mammal

Any of a large group of warm-blooded vertebrate animals characterized by having mammary glands in the female; these are used for suckling the young. Other features of mammals are hair (very reduced in some species, such as whales); a middle ear formed of three small bones (ossicles); a lower jaw consisting of two bones only; seven vertebrae in the neck; and no nucleus in the red blood cells. (Class Mammalia.)

Mammals are divided into three groups:

placental mammals, where the young develop inside the mother's body, in the uterus, receiving nourishment from the blood of the mother via the placenta;

marsupials, where the young are born at an early stage of development and develop further in a pouch on the mother's body where they are attached to and fed from a nipple; and

monotremes, where the young hatch from an egg outside the mother's body and are then nourished with milk.

The monotremes are the least evolved and have been largely displaced by more sophisticated marsupials and placentals, so that there are only a few types surviving (platypus and echidna). Placentals have spread to all parts of the globe, and where placentals have competed with marsupials, the placentals have in general displaced marsupial types. However, marsupials occupy many specialized niches in South America and, especially, Australasia.

According to the Red List of endangered species published by the World Conservation Union for 1996, 25% of mammal species are threatened with extinction.

The theory that marsupials succeed only where they do not compete with placentals was shaken in 1992, when a tooth, 55 million years old and belonging to a placental mammal, was found in Murgon, Australia, indicating that placental animals appeared in Australia at the same time as the marsupials. The marsupials, however, still prevailed.

There are over 4,000 species of mammals, adapted to almost every way of life. The smallest shrew weighs only 2 g/0.07 oz, the largest whale up to 140 tonnes. A 50-million-year-old jaw discovered in 1998 is from the smallest mammal ever to have lived, Batodonoides, an insectivore weighing only 1.3 g/0.046 oz.

Mammals include humans (order Primates).