Simpson (1945) provides systematics of mammal origins and relationships that had been taught universally until the end of the 20th century. McKenna & Bell (1997) and Wilson & Reeder (2005) provide useful recent compendiums. Mammal classification has been through several revisions since Carl Linnaeus initially defined the class, and at present, no classification system is universally accepted. Decline in numbers and extinction of many mammals is primarily driven by human poaching and habitat destruction, primarily deforestation. Mammals have been depicted in art since Paleolithic times, and appear in literature, film, mythology, and religion.

Mammals are also hunted and raced for sport, and are used as model organisms in science. Domesticated mammals provided, and continue to provide, power for transport and agriculture, as well as food ( meat and dairy products), fur, and leather. This led to a major restructuring of human societies from nomadic to sedentary, with more co-operation among larger and larger groups, and ultimately the development of the first civilizations. Most mammals are polygynous, but some can be monogamous or polyandrous.ĭomestication of many types of mammals by humans played a major role in the Neolithic Revolution, and resulted in farming replacing hunting and gathering as the primary source of food for humans. 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 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 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. 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-Early Jurassic. 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 therapsids during the beginning of the Middle Permian period. The early synapsids were sphenacodonts, a group that included the famous Dimetrodon. 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). Around 6,400 extant species of mammals have been described and divided into 29 orders. 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.

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