Mammals



Mammals (formally Mammalia /məˈmeɪli.ə/) are members of a class of air-breathing vertebrate animals characterized by the possession of hair, three middle ear bones, and mammary glands functional in mothers with young. Most mammals also possess sweat glands and specialised teeth, and the largest group of mammals, the placentals, have a placenta which feeds the offspring during gestation. The mammalian brain, with its characteristic neocortex, regulates endothermic and circulatory systems, including a four-chambered heart. Mammals range in size from the 30–40 millimetre (1- to 1.5-inch) Bumblebee Bat to the 33-metre (108-foot) Blue Whale.

The number of species accepted by the zoological community on any given calendar date depends on the classification scheme, and the figure changes continually as new species are discovered and the classification of others is revised. According to a major reference work, Mammal Species of the World, which is updated through periodic editions, 5676 species were known in 2005, distributed in 1,229 genera, 153 families and 29 orders.[1] In 2008 the IUCN completed a 5-year, 17,000 scientist Global Mammal Assessment for its IUCN Red List, which counted 5488 accepted species at the end of that period.[2] The class is divided into two subclasses (not counting fossils): the Prototheria (order of Monotremata) and the Theria, the latter containing the infraclasses Metatheria (including marsupials) and Eutheria (the placentals). The classification of mammals between the relatively stable class and family levels having changed often, different treatments of subclass, infraclass and order appear in contemporaneous literature, especially for Marsupialia.

Except for the five species of monotremes (which lay eggs), all living mammal species give birth to live young. Most mammals, including the six most species-rich orders, belong to the placental group. The three largest orders, in descending order, are Rodentia (mice, rats, porcupines, beavers, capybaras, and other gnawing mammals), Chiroptera (bats), and Soricomorpha (shrews, moles and solenodons). The next three largest orders include the Primates, to which the human species belongs, the Cetartiodactyla (including the even-toed hoofed mammals and the whales) and the Carnivora (dogs, cats, weasels, bears, seals, and their relatives).[1]

The early synapsid mammalian ancestors, a group which included pelycosaurs such as Dimetrodon, diverged from the amniote line that would lead to reptiles at the end of the Carboniferous period. Although they were preceded by many diverse groups of non-mammalian synapsids (sometimes referred to as mammal-like reptiles), the first true mammals appeared 220 million years ago in the Triassic period. Modern mammalian orders appeared in the Palaeocene and Eocene epochs of the Palaeogene period. Phylogenetically, the clade Mammalia is defined as all descendants of the most recent common ancestor of monotremes (e.g., echidnas and platypuses) and therian mammals (marsupials and placentals).[3] This means that some extinct groups of "mammals" are not members of the crown group Mammalia, even though most of them have all the characteristics that traditionally would have classified them as mammals.[4] These "mammals" are now usually placed in the unranked clade Mammaliaformes together with true mammals.