What Is Animal Classification?
How scientists organize millions of animal species into neat groups based on what they have in common.
Why Sort Animals at All?
Scientists have discovered and named over 1.5 million animal species — and there are probably millions more still undiscovered. With that many creatures, you need a system to organize them, just like a library needs a system to organize books. The science of sorting living things into groups is called taxonomy, and it was pioneered by a Swedish scientist named Carl Linnaeus back in the 1700s.
Classification isn't random — it groups animals based on their shared characteristics: body structure, how they reproduce, how they breathe, whether they're warm- or cold-blooded, and how they're related through evolution. Animals that share more traits are placed in closer groups.
The Big Divide: Vertebrates vs. Invertebrates
The first big split in the animal kingdom is whether an animal has a backbone (spine) or not. Vertebrates — animals with backbones — include fish, amphibians, reptiles, birds, and mammals. They tend to be the animals we think of first: dogs, eagles, sharks, frogs. But vertebrates are actually the minority. Invertebrates — animals without backbones — make up roughly 97% of all animal species. Insects, spiders, worms, jellyfish, octopuses, and crabs are all invertebrates.
The Five Vertebrate Classes
Fish live in water, breathe through gills, are cold-blooded, and most are covered in scales. They were the first vertebrates to evolve, over 500 million years ago.
Amphibians — frogs, toads, salamanders — live a double life. They start in water (breathing with gills as tadpoles) and usually move to land as adults (breathing with lungs). Their skin must stay moist, which is why you find them near water. They're cold-blooded.
Reptiles — snakes, lizards, turtles, crocodilians — have dry, scaly skin and breathe air with lungs their entire lives. They're cold-blooded, meaning their body temperature depends on their environment. Most lay eggs on land.
Birds are the only animals with feathers. They're warm-blooded, lay hard-shelled eggs, and most can fly (though penguins, ostriches, and kiwis cannot). Birds are actually living dinosaurs — they evolved from a group of small, feathered theropod dinosaurs.
Mammals are warm-blooded, have hair or fur, and feed their young with milk. Humans are mammals, along with dogs, whales, bats, and elephants. Most mammals give birth to live young, though a few (like the platypus) lay eggs.
The Hierarchy: From Kingdom to Species
Scientists organize life using a series of ranks, from broadest to most specific: Kingdom → Phylum → Class → Order → Family → Genus → Species. A helpful memory trick is: "King Philip Came Over For Good Spaghetti." For example, a domestic cat is classified as Kingdom Animalia, Phylum Chordata, Class Mammalia, Order Carnivora, Family Felidae, Genus Felis, Species catus. Each level narrows the group until you reach a single species.
The animal kingdom contains some classification surprises. A whale is not a fish — it's a mammal that breathes air, nurses its young, and is warm-blooded. A bat is not a bird — it's the only mammal that can truly fly. And a spider is not an insect — insects have six legs and three body parts, while spiders have eight legs and two body parts. Classification is all about looking past appearances to find what creatures truly have in common.
Last reviewed: April 2026