Animals are divided into two basic categories: having vertebrae and having no backbone at all. Furthermore, these two basic categories are even subdivided into another form of categorizing characteristic, much like another lower-level hierarchy division. For vertebrae, we have mammals, reptiles, amphibians, birds and fishes.

Mammals are differentiated from other vertebrates mainly because of their ability to feed their young from their bodies and give birth to live young. However, some mammals are giving birth to eggs such as the duck-billed platypus. Furthermore, mammals are warm-blooded, that is they can maintain their body temperature even if their surroundings change. There are about four thousand species of mammals in this world. Some mammals live in water unlike most of their counterparts. Whales and dolphins spend their life in water but breath air at the surface because like other mammals, they have lungs. Only one specie is known to possess the power of flight and they are bats.

Reptiles are descendants of the ancient dinosaurs that reigned the earth millions of years ago. They have the distinguishing characteristic or dry scales and eggs of leathery shells. There are four groups that still exist today: crocodiles, turtles, snakes and tuataras. Reptiles are cold-blooded, that is they gain their heat from their surroundings.

Fishes are water vertebrates. They have developed gills that enable them to thrive in the oceans and obtain oxygen through the water entering their mouths. There are quite a number of species under fishes. Like amphibians and reptiles, fishes are cold-blooded.

Birds are essentially, vertebrates that can fly, but some birds have to adapt to their environment, losing their power of flight for something that is more useful where they lived. Birds are differentiated from others because of the presence of wings, body covered with feathers and beaks for mouths.