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.With the exception of the five species of monotreme (egg-laying mammals), all modern mammals give birth to live young.
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.
Mammals can organize themselves into fission-fusion societies, harems, and hierarchies, but can also be solitary and territorial.
Most mammals are polygynous, but some can be monogamous or polyandrous.
The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that produced the non-mammalian Dimetrodon.