Most mammals are viviparous, giving birth to live young. However, the five species of monotreme, the platypuses and the echidnas, lay eggs. The monotremes have a sex determination system different from that of most other mammals.^[37] In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian mammal.^[38]

The mammary glands of mammals are specialized to produce milk, a liquid used by newborns as their primary source of nutrition. The monotremes branched early from other mammals and do not have the nipples seen in most mammals, but they do have mammary glands. The young lick the milk from a mammary patch on the mother's belly.

Viviparous mammals are in the subclass Theria; those living today are in the marsupial and placental infraclasses. A marsupial has a short gestationperiod, typically shorter than its estrous cycle, and gives birth to an undeveloped newborn that then undergoes further development; in many species, this takes place within a pouch-like sac, the marsupium, located in the front of the mother's abdomen. The placentals give birth to complete and fully developed young, usually after long gestation periods.

Mammalia

 air-breathing vertebrate animals characterised by the possession ofendothermy, hair, three middle ear bones, and mammary glands functional in mothers with young. Most mammals also possess sweat glandsand specialised teeth. 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, the latter featuring red blood cells lackingnuclei and a four-chambered heart. Mammals range in size from the 30–40 millimeter (1- to 1.5-inch) bumblebee bat to the 33-meter (108-foot)blue whale.

The word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma ("teat, pap"). All female mammals nurse their young with milk, which is secreted from special glands, the mammary glands. According to Mammal Species of the World, 5,702 species were known in 2005. These were grouped in 1,229 genera, 153 families and 29 orders.^[1] In 2008 the IUCNcompleted a five-year, 17,000-scientist Global Mammal Assessment for its IUCN Red List, which counted 5,488 accepted species at the end of that period.^[2] In some classifications, the mammals are divided into two subclasses (not counting fossils): the Prototheria (order ofMonotremata) and the Theria, the latter composed of the infraclasses Metatheria and Eutheria. The marsupials comprise the crown group of the Metatheria and therefore include all living metatherians as well as many extinct ones; the placentals likewise constitute the crown group of the Eutheria.

Except for the five species of monotremes (which lay eggs), all living mammals 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, depending on the classification scheme used, are the primates, to which the human species belongs, the Cetartiodactyla (including the even-toed hoofed mammals and the whales), and the Carnivora (cats, dogs, weasels, bears, seals, and their relatives).^[1] While the classification of mammals at the family level has been relatively stable, different treatments at higher levels—subclass, infraclass, and order—appear in contemporaneous literature, especially for the marsupials. Much recent change has reflected the results of cladistic analysis and molecular genetics. Results from molecular genetics, for example, have led to the adoption of new groups such as the Afrotheria and the abandonment of traditional groups such as the Insectivora.

The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that also included Dimetrodon. At the end of theCarboniferous period, this group diverged from the sauropsid line that led to today's reptiles and birds. Preceded by many diverse groups of non-mammalian synapsids (sometimes referred to as mammal-like reptiles), the first mammals appeared in the early Mesozoic era. The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era.