Mammals began to dominate the Earth about 65 million years ago. Without a doubt, modern humans are the most successful mammals—they occupy all the Earth’s habitats! Their domestic coexistence with other species began barely 10,000 years BC, when human culture transitioned from a world of nomadic hunters and gatherers to a society based on agriculture. At that time, humans began to benefit from the meat and milk products of small mammals and to use large animals for labor. The first animals to be domesticated were sheep (about 9000 BC) in the Middle East. Pigs, cows, goats, and dogs followed.
However, the great majority of mammal species continue, even today, to live in the wild. There are 5,416 known mammal species distributed over different land and aquatic environments. Despite the characteristics that make them part of the same class, their diversity is such that the smallest of them, the shrew, may weigh only one tenth of an ounce (3 g), and the largest, the blue whale, can reach 160 tons. But their diversity is also evident in their adaptation to different environments. There are mammals that run and others that glide—some fly, and others jump, swim, or crawl. Most aquatic mammals have suppressed the development of hair or fur, replacing it with thick layers of fat. The rigors of low temperatures have made some animals—such as polar bears, dormice, and certain bats—exceptions to the vital law of homeothermy, as they spend the winter sunk in deep sleep to save energy.
Seals, dolphins, bats, and chimpanzees all have upper limbs with similar bones, but the environmental niche they occupy has made seals develop flippers, dolphins fins, bats wings, and chimpanzees arms. Thus from the polar tundra to the dense tropical jungle, through the deep oceans and high mountain lakes, the whole Earth has been populated by thousands of mammal species.
But this marvelous animal world has been disturbed by its most numerous species—humankind. Indiscriminate hunting, illegal trade, deforestation, urbanization, massive tourism, and pollution have left more than a thousand species (many of them mammals) endangered or vulnerable. However, science allows us to understand nature’s many wonders, and it can help us respect the world’s ecological balance. In this book, which includes dazzling photographs and illustrations, we invite you to discover many details of mammals’ lives: their life cycles, their social lives, their special features, and their characteristics, from those of the greatest friend of them all, the dog, to the mysterious and solitary platypus.
