I.

Introduction

Mammal, animal that raises its young on milk. Most mammals are covered with hair or fur, and most have specialized teeth that help them to cut or chew their food. Compared to other vertebrates (animals with backbones), mammals have highly developed nervous systems, and they show an intelligence and resourcefulness that few other animals can match. Mammals include some of the most familiar members of the animal kingdom, such as cats, dogs, elephants, and whales, and also human beings—a species that now dominates life on earth.

With the exception of three highly unusual mammals called monotremes, all mammals give birth to live young. Some young mammals are completely helpless when they are born, while others are relatively well developed. Despite these differences, all young mammals initially rely on their mothers for food, and stay with them until they are ready to fend for themselves. This close link between mother and offspring produces strong family ties, and allows young mammals to learn by copying their parents' behavior.

Mammals evolved from a group of reptiles called therapsids. The earliest true mammals, appearing over 200 million years ago, were only 5 cm (2 in) long and probably spent at least some of their lives in trees. These tiny mammals resembled shrews with four roughly equal short legs and sharp claws. Since that time mammals have evolved an extraordinary variety of body shapes and sizes. Of the approximately 4600 species of mammals alive today, most live on land, and most still move about on all four legs. But modern mammals also include animals that hop on two legs, ones that live permanently in water, and ones that can fly. These swimming and flying species include the world's largest mammal—the blue whale, which can grow over 30 m (100 ft) long—and also the smallest, the Kitti's hog-nosed bat. Discovered in 1973 in the forests of Thailand, this tiny bat is about the size of a bumblebee, and weighs just 2 g (0.07 oz).

The life spans of mammals vary as enormously as their sizes. Most shrews survive for less than a year, exhausting themselves in a life of almost ceaseless activity. By contrast, horses can live about 20 years, chimpanzees can live to be over 50, and elephants can survive into their 60s. Humans have the longest life span of any mammal, with a few individuals living over 110 years.

Mammals have adapted to some of the most extreme habitats on earth. They are warm-blooded, or endothermic, meaning that they maintain their body temperature within a narrow range despite changes in the environment. Polar bears survive on Arctic ice, while Arctic foxes can sleep on open snow in temperatures as low as –68° C (-90° F). Camels and kangaroo rats live in deserts, and can tolerate blazing temperatures that would kill many animals from cooler habitats.

Mammals can also tolerate the thin air of the highest mountains as well as the crushing pressures of the ocean depths. Yaks, for example, forage for food on mountain slopes at altitudes of up to 6100 m (20,000 ft), while sperm whales can dive to depths of at least 2100 m (7000 ft), holding their breath for over an hour. Mammals also include some of the world's greatest animal migrants. During its yearly migration from Arctic waters to the coast of Mexico and back, a gray whale may travel 20,000 km (12,500 mi).