Of all the classes of animal life, mammals are considered the most advanced and probably the most popular class. Dogs and cats are mammals, squirrels and rabbits in our backyards are mammals as well. Horses, sheep, baboons, giraffes and elephants are mammals. For that matter, we human beings are mammals too.
Mammals vary greatly in size. The smallest mammal is the shrew with a body that is only a little more than 2 inches long and weighs less than some insects. In contrast, the largest mammal is the blue whale, which can sometimes measure up to 100 feet long and weigh as much as 130 tons. Mammals also differ in appearance in shape. Most of them walk on four legs, but not all of them do. Some mammals fly. Dolphins and whales have lost their hind limbs and now have taken fish-like shapes and spend their entire lives in the ocean.
What is a mammal? Mammals are vertebrates - they are animals with backbones. All mammals have lungs and breathe in air and all of them are warm-blooded (they are able to maintain a constant body temperature regardless of the outside temperature.) Mammals and birds share a common characteristic - the possession of four-chambered hearts that circulate blood efficiently to all parts of the body. All mammals, except two types that lay eggs, give birth to living young and provide protection and care for them before and after birth. Mammals are the only animals that possess true hair and the capacity to produce milk. In fact, the word "mammal" comes from the Latin word "mamma" which means "breast".
Mammals have other characteristics that are not so obvious. A mammal's heart and lungs are separated from the stomach by a wall of muscle called the diaphragm. A mammal's lower jaw has a single bone on each side. Mammals also have different types of teeth adapted to different uses. And most importantly, mammal brains are much more highly developed than the brains of any other animal.
In the struggle to survive, warm blood, improved methods of caring for young and superior intelligence have all given mammals great advantages through the ages. In fact, many scientists consider mammals as the dominant animals of the world.
Yet mammals did not start out immediately as mammals. Strange as it may seem, they rose from the reptile family. In the Mesozoic era, a branch of reptiles began to grow coats of hair instead of armor and slowly turned warm-blooded. Some of them began to keep eggs inside their bodies instead of laying eggs. The first mammals were probably very tiny and timid creatures, like today's rats and mice. They were nocturnal and hid for protection during the day. When conditions of the Earth gradually changed, the dinosaurs could not adapt to this dramatic shift, so their numbers grew smaller and smaller. When they died out, the more adaptable mammals later arose in the Miocene epoch.
Between 3500 and 5000 species of mammals live in the world today and more varieties under them. Many of these animals can trace their lineage to their ancestors that adapted and survived through the Ice Age. Now, mammals have developed in a bewildering variety of sizes and shapes, classified by scientists according to body structure and relationships. In all, under the mammal kingdom, there are now 18 different groups or orders.
