Animal Kingdom

The animal kingdom is the group of living things we know best. Animals are multi-cellular creatures that get their energy by eating other living things (or what is left of them), they can usually move at least part of their body, and they sense the world around them. The animal kingdom contains over 1.5 million known species, from microscopic mites to the giant blue whale, and biologists think the real number could be 7 to 9 million once everything is described.

• Known animal speciesapprox. 1.5 millionMost are insects
• Estimated total7 to 9 millionMost of the rest live in the ocean or rainforests
• BiggestBlue whaleUp to 30 m, 200 tonnes
• SmallestRotifers and tardigradesUnder 0.1 mm long
• First animalsapprox. 600 million years agoSoft-bodied creatures in the oceans
• Animal phylaapprox. 35Each a major branch of the family tree

The main animal groups

Biologists sort animals into phyla (the big branches), then into classes, orders, families, genera and species. The most important split is between animals with a backbone (vertebrates) and animals without (invertebrates). 97% of all animal species are invertebrates.

• Mammals: warm-blooded, fur or hair, feed milk to their young. Includes humans.
• Birds: warm-blooded, feathered, lay eggs, almost all can fly.
• Reptiles: cold-blooded, scaly skin, mostly lay eggs.
• Amphibians: cold-blooded, soft skin, often live partly in water.
• Fish: cold-blooded, live in water, breathe through gills.
• Insects: 6 legs, 3 body parts, often wings. The most numerous group on Earth.
• Arachnids: 8 legs, 2 body parts. Spiders, scorpions, mites, ticks.
• Molluscs: soft-bodied, often shelled. Snails, octopuses, mussels.
• Crustaceans: hard outer shell, mostly water-dwelling. Crabs, lobsters, shrimp.

Why insects are the most successful group

Of every 10 animal species on Earth, roughly 8 are insects. There are about 1 million known species and possibly several million more not yet described. Insects are tough, small, fast-breeding and able to live in almost every habitat. They have been around for about 400 million years (longer than the dinosaurs), and they almost certainly will outlast us. There are around 10 quintillion (10,000,000,000,000,000,000) individual insects on Earth at any moment.

Why animals are in trouble

The animal kingdom is going through one of the worst times in its 600 million-year history. Many scientists call the current period a sixth mass extinction, and (unlike previous mass extinctions, which were caused by volcanoes or asteroids) this one is being driven by humans. Major threats include habitat loss, climate change, pollution, hunting and over-fishing. About 1 million species are currently at risk of extinction. The good news is that many threatened species can recover quickly if humans give them a chance.

Fact The first animals appeared in Earth's oceans around 600 million years ago, long before plants colonised the land or dinosaurs roamed the planet. The famous Cambrian explosion 538 million years ago saw an enormous burst of new animal designs, and almost every modern animal phylum traces its roots back to this time.

Pick a topic below to find out more about a specific group.