warm-blooded, and they lay eggs with hard shells.

Not every bird can fly — an ostrich and a penguin cannot — but every bird has feathers. Remember from the dinosaur story that birds evolved from small feathered dinosaurs, so in a way birds are living dinosaurs.

Reptiles

Reptiles have dry, scaly skin that keeps water in. Snakes, lizards, crocodiles, tortoises and turtles are reptiles. They are cold-blooded, which means their body temperature follows the surroundings — this is why you often see a lizard basking in the sun to warm up. Most reptiles lay eggs with leathery shells on land.

Amphibians

Amphibians live a double life — the name even means "two lives". They start in water as eggs that hatch into young (like a frog's tadpole) breathing through gills. As they grow, they change shape, develop lungs, and can live partly on land. Frogs, toads, newts and salamanders are amphibians. They have smooth, moist skin and must stay near water. You can see this transformation in The Life Cycle of a Frog.

Fish

Fish live in water all their lives. They breathe using gills, which take oxygen out of the water so the fish never needs to surface for air. They have fins to swim and most are covered in shiny scales. Goldfish, sharks and salmon are all fish.

A quick clue chart

Here is a handy way to identify which group an animal belongs to:

Group	Body covering	Breathes with	Young
Mammals	Hair or fur	Lungs	Live young, fed milk
Birds	Feathers	Lungs	Hard-shelled eggs
Reptiles	Dry scales	Lungs	Leathery eggs
Amphibians	Moist smooth skin	Gills then lungs	Soft eggs in water
Fish	Wet scales	Gills	Eggs in water

If you can spot the body covering and how it breathes, you can usually name the group.

Watch out for tricky animals

Some animals try to fool you. A bat flies like a bird but has fur and feeds milk — it is a mammal. A whale swims like a fish but breathes air through a blowhole — it is a mammal too. A penguin swims and cannot fly, yet it has feathers and lays eggs — it is a bird. The trick is to look past where an animal lives or how it moves, and check its real body features.

Try it yourself: become an animal sorter

Turn classification into a game using things you already have.

1. Gather a pile of toy animals, or cut out animal pictures from old magazines.
2. Lay out five labels on the floor or table: Mammals, Birds, Reptiles, Amphibians, Fish (add a sixth for Invertebrates if you like).
3. Pick up each animal and ask yourself the clue questions: Does it have fur, feathers or scales? Does it breathe with lungs or gills? Where does it lay its eggs?
4. Place each animal under the right label. Check a tricky one (like a bat or dolphin) against the chart above.

For an extra outdoor challenge, take a notebook into a garden or park and try to spot a real animal from at least two different groups — perhaps a bird overhead and an invertebrate beetle on the ground. Write down the clues that told you which group each one belonged to. You are now classifying just like a scientist.

To see how these groups fit into the living world around them, explore Food Chains and Ecosystems.