Mammals can communicate and vocalize in several ways, including the production of ultrasound, scent-marking, alarm signals, singing, and echolocation. Most mammals are intelligent, with some possessing large brains, self-awareness, and tool use. The most species-rich group of mammals, the cohort called placentals, have a placenta, which enables the feeding of the fetus during gestation.

All modern mammals give birth to live young, except the five species of monotremes, which are egg-laying mammals. Maximum lifespan varies from two years for the shrew to 211 years for the bowhead whale. Mammals range in size from the 30–40 mm (1.2–1.6 in) bumblebee bat to the 30 m (98 ft) blue whale-possibly the largest animal to have ever lived. The basic body type is quadruped, and most mammals use their four extremities for terrestrial locomotion but in some, the extremities are adapted for life at sea, in the air, in trees, underground, or on two legs. The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of non-avian dinosaurs, and have been the dominant terrestrial animal group from 66 million years ago to the present. Mammals originated from cynodonts, an advanced group of therapsids, during the Late Triassic. The synapsids split into several diverse groups of non-mammalian synapsids - traditionally and incorrectly referred to as mammal-like reptiles or by the term pelycosaurs, and now known as stem mammals or protomammals - before giving rise to Therapsida in the Early Permian period. At the end of the Carboniferous period around 300 million years ago, synapsids and sauropsids diverged. The early synapsid mammalian ancestors were sphenacodonts, a group that included the non-mammalian Dimetrodon. In terms of cladistics, which reflects evolutionary history, mammals are the only living members of the Synapsida (synapsids) this clade, together with Sauropsida (reptiles and birds), constitutes the larger Amniota clade. The next three are the Primates (including humans, apes, monkeys, and others), the Artiodactyla ( cetaceans and even-toed ungulates), and the Carnivora ( cats, dogs, seals, and others). The largest orders are the rodents, bats and Eulipotyphla ( hedgehogs, moles, shrews, and others). Around 6,400 extant species of mammals have been described. These characteristics distinguish them from reptiles and birds, from which they diverged in the Carboniferous, over 300 million years ago. Mammals (from Latin mamma, 'breast') are a group of vertebrate animals constituting the class Mammalia ( / m ə ˈ m eɪ l i ə/), and characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or hair, and three middle ear bones.

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