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250px Mammals (class Mammalia) are vertebrate animals characterized by the presence of sweat glands, including milk producing sweat glands, and by the presence of: hair, three middle ear bones used in hearing, and a neocortex region in the brain. Mammals, other than the monotremes, give birth to live young instead of laying eggs. They also possess specialized teeth and use a placenta in the ontogeny. The mammalian brain regulates endothermic and circulatory systems, including a four-chambered heart. Mammals encompass approximately 5,400 species, ranging in size from the Bumblebee Bat, (30-40mm), to the Blue Whale, (33,000mm), distributed in about 1,200 genera, 153 families, and 29 orders, though this varies by classification scheme. Most mammals belong to the placental group. The four largest orders within the placental mammals are Rodentia (mice, rats, and other small, gnawing mammals), Chiroptera (bats), Carnivora (dogs, cats, bears, and other mammals that primarily eat meat), and Cetartiodactyla (including numerous herbivore species, such as deer, sheep, goats, and buffalos, plus whales). Phylogenetically, Mammalia is defined as all descendants of the most recent common ancestor of monotremes (e.g., echidnas and platypuses) and therian mammals (marsupials and placentals). This means that some extinct groups of "mammals" are not members of the crown group Mammalia, even though most of them have all the characteristics that traditionally would have classified them as mammals. These "mammals" are now usually placed in the unranked clade Mammaliaformes. The mammalian line of descent diverged from the sauropsid line at the end of the Carboniferous period. The sauropsids would evolve into modern-day reptiles and birds, while the synapsid branch led to mammals. The first true mammals appeared in the Jurassic period. Modern mammalian orders appeared in the Palaeocene and Eocene epochs of the Palaeogene period. The elk, or wapiti (Cervus canadensis), is the second largest species of deer in the world and one of the largest mammals in North America and eastern Asia. In the deer family (Cervidae), only the moose, Alces alces (called an "elk" in Europe), is larger. Wapiti are almost identical to red deer found in Europe, of which they were long believed to be a subspecies; they have recently been determined to be a distinct species based on DNA evidence. Elk range in forest and forest-edge habitat, feeding on grasses, plants, leaves and bark. Although native to North America and eastern Asia, they have adapted well to countries where they have been introduced, including New Zealand and Argentina. Their high level of adaptability poses a threat to endemic species and ecosystems where they have been introduced. Male elk have large antlers which are shed each year. Males engage in ritualized mating behaviors during the rut, including posturing, antler wrestling and bugling, a loud series of screams which establishes dominance over other males and attracts females. The bugle call is one of the most distinctive calls in nature. Elk are susceptible to a number of infectious diseases, some of which can be transmitted to livestock. Efforts to eliminate infectious diseases from elk populations, largely through vaccination, have had mixed success. Some cultures revere the elk as a spiritual force. In parts of Asia, antlers and their velvet are used in traditional medicines. Elk are hunted as a game species; the meat is leaner and higher in protein than beef or chicken. ...that one burrow of the eastern chipmunk (pictured) was found to contain 390 acorns? .. that the Hare Indian dog, now extinct, was not known to bark, but puppies learned to imitate the barking of other dogs when the breed was introduced to Europe? ... that the rodents Thomasomys ucucha from Ecuador and Oxymycterus hucucha from Bolivia were both named after the local Quechua word for "mouse"? ...that the incisors of blesmols are visible even when their mouths are closed? ... that the pebble-mound mice of northern Australia construct mounds of stones around their burrows up to 10 m2 in area? ... that the stationmaster of the Kinokawa train station in Kinokawa, Japan is a cat named Tama? A pod of hippopotamuses (Hippopotamus amphibius) in South Luangwa National Park, Zambia, the southernmost of three national parks in the valley of the Luangwa River. Hippo pods generally number up to about 40 individuals. Hippos spend most of their time in shallow water, rarely coming out of that depth. Most hippos that look as though they are floating are in fact standing or lying on the bottom. Monotremata Didelphimorphia Paucituberculata Microbiotheria Notoryctemorphia Dasyuromorphia Peramelemorphia Diprotodontia Afrosoricida Macroscelidea Tubulidentata Hyracoidea Proboscidea Sirenia Cingulata Pilosa Scandentia Dermoptera Primates Rodentia Lagomorpha Erinaceomorpha Soricomorpha Chiroptera Pholidota Carnivora Perissodactyla Artiodactyla Cetacea Biology Tree of Life Prokaryotes and Protists Plants Arthropods Cephalopods Gastropods Fishes Amphibians and Reptiles Birds Mammals WikiProject Dogs WikiProject Dog breeds WikiProject Rodents WikiProject Cats WikiProject Equine WikiProject Horse breeds WikiProject Cetaceans WikiProject Primates WikiProject Monotremes and Marsupials Viruses A complete list of scientific WikiProjects can be found here. See also Wikispecies, a Wikimedia project dedicated to classification of biological species. Articles needing attention Biology pages needing attention (cleanup, expansion, wikification, expert needed, etc.) Help Wikipedia by uploading required images Help Wikipedia by creating pages or redirects Help Wikipedia by expanding mammal-related stub articles. Biology • Agriculture and Agronomy • Animals  • Arthropods • Amphibians and Reptiles • Biotechnology • Birds • Cats • Cetaceans • Dinosaurs • Dogs • Extinct and Endangered Species • Evolutionary biology • Fish • Marine life • Medicine • Metabolism • Mind and Brain • Molecular and Cellular Biology • Neuroscience • Plants • Primates • Sharks • Turtles What are portals? List of portals Featured portals