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==Etymology==
New Latin Mammalia, from Late Latin, neuter plural of mammalis of the [[breast]], from Latin mamma breast
*Date: [http://www.wikipedia.org/wiki/19th_Century 1826]
==Definition==
1 : any of a class (Mammalia) of warm-blooded higher [http://en.wikipedia.org/wiki/Vertebrate vertebrates] (as placentals, marsupials, or monotremes) that nourish their young with [[milk]] secreted by [http://en.wikipedia.org/wiki/Mammary_gland mammary glands], have the skin usually more or less covered with hair, and include [[humans]]
==Description==
Mammals (formally Mammalia) are a class of [http://en.wikipedia.org/wiki/Vertebrate vertebrate], [[air]]-[[breathing]] [[animals]] whose [[females]] are characterized by the [[possession]] of [http://en.wikipedia.org/wiki/Mammary_gland mammary glands] while both [[males]] and [[females]] are characterized by [http://en.wikipedia.org/wiki/Sweat_gland sweat glands], [http://en.wikipedia.org/wiki/Hair hair] and/or fur, three middle ear bones used in hearing, and a [http://en.wikipedia.org/wiki/Neocortex neocortex] region in the [[brain]].
Mammals are divided into three main infraclass taxa depending how they are born. These taxa are: [http://en.wikipedia.org/wiki/Monotremes monotremes], [http://en.wikipedia.org/wiki/Marsupials marsupials] and [http://en.wikipedia.org/wiki/Placentals placentals]. Except for the five species of monotremes (which lay eggs), all mammal [[species]] give [[birth]] to live young. Most mammals also [[possess]] specialized [http://en.wikipedia.org/wiki/Teeth teeth], and the largest group of mammals, the placentals, use a [http://en.wikipedia.org/wiki/Placenta placenta] during gestation. The mammalian [[brain]] regulates [http://en.wikipedia.org/wiki/Warm-blooded endothermic] and [http://en.wikipedia.org/wiki/Circulatory_system circulatory systems], including a four-chambered [[heart]].
There are approximately 5,400 species of mammals, distributed in about 1,200 [http://en.wikipedia.org/wiki/Genus genera], 153 [http://en.wikipedia.org/wiki/Family_(biology) families], and 29 [http://en.wikipedia.org/wiki/Order_(biology) orders] (though this varies by [[classification]] [[scheme]]). Mammals range in size from the 30–40 millimeter (1- to 1.5-inch) Bumblebee Bat to the 33-meter (108-foot) Blue Whale.
Mammals are divided into two subclasses: the [http://en.wikipedia.org/wiki/Prototheria Prototheria], which includes the oviparous monotremes, and the [http://en.wikipedia.org/wiki/Theria Theria], which includes the placentals and live-bearing marsupials. Most mammals, including the six largest orders, belong to the [http://en.wikipedia.org/wiki/Placental placental] group. The three largest orders, in descending order, are [http://en.wikipedia.org/wiki/Rodentia Rodentia] (mice, rats, porcupines, beavers, capybaras, and other gnawing mammals), [http://en.wikipedia.org/wiki/Chiroptera Chiroptera] (bats), and [http://en.wikipedia.org/wiki/Soricomorpha Soricomorpha] (shrews, moles and solenodons). The next three largest orders include the [http://en.wikipedia.org/wiki/Carnivora Carnivora] (dogs, cats, weasels, bears, seals, and their relatives), the [http://en.wikipedia.org/wiki/Cetartiodactyla Cetartiodactyla] (including the even-toed hoofed mammals and the whales) and the [http://en.wikipedia.org/wiki/Primates Primates] to which the [[human]] [[species]] belongs. The [[relative]] size of these latter three orders differs according to the [[classification]] scheme and definitions used by various [[authors]].
[http://en.wikipedia.org/wiki/Phylogenetics 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).[2] This means that some extinct groups of "mammals" are not members of the [http://en.wikipedia.org/wiki/Crown_group 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 [http://en.wikipedia.org/wiki/Mammaliaformes Mammaliaformes].
The mammalian line of [[descent]] diverged from an amniote line at the end of the [http://en.wikipedia.org/wiki/Carboniferous Carboniferous] period. One line of amniotes would lead to [http://en.wikipedia.org/wiki/Reptile reptiles], while the other would lead to synapsids. According to cladistics, mammals are a sub-group of [http://en.wikipedia.org/wiki/Synapsid synapsids]. Although they were preceded by many [[diverse]] groups of non-mammalian synapsids (sometimes misleadingly referred to as mammal-like reptiles), the first true mammals appeared in the [http://en.wikipedia.org/wiki/Triassic Triassic] period. Modern mammalian orders appeared in the [http://en.wikipedia.org/wiki/Palaeocene Palaeocene] and [http://en.wikipedia.org/wiki/Eocene Eocene] [[epochs]] of the [http://en.wikipedia.org/wiki/Palaeogene Palaeogene] period.[http://en.wikipedia.org/wiki/Mammal]
[[Category: Biology]]
==Etymology==
New Latin Mammalia, from Late Latin, neuter plural of mammalis of the [[breast]], from Latin mamma breast
*Date: [http://www.wikipedia.org/wiki/19th_Century 1826]
==Definition==
1 : any of a class (Mammalia) of warm-blooded higher [http://en.wikipedia.org/wiki/Vertebrate vertebrates] (as placentals, marsupials, or monotremes) that nourish their young with [[milk]] secreted by [http://en.wikipedia.org/wiki/Mammary_gland mammary glands], have the skin usually more or less covered with hair, and include [[humans]]
==Description==
Mammals (formally Mammalia) are a class of [http://en.wikipedia.org/wiki/Vertebrate vertebrate], [[air]]-[[breathing]] [[animals]] whose [[females]] are characterized by the [[possession]] of [http://en.wikipedia.org/wiki/Mammary_gland mammary glands] while both [[males]] and [[females]] are characterized by [http://en.wikipedia.org/wiki/Sweat_gland sweat glands], [http://en.wikipedia.org/wiki/Hair hair] and/or fur, three middle ear bones used in hearing, and a [http://en.wikipedia.org/wiki/Neocortex neocortex] region in the [[brain]].
Mammals are divided into three main infraclass taxa depending how they are born. These taxa are: [http://en.wikipedia.org/wiki/Monotremes monotremes], [http://en.wikipedia.org/wiki/Marsupials marsupials] and [http://en.wikipedia.org/wiki/Placentals placentals]. Except for the five species of monotremes (which lay eggs), all mammal [[species]] give [[birth]] to live young. Most mammals also [[possess]] specialized [http://en.wikipedia.org/wiki/Teeth teeth], and the largest group of mammals, the placentals, use a [http://en.wikipedia.org/wiki/Placenta placenta] during gestation. The mammalian [[brain]] regulates [http://en.wikipedia.org/wiki/Warm-blooded endothermic] and [http://en.wikipedia.org/wiki/Circulatory_system circulatory systems], including a four-chambered [[heart]].
There are approximately 5,400 species of mammals, distributed in about 1,200 [http://en.wikipedia.org/wiki/Genus genera], 153 [http://en.wikipedia.org/wiki/Family_(biology) families], and 29 [http://en.wikipedia.org/wiki/Order_(biology) orders] (though this varies by [[classification]] [[scheme]]). Mammals range in size from the 30–40 millimeter (1- to 1.5-inch) Bumblebee Bat to the 33-meter (108-foot) Blue Whale.
Mammals are divided into two subclasses: the [http://en.wikipedia.org/wiki/Prototheria Prototheria], which includes the oviparous monotremes, and the [http://en.wikipedia.org/wiki/Theria Theria], which includes the placentals and live-bearing marsupials. Most mammals, including the six largest orders, belong to the [http://en.wikipedia.org/wiki/Placental placental] group. The three largest orders, in descending order, are [http://en.wikipedia.org/wiki/Rodentia Rodentia] (mice, rats, porcupines, beavers, capybaras, and other gnawing mammals), [http://en.wikipedia.org/wiki/Chiroptera Chiroptera] (bats), and [http://en.wikipedia.org/wiki/Soricomorpha Soricomorpha] (shrews, moles and solenodons). The next three largest orders include the [http://en.wikipedia.org/wiki/Carnivora Carnivora] (dogs, cats, weasels, bears, seals, and their relatives), the [http://en.wikipedia.org/wiki/Cetartiodactyla Cetartiodactyla] (including the even-toed hoofed mammals and the whales) and the [http://en.wikipedia.org/wiki/Primates Primates] to which the [[human]] [[species]] belongs. The [[relative]] size of these latter three orders differs according to the [[classification]] scheme and definitions used by various [[authors]].
[http://en.wikipedia.org/wiki/Phylogenetics 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).[2] This means that some extinct groups of "mammals" are not members of the [http://en.wikipedia.org/wiki/Crown_group 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 [http://en.wikipedia.org/wiki/Mammaliaformes Mammaliaformes].
The mammalian line of [[descent]] diverged from an amniote line at the end of the [http://en.wikipedia.org/wiki/Carboniferous Carboniferous] period. One line of amniotes would lead to [http://en.wikipedia.org/wiki/Reptile reptiles], while the other would lead to synapsids. According to cladistics, mammals are a sub-group of [http://en.wikipedia.org/wiki/Synapsid synapsids]. Although they were preceded by many [[diverse]] groups of non-mammalian synapsids (sometimes misleadingly referred to as mammal-like reptiles), the first true mammals appeared in the [http://en.wikipedia.org/wiki/Triassic Triassic] period. Modern mammalian orders appeared in the [http://en.wikipedia.org/wiki/Palaeocene Palaeocene] and [http://en.wikipedia.org/wiki/Eocene Eocene] [[epochs]] of the [http://en.wikipedia.org/wiki/Palaeogene Palaeogene] period.[http://en.wikipedia.org/wiki/Mammal]
[[Category: Biology]]
