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'''Mammals''' are members of class '''Mammalia''' ({{IPAc-en|icon|m|ə|ˈ|m|eɪ|l|i|.|ə}}), air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young.  Most mammals also possess sweat glands and specialised teeth. The largest group of mammals, the placentals, have a placenta which feeds the offspring during gestation.  The mammalian brain, with its characteristic neocortex, regulates endothermic and circulatory systems, the latter featuring red blood cells lacking nuclei and a large, four-chambered heart maintaining the very high metabolism rate they have.  Mammals range in size from the 30–40 millimeter (1- to 1.5-inch) bumblebee bat to the 33-meter (108-foot) blue whale.

The word "[[:wikt:mammal|mammal]]" is modern, from the scientific name ''Mammalia'' coined by Carl Linnaeus in 1758, derived from the  [[Latin]] ''[[:wikt:mamma#Latin|mamma]]'' ("teat, pap"). All female mammals nurse their young with milk, which is secreted from special [[gland]]s, the [[mammary gland]]s. According to ''Mammal Species of the World'', which is updated through periodic editions, 5,676 [[species]] were known in 2005. These were distributed in 1,229 [[genus|genera]], 153 [[family (biology)|families]] and 29 [[order (biology)|orders]].<ref name=MSW3intro>{{MSW3|heading=Preface and introductory material|page=xxvi}}</ref> In 2008 the [[International Union for Conservation of Nature|IUCN]] completed a five-year, 17,000-scientist Global Mammal Assessment for its [[IUCN Red List]], which counted 5488 accepted species at the end of that period.<ref>{{cite web | title=Initiatives |  work=The IUCN Red List of Threatened Species | date=April, 2010 | publisher=IUCN | url=http://www.iucnredlist.org/initiatives}}</ref> In some classifications, the class is divided into two subclasses (not counting fossils): the [[Prototheria]] (order of [[monotremes|Monotremata]]) and the [[Theria]], the latter composed of the infraclasses [[Metatheria]] and [[Eutheria]]. The [[marsupial]]s are the [[crown group]] of the Metatheria and therefore include all living metatherians as well as many extinct ones; the [[Eutheria|placentals]] are likewise the crown group of the Eutheria.

The [[Mammal classification|classification of mammals]] between the relatively stable class and family levels has changed often; different treatments of subclass, infraclass and order appear in contemporaneous literature, especially for [[Marsupialia]]. Much recent change has reflected the results of [[cladistic analysis]] and [[molecular genetics]].  Results from molecular genetics, for example, have led to the adoption of new groups such as the [[Afrotheria]] and the abandonment of traditional groups such as the [[Insectivora]].

Except for the five species of [[monotreme]]s (which lay eggs), all living mammals give birth to live young.  Most mammals, including the six most species-rich orders, belong to the [[placental]] group. The three largest orders, in descending order, are [[Rodentia]] ([[Mouse|mice]], [[rat]]s, [[porcupine]]s, [[beaver]]s, [[capybara]]s, and other gnawing mammals), [[Chiroptera]] (bats), and [[Soricomorpha]] ([[shrew]]s, [[Mole (animal)|moles]] and [[solenodon]]s). The next three largest orders, depending on the [[biological classification|classification scheme]] used, are the [[primate]]s, to which the [[human]] species belongs, the [[Cetartiodactyla]] (including the [[Even-toed ungulate|even-toed hoofed mammals]] and the [[whale]]s), and the [[Carnivora]] ([[dog]]s, [[cat]]s, [[weasel]]s, [[bear]]s, [[Pinniped|seals]], and their relatives).<ref name=MSW3intro/>

The early [[synapsid]] mammalian ancestors were [[Sphenacodontia|sphenacodont]] [[pelycosaur]]s, a group that also included ''[[Dimetrodon]]''.  At the end of the [[Carboniferous]] period, this group diverged from the [[Sauropsida|sauropsid]] line that led to today's [[reptile]]s and [[bird]]s.  Preceded by many diverse groups of non-mammalian synapsids (sometimes referred to as mammal-like reptiles), the first mammals appeared in the early [[Mesozoic]] era. The modern mammalian orders arose in the [[Paleogene]] and [[Neogene]] periods of the [[Cenozoic]] era.

==References==

{{Reflist|2}}

==External links==
{{Sister project links|Mammal}}

*[http://www.bbc.co.uk/nature/class/Mammal BBC Wildlife Finder – video clips from the BBC's natural history archive]
*[http://www.animalias.com/mammals Mammal Species] Thousands of species, showing taxonomic classification, images, and other information, animalias.com
*[http://www.globaltwitcher.com/taxa_order.asp?classid=3 GlobalTwitcher.com – All species in the world with distribution maps and images]
*[http://paleocene-mammals.de/ Paleocene Mammals], a site covering the rise of the mammals, paleocene-mammals.de
*[http://www.enchantedlearning.com/subjects/mammals/Evolution.shtml Evolution of Mammals], a brief introduction to early mammals, enchantedlearning.com
*[http://tellapallet.com/tree_of_life.htm Tree of Life poster] – Shows mammals' evolutionary relation to other organisms, tellapallet.com
*[http://home.arcor.de/ktdykes/mesomamm.htm The Evolution of Mesozoic Mammals, a Rough Sketch], an informal introduction, home.arcor.de
*[http://www.carnegiemnh.org/research/news.html Carnegie Museum of Natural History], some discoveries of early mammal fossils, carnegiemnh.org
*[http://brainmaps.org/index.php?p=datasets-species High-Resolution Images of various Mammalian Brains], brainmaps.org
*[http://www.learnanimals.com/mammals.php Mammal Species], collection of information sheets about various mammal species, learnanimals.com
*[http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0040111 Summary of molecular support for Epitheria], biology.plosjournals.org
*[http://www.fmnh.helsinki.fi/users/haaramo/Metazoa/Deuterostoma/Chordata/Synapsida/Basal_Mammalia/Mammaliaformes_2.htm#Australophenida Mikko's Phylogeny Archive], fmnh.helsinki.fi
*[http://www.european-mammals.org/php/mapmaker.php European Mammal Atlas EMMA] from Societas Europaea Mammalogica, European-mammals.org
*[http://nlbif.eti.uva.nl/bis/marine_mammals.php Marine Mammals of the World]—An overview of all marine mammals, including descriptions, multimedia and a key, eti.uva.nl
*[http://www.mammalogy.org Mammalogy.org] The American Society of Mammalogists was established in 1919 for the purpose of promoting the study of mammals, and this website includes a mammal image library


[[Category:Mammals| ]]