AnimalsVertebrates and invertebrates
The first big split is the backbone. Animals with a backbone are vertebrates: mammals, birds, reptiles, amphibians and fish. Animals without one are invertebrates. Vertebrates are the animals we notice most, but invertebrates are far greater in number and variety.
Warm-blooded and cold-blooded
Mammals and birds are warm-blooded: their bodies make heat and keep a steady temperature, so they can stay active in the cold, but they need a lot of food. Reptiles, amphibians, fish and invertebrates are cold-blooded: their temperature follows their surroundings, so they need far less food but slow down when it is cold.
A quick comparison
| Group | Body covering | Breathes with | Young | Blood |
|---|---|---|---|---|
| Mammals | Hair or fur | Lungs | Born live, fed on milk | Warm |
| Birds | Feathers | Lungs | Hard-shelled eggs | Warm |
| Reptiles | Dry scales | Lungs | Mostly leathery eggs on land | Cold |
| Amphibians | Moist bare skin | Gills when young, then lungs and skin | Soft eggs in water | Cold |
| Fish | Mostly scales | Gills | Mostly eggs in water | Cold |
These school groups are a simple way to begin. Scientists use a more detailed system. See how animals are classified.