I.

Introduction

Mammal, common name applied to any warm-blooded animal belonging to the class that includes human beings and all other animals that nourish their young with milk, are covered with varying amounts of hair, and possess a muscular diaphragm. The class Mammalia, which is represented by about 4,600 living species, is usually divided into three subclasses: the monotremes (egg-laying mammals), the marsupials (usually mammals with pouches), and the placental mammals. The majority of mammals are placental mammals. Mammals have the most highly developed nervous systems, including the brain, of all animals. Most members of the group have four appendages, usually legs. These may be adapted for use as swimming appendages, as in seals, or as wings, as in bats. Some types, however, have two limbs that have been reduced to small vestiges beneath the skin, as in whales, or have been lost altogether, as in sea cows. All mammals, except the monotremes, produce live young that undergo the early stages of development within the body cavity of the mother (see Development). Some mammals are helpless at birth; others are able to walk and even run immediately, and may be born fully furred and with their eyes and ears open. The range in size of mammals is extraordinarily wide; the largest mammal, the blue whale, often exceeds 30 m (100 ft) in length, and the smallest shrews, mice, and bats are less than 5 cm (2 in) in length, excluding the tail.