The six big animal groups

Mammals are warm-blooded vertebrates that nurse their young with milk and, in almost every case, grow hair or fur. They range from the 30-gram bumblebee bat to the 150-tonne blue whale and live on every continent and ocean on Earth.

Birds are the only living dinosaurs. Every one of the ~11,000 species has feathers, a toothless beak, and lays hard-shelled eggs. Most fly, but some — like penguins and ostriches — have traded flight for swimming or running.

Reptiles are air-breathing vertebrates covered in scales or scutes. This group includes turtles, lizards, snakes, crocodilians, and the lizard-like tuatara. Most lay eggs on land, and all rely on the environment to regulate body heat.

Amphibians — frogs, salamanders, and the worm-like caecilians — typically begin life in water as gilled larvae, then metamorphose into air-breathing adults. Their permeable skin makes them sensitive barometers of environmental health.

Fish are the most diverse group of vertebrates, with well over 34,000 species. They breathe through gills, move with fins, and occupy nearly every aquatic habitat — from Himalayan streams to the crushing dark of ocean trenches. Sharks and rays belong here too, in the cartilaginous branch.

Invertebrates are animals without a backbone: insects, spiders, crustaceans, molluscs, corals, worms, jellyfish and more. They make up the overwhelming majority of animal life and run the planet’s pollination, decomposition, and ocean food webs.