Mammals (Class Mammalia) are warmblooded, fur-bearing tetrapods (vertebrates with four limbs) that lactate (produce milk) to nourish their young. Humans are especially interested in the biology and behavior of mammals because we are also members of Class Mammalia, and because mammals are aesthetically, economically, and ecologically important.

Mammals are differentiated from other tetrapods by a suite of unique characteristics that include: mammary glands providing milk to nourish offspring; skin that produces unique structures such as hair, nails, horns, and hooves; a diaphragm separating the thoracic and abdominal cavities; endothermy (warm-bloodedness); a lower jaw articulating directly with the cranium; a dentary (one side of the lower jaw) composed of a single bone; and a middle ear containing three ear ossicles. These and other unique shared characteristics support the widely accepted hypothesis that all living and extinct mammals are more closely related to each other (monophyletic) than to any other tetrapod group, such as birds or reptiles.

Class Mammalia is composed of Subclass Prototheria (Monotremata), which includes the two orders of egg-laying monotremes, platypuses and echidnas; and Subclass Theri-iformes, which includes the remaining twenty-three orders exhibiting viviparity (live birth). Subclass Theriiformes is divided into two cohorts. Cohort Marsupiala includes the usually pouched mammals (most females possess a pouch, or marsupium) that give birth to altricial (relatively undeveloped) young. Marsupials have extended lactation with associated endocrine controls to nourish their altricial offspring. They also lack a complete placenta, the membranous structure that aids in the transfer of nutrients from the mother's circulation to the developing embryo. Cohort Placen-talia is composed of the nonpouched mammals that give birth to precocious (relatively developed) young. Placentals have extended embryonic gestation (longer pregnancy), and a complete placenta that facilitates a more efficient transfer of nutrients from the mother's circulation to the developing embryo.

Mammals are amniotes. Amniota is a grouping of terrestrial tetrapods (mammals, reptiles, and birds) with watertight eggs. Amniote eggs have three membranes (including the amni-otic membrane) that aid in protection, water retention, and gas exchange. These membranes, and a protective shell, surround the

Gorillas holding their babies (William Dow/Corbis)

embryo. Amphibian eggs lack extraembryonic membranes and a protective shell, and can be laid only in or near water, so the eggs do not dry out. The watertight egg is an important evolutionary novelty that reduced the necessity for proximity to standing water and allowed extensive land colonization and migration, otherwise impossible.

The first amniotes were small, insectivorous, terrestrial tetrapods and are known from the Early Pennsylvanian Period of the Paleozoic Era, around 310 million years ago. By around 300 million years ago, the reptilian ancestors of mammals had diverged from the evolutionary lineages leading to turtles, lepidosaurs (snakes and lizards), and archosaurs (crocodiles, dinosaurs [including birds], and pterosaurs). These early reptilian ancestors of mammals, Subclass Synapsida, are commonly referred to as "mammal-like reptiles." Although some synapsids exhibit mammalian characteristics, mammals are distinguished from synapsids by the suite of unique characteristics outlined above.

The earliest mammals (and the first dinosaurs) appear in the Late Triassic Period of the Mesozoic Era, approximately 200 million years ago. These first mammals were small and probably reminiscent of living shrews. Although dinosaurs dominated the land during the Jurassic and Cretaceous periods of the Mesozoic, mammals thrived and diversified into more than a dozen families during this era. Many Mesozoic mammal lineages became extinct, but the lineages leading to the three major groups of living mammals (monotremes, marsupials, and placentals) were successful. Marsupials and placentals are more closely related to each other than either is to monotremes, as they share several derived characteristics, including viviparity (live birth). Relationships among the fossil Mesozoic mammal groups, and their relationship to the lineages of extant mammals, are poorly understood.

At the end of the Mesozoic Era, the extinction of the dinosaurs and many other groups of animals and plants opened many previously occupied ecological niches, and also changed the parameters of those niches. In the Early Cenozoic Era (the era we live in, also known as "the age of mammals"), the theri-iform mammals (marsupials and placentals) diversified and occupied these newly available and redefined niches. The monotremes apparently did not diversify, and were isolated on southern continents (former Gondwana-land), perhaps since the Early Jurassic. Although the fossil record from the earliest epoch (Paleocene) of the Cenozoic is fragmentary, most of the living orders and many extinct groups are represented by the Early Eocene Epoch (around 50 million years ago).

The nearly 5,000 species of living mammals are arranged into the following twenty-five living mammalian orders: Platypoda (platypuses); Tachyglossa (echidnas); Notorycte-morpha (marsupial carnivores, mice, and moles); Peramelia (bandicoots); Diprotodon-tia (wombats, kangaroos, koalas, and kin); Didelphimorphia (opossums and kin); Pauci-tuberculata (rat opossums and kin); Cingulata (armadillos and kin); Pilosa (anteaters and sloths); Lagomorpha (pikas, hares, and rabbits); Rodentia (rodents); Cimolesta (pangolins); Carnivora (carnivores, including seals and walruses); Chrysochloridea (golden moles); Erinaceomorpha (hedgehogs and gymnures); Soricomorpha (shrews, moles, tenrecs, and kin); Chiroptera (bats); Primates (primates); Scandentia (tree shrews); Tubulidentata (aard-varks); Cete (whales and dolphins); Artio-dactyla (pigs, camels, deer, giraffes, cattle, and kin); Perissodactyla (horses, rhinoceroses, and tapirs); Uranotheria (hyraxes, manatees, dugongs, elephants); and Macroscelidea (elephant shrews).

Many wild and domesticated mammals serve as significant food sources for humans. Some species, such as oxen, aid in vegetable crop production by pulling plows and other equipment; horses, camels, and llamas provide transportation. Hides and fur are used for clothing and adornment. Some species are important pollinators; others incidentally control insect populations. All mammals are integral links in the delicate ecological web; predators control prey populations, whose abundance in turn impacts predator populations, and both predator and prey are links in many other complex biological interactions. Humans value the beauty and intelligence of mammals and enjoy viewing, interacting with, and hunting certain species. Many species are kept as pets, and mammals are also utilized in medical and cosmetic research, the ethical and epidemiological ramifications of which are hotly debated.

—Mary Ellen Holden

See also: Artiodactyls; Carnivora; Cetacea (Whales, Dolphins, Porpoises); Chiroptera (Bats); Cretaceous-Tertiary Extinction; Great Apes; Lagomorpha; Lemurs and Other Lower Primates; Monkeys; Order Uran-otheria; Perissodactyls; Permo-Triassic Extinction; Phylogeny; Primates; Rodents


Benton, Michael J., ed. 1988. The Phylogeny and Classification of the Tetrapods. Vol. 2: Mammals. Oxford: Clarendon; Carroll, Robert. 1988. Vertebrate Paleontology and Evolution. New York: W. H. Freeman; McKenna, Malcolm C., and Susan K. Bell. 1997. Classification of Mammals above the Species Level. New York: Columbia University Press; Nowak, Ronald M. 1999. Walker's Mammals of the World, 6th ed. Vols. 1 and 2. Baltimore: Johns Hopkins University Press; Vaughan, Terry A., James M. Ryan, and Nicholas J. Czaplewski. 2000. Mammalogy, 4th ed. Orlando: Harcourt.

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