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We are presently living in the Age of Mammals, which began more than 60 million years ago. During this period, mammals have spread over the entire earth and can now be found in almost every kind of climate and habitat. They live where it is wet, dry, hot, cold, or in-between. They can be found in the air, in the waters, on the ground, and underground. However, most of them are primarily land creatures – living in jungles, deserts, prairies, plains, woodlands, mountainous areas, or on frozen icecaps.
Humans, the most advanced mammals of all, can be found on all inhabitable land masses of earth and have even managed to visit the moon. One day technology may even make it possible for humans to live comfortably in the driest deserts, on the coldest mountain tops, beneath the deepest oceans, and in the far reaches of space.
There is no such thing as a typical mammal. Mammals range in size from the hundred-foot blue whale, which weighs many tons, to the pigmy shrew, which is less than four inches long, including its tail, and weighs about as much as a penny. Mammals can walk, run, hop, swim, climb, glide, and fly.
Scientists believe the first tiny, primitive mammals appeared on the earth about 280 million years ago during the Age of Reptiles and that they evolved from mammal-like reptiles. As the larger reptiles began to decrease in number and eventually became extinct, mammals began to increase and evolve. When mammals became the dominant animal on earth, the Age of Reptiles came to an end. Mammals are still the dominant animals and the most advanced or highly evolved of all creatures, although not the most numerous; that distinction belongs to the insects.
All mammals are warm-blooded, air-breathing vertebrates (animals with backbones) with internal skeletons made of bone. However, these characteristics are shared by other groups of animals. Birds also are warm-blooded; birds and reptiles also breathe air with lungs all their lives; and birds, reptiles, amphibians, and most fishes are vertebrates with skeletons made of bones. (Sharks have skeletons made of cartilage instead of bones.) However, there are two special characteristics that separate mammals from all other warm-blooded, air-breathing vertebrates.
The first characteristic is the one that gives mammals their name. "Mammal" comes from the Latin word mamma, which means breast, and indicates that the females of this group produce milk within their bodies to feed their young. This milk is removed as the young suck on the female's mammary glands. The location and number of these glands varies with the species, as does the length of time the young are fed this milk. Depending on the animal, the young may nurse for only a few days or they may rely on their mother's milk for several months. The fat, protein, sugar, and water content of the milk also varies according to the species. The richest milk produced by any mammal comes from the seal and is about 53 percent fat. Milk produced by the desert-dwelling kangaroo rat is about 50 percent water to meet the moisture requirements of the young.
The other characteristic that sets mammals apart is the fact that they all have true hair. The presence of hair has made it possible for mammals to live in almost any climate. Hair not only insulates their bodies from the cold, but it also protects their tender skin from the sun's rays. Although all mammals have hair, it is not all alike. It may be long or short, thick or thin, coarse or soft, bristly or velvety. Some mammals, such as the elephant and whale, may have only a little. Others, such as the sheep, have hair so woolly and thick it is difficult to find the skin beneath it. Hair may serve as feelers, like the whiskers on a cat; as weapons, like the prickly quills on a porcupine; or as camouflage to help the mammal blend into its surroundings.
Most of the approximately four thousand types of mammals give birth to live young; however, there are two kinds of mammals that lay eggs – Australia's duckbilled platypus and the spiny anteater. Although their egg-laying habits may set them apart from other mammals, they still are included in this category because once their young hatch, the mothers produce milk to feed them and both species have hair.
Most mammals are carnivores, which means they eat other animals. Some, though, are herbivores, which means they eat only plants. Those which eat both plants and animals are called omnivores. Even if humans were not mammals, we would consider this group of animals very important. Much of our meat, all of our milk, and all of the natural fur we use, including wool, comes from mammals. All of our work animals and many of our pets also are mammals.
In the following pages you will get to know a little more about a few of our wild mammals as we take a look at their private lives.
1990 – Introduction: Introducing Mammals to Young Naturalists. The Louise Lindsey Merrick Texas Environment Series, No. 10, pp. 13-14. Texas A&M University Press, College Station.