How animals are classified
Backbone or no backbone, and then the five classes of vertebrate — fish, amphibians, reptiles, birds and mammals — with the traits that give each one away, plus the animals that break every rule.
Animals
Scientists have named around one and a half million kinds of animal, and they think most species are still waiting to be found. To make sense of that crowd, biologists sort animals by what they share, starting with one simple question about the back.
Invertebrates: no backbone
The first group has no backbone at all. Invertebrates are about ninety seven of every hundred animal species, from a jellyfish with no brain to an octopus clever enough to open a screw top jar. Having no spine is the normal way to be an animal.
Insects
Insects alone account for more than a million named species, more than every other animal group put together. Each one wears its skeleton on the outside, has six legs and three body sections, and has to climb out of that armour to grow any bigger.
Worms, snails and jellyfish
Invertebrates go far beyond insects. A snail builds its shell out of its own body, an earthworm swallows soil and passes it out richer than it found it, and jellyfish have been drifting the oceans for more than five hundred million years with no heart and no brain.
Vertebrates: a backbone
Only about three in every hundred animal species have a backbone, yet they include almost every animal you could name. A spine is a chain of small bones that shields the nerve cord and lets a body grow large without folding under its own weight.
Fish
Fish were the first vertebrates, swimming more than five hundred million years ago. They pull oxygen straight out of the water with gills, they stay the temperature of whatever surrounds them, and most lay soft eggs by the thousand and simply swim away.
Amphibians
Amphibian means double life, and that is exactly the deal. A frog begins as a tadpole with gills and a tail, then rebuilds itself with lungs and legs. Adults still breathe partly through damp skin, which is why you never find one far from water.
Reptiles
Reptiles solved the water problem twice over, with waterproof scaly skin and a leathery egg that carries its own private pond inside the shell. Because a crocodile never spends energy heating itself, it can go for months between meals and barely notice.
Birds
Birds are the dinosaurs that made it through. Feathers came first for warmth and for showing off, and flight arrived later. Their bones are hollow and braced inside like a bridge, so a great frigatebird can spread wings over two metres wide on a skeleton lighter than its own feathers.
Mammals
Mammals are the fur and milk club. Every single one, from a two gram bumblebee bat to a blue whale of well over one hundred tonnes, feeds its young on milk, and all but five species give birth to live young instead of laying eggs.
Warm blooded or cold blooded
Birds and mammals burn food constantly to hold a steady inner temperature, which is why they have to eat so often. Fish, amphibians and reptiles take whatever temperature is around them, so a lizard sits on a warm rock each morning to get its engine started.
Eggs or live young
Most animals lay eggs and leave. An ocean sunfish can release hundreds of millions of them in a season and abandon every one. Mammals took the opposite bet, growing a few young inside the body, and an elephant spends nearly two years pregnant to produce a single calf.
Scales, feathers, fur
You can often name the class just by touching it. Fish wear thin bony scales, reptiles wear dry horny ones, birds wear feathers, mammals wear hair, and amphibians wear bare damp skin they can breathe through. Feathers, hair and reptile scales are all keratin, the same stuff as your fingernails.
The rule breakers
Every rule here has an exception. The platypus is a mammal that lays eggs and hunts with its eyes shut, sensing the faint electricity of a shrimp with its bill. In sea horses it is the male that gets pregnant. The groups are useful, but nature never agreed to obey them.
A family tree, not a filing cabinet
Classification is not a filing cabinet, it is a family tree. Two animals share a group because they share an ancestor, which is why a whale belongs beside the mice rather than the sharks, and why birds sit inside the dinosaurs rather than next to them.
Watch it explain itself
Every step above is narrated aloud. Play it, or open it in the editor and make it yours — no account needed.