How Scientists Classify Animals: A Beginner’s Guide

Scientists classify animals by placing each one into a nested set of groups, from broad to specific, based on shared body features. This page covers the basic groups: vertebrates and invertebrates, and the five main classes of vertebrates.

What does it mean to classify an animal?

Classifying an animal means sorting it into a group with other animals that share key features, then narrowing that group step by step.

Taxonomists (scientists who classify living things) use a standard set of ranks, from broadest to most specific: kingdom, phylum, class, order, family, genus and species (Encyclopaedia Britannica). A house cat, for example, is in the animal kingdom, the phylum that includes animals with a backbone, the class Mammalia, and eventually its own species. This page stops at the class level. It does not walk through every rank down to genus and species.

What makes an animal an animal, not a plant?

An animal is a living thing that breathes, eats other organisms for food, grows and reproduces, and most animals can move under their own power.

Plants are also living things that breathe, grow and reproduce, but plants make their own food using sunlight instead of eating other organisms. This page does not cover fungi, bacteria or other kingdoms of life, which follow different rules again.

What is the difference between a vertebrate and an invertebrate?

A vertebrate is an animal with a backbone, and an invertebrate is an animal without one. This single feature is the first major split in animal classification.

Insects, spiders, worms, jellyfish, crabs and octopuses are all invertebrates. This page covers only two invertebrate groups in detail, insects and spiders, and does not cover the many other invertebrate groups, such as mollusks and worms.

• Insects have three main body parts and six legs.
• Spiders have two main body parts and eight legs, and belong to a different group called arachnids, not insects.

What are the five main classes of vertebrates?

The five main vertebrate classes are mammals, birds, fish, reptiles and amphibians, and each class has a defining body covering and way of breathing.

• Mammals have hair or fur, breathe air and feed their young milk. Whiskers count as fur, so a whiskered animal meets that test even without a full coat.
• Birds have feathers and wings, breathe air and lay eggs.
• Fish have scales and fins, and breathe underwater using gills.
• Reptiles have dry, scaly skin, breathe air and lay eggs.
• Amphibians have smooth, moist skin and spend part of their life in water and part on land.

Body covering is one of the simplest ways to tell these classes apart, though it is not the only difference (New York State Department of Environmental Conservation). Mammals and birds are warm-blooded, meaning their body temperature stays steady. Fish, reptiles and amphibians are cold-blooded, meaning their body temperature changes with their surroundings. This page does not cover every exception within each class, such as egg-laying mammals.

Practice: classify these animals

Read each clue and name the class or group it describes.

1. Has feathers, lays eggs and is warm-blooded.
2. Has six legs and three main body parts.
3. Has smooth, moist skin and lives part of its life in water and part on land.
4. Has fur, breathes air and feeds its young milk.
5. Has eight legs and two main body parts.

Where can you learn more?

See the animals hub for more animal topics, and how animals adapt to their environment once you know which class they belong to. For printable practice, browse the science printables page. This page is an introduction and does not replace a full biology course.

Sources

• Encyclopaedia Britannica. Taxonomy: Ranks
• New York State Department of Environmental Conservation. The Five Classes of Vertebrates