Mammals, the group of vertebrates (animals with backbones) to which human beings belong. Mammals are distinguished from all other animals by their mammary glands, glands that produce milk to feed the young. Mammals are also unique in having hair, although it may be scant. They are warm-blooded and air-breathing, and all but the whales and sirenians, or sea cows, have four limbs.

The Cenozoic Era,The Cenozoic Era, from sixty-five million years ago to the present, is the Age of Mammals.

Mammals range in size from shrews that weigh 1/14 ounce (2 g) to whales that weigh 140 tons (127,000 kg). They are found in almost all parts of the world—in the tropics and the Arctic, in rain forests and deserts. Most kinds of mammals live on land. Some kinds, including whales and walruses, live in the water. Bats are the only flying mammals.

All mammals except the duckbill platypus and the echidna, which lay eggs, give birth to live young. Mammals care for their young for longer periods than other animals do. Mammals have a more highly developed brain than other animals, with correspondingly higher intelligence.

Some mammals are the source of products used for food, shelter, clothing, and other purposes. Some are trained to do work, and others are enjoyed as pets. By studying other mammals, people have gained insight into human behavior and learned how to prevent and cure certain diseases.

Mammals appeared more than 200,000,000 years ago; they evolved from reptile-like animals called therapsids. The earliest known mammals, called triconodonts, resembled shrews, but layed eggs. Marsupials (mammals with external pouches in which young are nourished) and placentals (mammals with wombs in which unborn young are nourished) appeared about 100 million years ago. About 65 million years ago (the beginning of the Cenozoic Era, or Age of Mammals), mammals increased in number, diversity, and size. About 10,000 years ago, a number of very large land mammals, including ground sloths and woolly mammoths, became extinct.

Interesting facts about mammals
The largest mammal --and the largest animal that has ever lived--is the blue whale. It measures up to 100 feet (30 meters) long when fully grown.
The Kitti's hog-nosed bat of Thailand is probably the smallest mammal. It is about the size of a bumble bee and weighs no more than a penny.
The rhinoceros has horns that look like closely packed hairs. Actually, they consist of many fibers of keratin, the horny substance that makes up the nails of people. African rhinos have two horns. Indian rhinos have one.
A young lion tamarin rides on its father's back. Most male mammals have little to do with raising their offspring. But among these South American monkeys, the father and mother share the job of carrying and protecting their babies.
The hyrax is a small mammal that looks much like a guinea pig. But scientists believe that its nearest relatives are actually elephants. Hyraxes, which are also known as conies, live in Africa and the Middle East.
How Mammals Are Classified

Mammals form the class Mammalia of the subphylum Vertebrata of the phylum Chordata. There are four subclasses, two of which have only extinct members. Subclass Prototheria consists of only one living order, Monotremata, the only egg-laying mammals. All other orders of living mammals belong to subclass Theria, which also includes extinct members. Many zoologists recognize the following 19 orders of Theria (listed in alphabetical order). Some, however, distinguish a 20th order, the Pinnipedia (seals and walruses); others include the Pinnipedia with the Carnivora.

1. Artiodactyla, hoofed mammals with an even number of toes.

2. Carnivora, the flesh-eaters.

3. Cetacea, mammals completely adapted to life in water.

4. Chiroptera, flying mammals.

5. Dermoptera, gliding mammals. There are only two species.

6. Hyracoidea, the conies, of Asia and Africa.

7. Insectivora, primitive, insect-eating mammals.

8. Lagomorpha, small, leaping mammals.

9. Macroscelidea, the elephant shrews, of Africa.

10. Marsupialia, mammals that carry their young in a pouch.

11. Perissodactyla, hoofed mammals with an odd number of toes.

12. Pholidota, the pangolins, or scaly anteaters, of Asia and Africa.

13. Primates, mammals with the most highly developed brain.

14. Proboscidea, heavy-footed mammals with proboscises.

15. Rodentia, gnawing mammals. This is the largest order of mammals.

16. Scandentia, the tree shrews.

17. Sirenia, the sea cows.

18. Tubulidentata, the aardvark, found only in Africa.

19. Xenarthra, a primitive group found in the Americas.