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