from Latin mamma "breast"), a clade of endothermic amniotes distinguished from reptiles (including birds) by the possession of a neocortex (a region of the brain), hair, three middle ear bones, and mammary glands.
Females of all mammal species nurse their young with milk, secreted from the mammary glands.
Mammals include the largest animal on the planet, the blue whale.
The basic body type is a terrestrial quadruped, but some mammals are adapted for life at sea, in the air, in trees, underground or on two legs.
The largest group of mammals, the placentals, have a placenta, which enables the feeding of the fetus during gestation.
Mammals range in size from the 30–40 mm (1.2–1.6 in) bumblebee bat to the 30-meter (98 ft) blue whale.
With the exception of the five species of monotreme (egg-laying mammals), all modern mammals give birth to live young.
The next three biggest orders, depending on the biological classification scheme used, are the Primates (apes and monkeys), the Cetartiodactyla (whales and even-toed ungulates), and the Carnivora (cats, dogs, seals, and allies).
Living mammals are divided into the Yinotheria (platypus and echidnas) and Theriiformes (all other mammals).
There are around 5450 species of mammal, depending on which authority is cited.
In some classifications, extant mammals are divided into two subclasses: the Prototheria, that is, the order Monotremata; and the Theria, or the infraclasses Metatheria and Eutheria.
The marsupials constitute the crown group of the Metatheria, and include all living metatherians as well as many extinct ones; the placentals are the crown group of the Eutheria.