Mammals can organize themselves into fission–fusion societies, harems, and hierarchies-but can also be solitary and territorial. Mammals can communicate and vocalize in several ways, including the production of ultrasound, scent-marking, alarm signals, singing, echolocation and, in the case of humans, complex language. Most mammals are intelligent, with some possessing large brains, self-awareness, and tool use. The most species-rich group of mammals, the infraclass 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 mammalian 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. Modern mammalian achieved their modern diversity 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 to Early Jurassic. The more advanced therapsids became dominant during the Middle Permian. Early synapsids are referred to as " pelycosaurs". Mammals are the only living members of Synapsida this clade, together with Sauropsida (reptiles and birds), constitutes the larger Amniota clade. The next three are the primates (including humans, monkeys and lemurs), the even-toed ungulates (including pigs, camels, and whales), and the Carnivora (including cats, dogs, and seals). The largest orders of mammals, by number of species, are the rodents, bats, and Eulipotyphla (including hedgehogs, moles and shrews).
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Around 6,400 extant species of mammals have been described and divided into 29 orders.
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These characteristics distinguish them from reptiles and birds, from which their ancestors diverged in the Carboniferous Period over 300 million years ago. Mammals are characterized by the presence of milk-producing mammary glands for feeding their young, a neocortex region of the brain, fur or hair, and three middle ear bones. A mammal (from Latin mamma 'breast') is a vertebrate animal of the class Mammalia ( / m ə ˈ m eɪ l i.