Mammals - 4600 species

Animals that raise their 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. 

(Encarta Encyclopedia)