Mammals are a class of vertebrate animals whose females are characterized by the possession of mammary glands while both males and females are characterized by sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain.
Except for the five species of monotremes (which lay eggs), all mammal species give birth to live young. Most mammals also possess specialized teeth, and the largest group of mammals, the placentals, use a placenta during gestation. The mammalian brain regulates endothermic and circuolatory systems, including a four-chambered heart.
There are approzimately 5,400 species of mammals, distributed in abouy 1,200 genera, 153 families, and 29 orders (though this varies by classification scheme). Mammals range in size from the 30-40 millimetre (1.2-1.6 in) Bumblebee Bat to the 33-metre (110 ft) Blue Whale.
The mammalian line of descent diverged from an amniote line at the end of the Carboniferous period. One line of amniotes would lead to reptiles, while the other would lead to synapsids, including mammals. The first true mammals appeared in the Triassic period. Modern Mammalian orders appeared in the Palaeocene and Eocene epochs of the Palaeogene period.
