Mammals Temporal range: See text PreЄ Є O S D C P T J K Pg N
Examples of various mammalian orders, click the image and scroll down for individual descriptions
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Infraphylum:	Gnathostomata
clade:	Eugnathostomata
clade:	Teleostomi
Superclass:	Tetrapoda
clade:	Amniota
clade:	Synapsida
Class:	Mammalia Linnaeus, 1758
Subgroups
Order Monotremata Clade Theriiformes Order †Triconodonta Order †Multituberculata Subclass Theria Infraclass Marsupialia Infraclass Placentalia

Mammals are members of class Mammalia ( /məˈmeɪli.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 four-chambered heart. 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 "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma ("teat, pap"). All female mammals nurse their young with milk, which is secreted from special glands, the mammary glands. 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 genera, 153 families and 29 orders.^[1] In 2008 the 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.^[2] In some classifications, the class is divided into two subclasses (not counting fossils): the Prototheria (order of Monotremata) and the Theria, the latter composed of the infraclasses Metatheria and Eutheria. The marsupials are the crown group of the Metatheria and therefore include all living metatherians as well as many extinct ones; the placentals are likewise the crown group of the Eutheria.

The 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 monotremes (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 (mice, rats, porcupines, beavers, capybaras, and other gnawing mammals), Chiroptera (bats), and Soricomorpha (shrews, moles and solenodons). The next three largest orders, depending on the classification scheme used, are the primates, to which the human species belongs, the Cetartiodactyla (including the even-toed hoofed mammals and the whales), and the Carnivora (dogs, cats, weasels, bears, seals, and their relatives).^[1]

The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that also included Dimetrodon. At the end of the Carboniferous period, this group diverged from the sauropsid line that led to today's reptiles and birds. 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.

Varying definitions, varying dates[link]

In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group mammals, the clade consisting of the most recent common ancestor of living monotremes (echidnas and platypuses) and therian mammals (marsupials and placentals) and all descendants of that ancestor.^[3] A broader phylogenetic definition was provided in a 2004 book by Kielan-Jaworowska, Cifelli, and Luo, who defined Mammalia as the clade originating with the most recent common ancestor, not only of the monotremes and the therians, but also of Sinoconodon, the morganucodonts, and the docodonts.^[4] The morganucodonts and the docodonts, included by Rowe in the unranked clade Mammaliaformes, had a widespread distribution in the northern continents and had many of the characteristics that traditionally would have classified them as mammals.^[5] In particular, some docodonts were furry.

Mammalia, considered as the crown group, appeared in the Pliensbachian age of the early Jurassic period.^[6] In the broader sense given to the term by Kielan-Jaworowska et al., the group arose in the Norian age in the middle of the Late Triassic.^[4] Finally, some writers consider Adelobasileus to be a mammal; as this animal lived in the Carnian age at the beginning of the Late Triassic, this would mean that mammals appeared even earlier.^[7] In any case, the temporal range of the group extends to the present day.

Distinguishing features[link]

Living mammal species can be identified by the presence of sweat glands, including those that are specialized to produce milk. In classifying fossils, however, other features must be used, since soft tissue glands and many other features are not visible in fossils.

Among the many traits shared by all living mammals, but not present in any of the early Triassic synapsids, are:

• Jaw joint - The dentary (the lower jaw bone which carries the teeth) and the squamosal (another small skull bone) meet to form the joint. In most gnathostomes, including early therapsids, the joint consists of the articular (a small bone at the back of the lower jaw) and the quadrate (a small bone at the back of the upper jaw).

• Middle ear - Sound is carried from the eardrum by a chain of three bones, the malleus, the incus, and the stapes. Ancestrally, the malleus and the incus are derived from the articular and the quadrate bones that constituted the jaw joint of early therapsids.

• Tooth replacement - Teeth are replaced once or (as in toothed whales and murid rodents) not at all, rather than being replaced continually throughout life.^[8]

• Prismatic enamel - The enamel coating on the surface of a tooth consists of prisms, solid, rod-like structures extending from the dentin to the tooth's surface.

• Occipital condyles - Two knobs at the base of the skull fit into the topmost neck vertebra; most tetrapods, in contrast, have only one such knob.

For paleontologists who define Mammalia phylogenetically, no limit can be set on the features used to distinguish the group. Any feature may be relevant to a fossil's phylogenetic position. Paleontologists defining Mammalia in terms of traits, on the other hand, need only consider those features that appear in the definition. The dentary-squamosal jaw joint is generally included.

Classification[link]

Over 70% of mammal species are in the orders Rodentia (blue), Chiroptera (red), and Soricomorpha (yellow)

George Gaylord Simpson's "Principles of Classification and a Classification of Mammals" (AMNH Bulletin v. 85, 1945) was the original source for the taxonomy listed here. Simpson laid out a systematics of mammal origins and relationships that was universally taught until the end of the 20th century. Since Simpson's classification, the paleontological record has been recalibrated, and the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself, partly through the new concept of cladistics. Though field work gradually made Simpson's classification outdated, it remained the closest thing to an official classification of mammals.

McKenna/Bell classification[link]

In 1997, the mammals were comprehensively revised by Malcolm C. McKenna and Susan K. Bell, which has resulted in the McKenna/Bell classification. Their 1997 book, Classification of Mammals: Above the species level,^[5] is the most comprehensive work to date on the systematics, relationships, and occurrences of all mammal taxa, living and extinct, down through the rank of genus, though recent molecular genetic data challenge several of the higher level groupings. The authors worked together as paleontologists at the American Museum of Natural History, New York. McKenna inherited the project from Simpson and, with Bell, constructed a completely updated hierarchical system, covering living and extinct taxa that reflects the historical genealogy of Mammalia.

The McKenna/Bell hierarchical listing of many terms used for mammal groups above the species includes extinct mammals, as well as modern groups, and introduces some fine distinctions such as legions and sublegions (ranks which fall between classes and orders) that are likely to be glossed over by the nonprofessionals.

The published reclassification forms both a comprehensive and authoritative record of approved names and classifications and a list of invalid names.

Extinct groups are represented by a dagger (†).

Class Mammalia

• Subclass Prototheria: monotremes: echidnas and the platypus
• Subclass Theriiformes: live-bearing mammals and their prehistoric relatives
  • Infraclass †Allotheria: multituberculates
  • Infraclass †Triconodonta: triconodonts
  • Infraclass Holotheria: modern live-bearing mammals and their prehistoric relatives
    • Supercohort Theria: live-bearing mammals
      • Cohort Marsupialia: marsupials
        • Magnorder Australidelphia: Australian marsupials and the monito del monte
        • Magnorder Ameridelphia: New World marsupials
      • Cohort Placentalia: placentals
        • Magnorder Xenarthra: xenarthrans
        • Magnorder Epitheria: epitheres
          • Grandorder Anagalida: lagomorphs, rodents, and elephant shrews
          • Grandorder Ferae: carnivorans, pangolins, †creodonts, and relatives
          • Grandorder Lipotyphla: insectivorans
          • Grandorder Archonta: bats, primates, colugos, and treeshrews
          • Grandorder Ungulata: ungulates
            • Order Tubulidentata incertae sedis: aardvark
            • Mirorder Eparctocyona: †condylarths, whales, and artiodactyls (even-toed ungulates)
            • Mirorder †Meridiungulata: South American ungulates
            • Mirorder Altungulata: perissodactyls (odd-toed ungulates), elephants, manatees, and hyraxes

Molecular classification of placentals[link]

Molecular studies based on DNA analysis have suggested new relationships among mammal families over the last few years. Most of these findings have been independently validated by retrotransposon presence/absence data. The most recent classification systems based on molecular studies have proposed four groups or lineages of placental mammals. Molecular clocks suggest that these clades diverged from early common ancestors in the Cretaceous, but fossils have not yet been found to corroborate this hypothesis. These molecular findings are consistent with mammal zoogeography:

Following molecular DNA sequence analyses, the first divergence was that of the Afrotheria 110–100 million years ago (mya). The Afrotheria proceeded to evolve and diversify in the isolation of the African-Arabian continent. The Xenarthra, isolated in South America, diverged from the Boreoeutheria approximately 100–95 million years ago. According to an alternative view, the Xenarthra has the Afrotheria as closest allies, forming the Atlantogenata as sister group to Boreoeutheria. The Boreoeutheria split into the Laurasiatheria and Euarchontoglires between 95 and 85 mya; both of these groups evolved on the northern continent of Laurasia. After tens of millions of years of relative isolation, Africa-Arabia collided with Eurasia, exchanging Afrotheria and Boreoeutheria. The formation of the Isthmus of Panama linked South America and North America, which facilitated the exchange of mammal species in the Great American Interchange. The traditional view that no placental mammals reached Australasia until about 5 million years ago, when bats and murine rodents arrived, has been challenged by recent evidence and may need to be reassessed. These molecular results are still controversial because they are not reflected by morphological data and therefore not accepted by many systematists. Further, there is some indication from retrotransposon presence/absence data that the traditional Epitheria hypothesis, suggesting Xenarthra as the first divergence, might be true. With the old order Insectivora shown to be polyphylectic and more properly subdivided (as Afrosoricida, Erinaceomorpha, and Soricomorpha), the following classification for placental mammals contains 21 orders:

• Clade Atlantogenata
  • Group I: Afrotheria
    • Clade Afroinsectiphilia
      • Order Macroscelidea: elephant shrews (Africa)
      • Order Afrosoricida: tenrecs and golden moles (Africa)
      • Order Tubulidentata: aardvark (Africa south of the Sahara)
    • Clade Paenungulata
      • Order Hyracoidea: hyraxes or dassies (Africa, Arabia)
      • Order Proboscidea: elephants (Africa, Southeast Asia)
      • Order Sirenia: dugong and manatees (cosmopolitan tropical)
  • Group II: Xenarthra
    • Order Pilosa: sloths and anteaters (neotropical)
    • Order Cingulata: armadillos (Americas)
• Clade Boreoeutheria
  • Group III: Euarchontoglires (Supraprimates)
    • Superorder Euarchonta
      • Order Scandentia: treeshrews (Southeast Asia).
      • Order Dermoptera: flying lemurs or colugos (Southeast Asia)
      • Order Primates: lemurs, bushbabies, monkeys, apes, human (cosmopolitan)
    • Superorder Glires
      • Order Lagomorpha: pikas, rabbits, hares (Eurasia, Africa, Americas)
      • Order Rodentia: rodents (cosmopolitan)
  • Group IV: Laurasiatheria
    • Order Erinaceomorpha: hedgehogs
    • Order Soricomorpha: moles, shrews, solenodons
    • Clade Ferungulata
      • Clade Cetartiodactyla
        • Order Cetacea: whales, dolphins and porpoises
        • Order Artiodactyla: even-toed ungulates, including pigs, hippopotamus, camels, giraffe, deer, antelope, cattle, sheep, goats
      • Clade Pegasoferae
        • Order Chiroptera: bats (cosmopolitan)
        • Clade Zooamata
          • Order Perissodactyla: odd-toed ungulates, including horses, donkeys, zebras, tapirs, and rhinoceroses
          • Clade Ferae
            • Order Pholidota: pangolins or scaly anteaters (Africa, South Asia)
            • Order Carnivora: carnivores (cosmopolitan), including cats and dogs

Evolutionary history[link]

Cladogram following,^[9] which takes Mammalia to be the crown group.

Synapsida, the group which contains mammals and their extinct relatives, originated during the Pennsylvanian subperiod, when they split from the lineage that led to reptiles and birds. Nonmammalian synapsids are sometimes called "mammal-like reptiles",^[10][11] although they are usually no longer considered reptiles. Crown group mammals evolved from earlier mammaliaforms during the Early Jurassic.

Evolution from amniotes in the Paleozoic[link]

The original synapsid skull structure contains one temporal opening behind the orbitals, in a fairly low position on the skull (lower right in this image). This might have assisted in the containing the jaw muscles of these organisms that could have increased their biting strength.

The first fully terrestrial vertebrates were amniotes. Like their amphibian predecessors, they have lungs and limbs. Amniotes' eggs, however, have internal membranes which allow the developing embryo to breathe but keep water in. Hence, amniotes can lay eggs on dry land, while amphibians generally need to lay their eggs in water.

The first amniotes apparently arose in the Late Carboniferous. They descended from earlier reptiliomorph amphibians,^[12] which lived on land already inhabited by insects and other invertebrates, and by ferns, mosses and other plants. Within a few million years, two important amniote lineages became distinct: the synapsids, which include mammals; and the sauropsids, which include turtles, lizards, snakes, crocodilians, dinosaurs and birds.^[13] Synapsids have a single hole (temporal fenestra) low on each side of the skull.

One synapsid group, the pelycosaurs, included the largest and fiercest animals of the early Permian.^[14]

Therapsids descended from pelycosaurs in the middle Permian, about 265 million years ago, and took over their position as the dominant land vertebrates.^[10] They differ from pelycosaurs in several features of the skull and jaws, including: larger temporal fenestrae and incisors which are equal in size.^[15] The therapsid lineage leading to mammals went through a series of stages, beginning with animals that were very like their pelycosaur ancestors and ending with probainognathian cynodonts, some of which could easily be mistaken for mammals. Those stages were characterized by:

• gradual development of a bony secondary palate.^[16]
• Progress was made towards an erect limb posture, which would increase the animals' stamina by avoiding Carrier's constraint. But this process was slow and erratic: for example, all herbivorous nonmammaliaform therapsids retained sprawling limbs (some late forms may have had semierect hind limbs); Permian carnivorous therapsids had sprawling forelimbs, and some late Permian ones also had semisprawling hindlimbs. In fact, modern monotremes still have semisprawling limbs.
• The dentary gradually became the main bone of the lower jaw and, in the Triassic, progressed towards the fully mammalian jaw (the lower consisting only of the dentary) and middle ear (which is constructed by the bones that were previously used to construct the jaws of reptiles).
• There is some evidence of hair in Triassic therapsids but none for Permian therapsids.
• Some Triassic therapsids also show signs of lactation.

The mammals appear[link]

The Permian–Triassic extinction event, which was a prolonged event due to the accumulation of several extinction pulses, ended the dominance of the carnivores among the therapsids. In the Early Triassic, all the medium to large land carnivore niches were taken over by early archosaurs, which over an extended period of time (35 million years) evolved into crocodilians, pterosaurs, dinosaurs and birds. By the Jurassic, the dinosaurs had come to dominate the large terrestrial herbivore niches as well.

The first mammals (in the sense given to the term by Kielan-Jawarowska et al.)^[4] appeared in the Late Triassic epoch (about 210 million years ago), 60 million years after the first therapsids. They expanded out of their nocturnal insectivore niche from the mid-Jurassic onwards; Castorocauda, for example, had adaptations for swimming, digging and catching fish.^[17]

The majority of the mammal species that existed in the Mesozoic Era were multituberculates, triconodonts and spalacotheriids.^[18]

The earliest known monotreme is Teinolophos, which lived about 123 million years ago in Australia. Monotremes have some features which may be inherited from the original amniotes:

• They use the same orifice to urinate, defecate and reproduce ("monotreme" means "one hole") – as lizards and birds also do.
• They lay eggs which are leathery and uncalcified, like those of lizards, turtles and crocodilians.

Unlike other mammals, female monotremes do not have nipples and feed their young by "sweating" milk from patches on their bellies.

The earliest known metatherian is Sinodelphys, found in 125 million-year-old Early Cretaceous shale in China's northeastern Liaoning Province. The fossil is nearly complete and includes tufts of fur and imprints of soft tissues.^[19]

The oldest known fossil among the Eutheria ("true beasts") is the small shrewlike Juramaia sinensis, or "Jurassic mother from China," dated to 160 million years ago in the Upper Jurassic.^[6] A later eutherian, Eomaia, dated to 125 million years ago in the Lower Cretaceous, possessed some features in common with the marsupials but not with the placentals, evidence that these features were present in the last common ancestor of the two groups but were later lost in the placental lineage.^[20] In particular:

• Epipubic bones extend forwards from the pelvis. These are not found in any modern placental, but they are found in marsupials, monotremes, and nontherian mammals like the multituberculates as well as in Ukhaatherium, an Upper Cretaceous animal in the eutherian order Asioryctitheria.^[21] They are apparently an ancestral feature which subsequently disappeared in the placental lineage. These epipubic bones seem to function by stiffening the muscles of these animals during locomotion, reducing the amount of space being presented, which placentals require to contain their fetus during gestation periods.

• A narrow pelvic outlet indicates that the young were very small at birth and therefore pregnancy was short, as in modern marsupials. This suggests that the placenta was a later development.

When true placental mammals evolved is uncertain – the earliest undisputed fossils of placentals come from the early Paleocene, after the extinction of the dinosaurs.^[22]

Rise to dominance in the Cenozoic[link]

Mammals took over the medium- to large-sized ecological niches in the Cenozoic, after the Cretaceous–Paleogene extinction event emptied ecological space once filled by reptiles.^[23] Then mammals diversified very quickly; both birds and mammals show an exponential rise in diversity.^[23] For example, the earliest known bat dates from about 50 million years ago, only 15 million years after the extinction of the dinosaurs.^[24]

Recent molecular phylogenetic studies suggest that most placental orders diverged about 100 to 85 million years ago and that modern families appeared in the period from the late Eocene through the Miocene.^[25] But paleontologists object that no placental fossils have been found from before the end of the Cretaceous.^[22]

During the Cenozoic, several groups of mammals appeared which were much larger than their nearest modern equivalents, but none was even close to the size of the largest dinosaurs with similar feeding habits.

Earliest appearances of features[link]

Hadrocodium, whose fossils date from the early Jurassic (approximately 195 million years ago, in the Lower Jurassic), provides the first clear evidence of a jaw joint formed solely by the squamosal and dentary bones; there is no space in the jaw for the articular, a bone involved in the jaws of all early synapsids.

It has been suggested that the original function of lactation (milk production) was to keep eggs moist. Much of the argument is based on monotremes (egg-laying mammals).^[26][27][28]

The earliest clear evidence of hair or fur is in fossils of Castorocauda, from 164 million years ago in the Middle Jurassic. In the past, some scientists interpreted the foramina (passages) in the maxillae (upper jaws) and premaxillae (small bones in front of the maxillae) of cynodonts as channels which supplied blood vessels and nerves to vibrissae (whiskers) and suggested that this was evidence of hair or fur.^[29][30] Foramina do not necessarily show that an animal had vibrissae, however; the modern lizard Tupinambis has foramina which are almost identical to those found in the nonmammalian cynodont Thrinaxodon.^[11][31]

The evolution of erect limbs in mammals is incomplete — living and fossil monotremes have sprawling limbs. Some scientists think that the parasagittal (nonsprawling) limb posture is a synapomorphy (distinguishing characteristic) of the Boreosphenida, a group which contains the Theria and therefore includes all eutherians (including the placentals).^[32] Since Juramaia, the earliest known eutherian, lived about 160 million years ago in the Jurassic, this implies that erect limbs must have evolved before then.

When endothermy first appeared in the evolution of mammals is uncertain. Modern monotremes have lower body temperatures and more variable metabolic rates than marsupials and placentals,^[33] but there is evidence that some of their ancestors, perhaps including ancestors of the therians, may have had body temperatures like those of modern therians.^[34] Some of the evidence found so far suggests that Triassic cynodonts had fairly high metabolic rates, but it is not conclusive. For small animals, an insulative covering like fur is necessary for the maintenance of a high and stable body temperature.

Anatomy and morphology[link]

Skeletal system[link]

The majority of mammals have seven cervical vertebrae (bones in the neck), including bats, giraffes, whales, and humans. The exceptions are the manatee and the two-toed sloth, which have only six cervical vertebrae, and the three-toed sloth with nine cervical vertebrae.^[35]

Respiratory system[link]

The lungs of mammals have a spongy texture and are honeycombed with epithelium having a much larger surface area in total than the outer surface area of the lung itself. The lungs of humans are typical of this type of lung.

Breathing is largely driven by the muscular diaphragm, which divides the thorax from the abdominal cavity, forming a dome with its convexity towards the thorax. Contraction of the diaphragm flattens the dome, increasing the volume of the cavity in which the lung is enclosed. Air enters through the oral and nasal cavities; it flows through the larynx, trachea and bronchi and expands the alveoli. Relaxation of the diaphragm has the opposite effect, passively recoiling during normal breathing. During exercise, the abdominal wall contracts, increasing visceral pressure on the diaphragm, thus forcing the air out more quickly and forcefully. The rib cage itself also is able to expand and contract the thoracic cavity to some degree, through the action of other respiratory and accessory respiratory muscles. As a result, air is sucked into or expelled out of the lungs, always moving down its pressure gradient. This type of lung is known as a bellows lung as it resembles a blacksmith's bellows. Mammals take oxygen into their lungs, and discard carbon dioxide.

Nervous system[link]

All mammalian brains possess a neocortex, a brain region unique to mammals. Placental mammals have a corpus callosum, unlike monotremes and marsupials. The size and number of cortical areas (Brodmann's areas) is least in monotremes (about 8-10) and most in placentals (up to 50).

Integumentary system[link]

The integumentary system is made up of three layers: the outermost epidermis, the dermis, and the hypodermis.

The epidermis is typically 10 to 30 cells thick; its main function is to provide a waterproof layer. Its outermost cells are constantly lost; its bottommost cells are constantly dividing and pushing upward. The middle layer, the dermis, is 15 to 40 times thicker than the epidermis. The dermis is made up of many components, such as bony structures and blood vessels. The hypodermis is made up of adipose tissue. Its job is to store lipids, and to provide cushioning and insulation. The thickness of this layer varies widely from species to species.

Although other animals have features such as feathers, whiskers, setae, or cilia that superficially resemble it, no animals other than mammals have hair. It is a definitive characteristic of the class. Though some mammals have very little, careful examination reveals the characteristic, often in obscure parts of their bodies.

Some primates and marsupials have shades of violet, green, or blue skin on parts of their bodies.^[36] The two-toed sloth and the polar bear sometimes appear to have green fur, but this color is caused by algae growths.

Reproductive system[link]

Goat kids will stay with their mother until they are weaned.

Most mammals are viviparous, giving birth to live young. However, the five species of monotreme, the platypuses and the echidnas, lay eggs. The monotremes have a sex determination system different from that of most other mammals.^[37] In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian mammal.^[38]

The mammary glands of mammals are specialized to produce milk, a liquid used by newborns as their primary source of nutrition. The monotremes branched early from other mammals and do not have the nipples seen in most mammals, but they do have mammary glands. The young lick the milk from a mammary patch on the mother's belly.

Viviparous mammals are in the subclass Theria; those living today are in the marsupial and placental infraclasses. A marsupial has a short gestation period, typically shorter than its estrous cycle, and gives birth to an undeveloped newborn that then undergoes further development; in many species, this takes place within a pouch-like sac, the marsupium, located in the front of the mother's abdomen. The placentals give birth to complete and fully developed young, usually after long gestation periods.

Physiology[link]

Endothermy[link]

Nearly all mammals are endothermic ("warm-blooded"). Most mammals also have hair to help keep them warm. Like birds, mammals can forage or hunt in weather and climates too cold for nonavian reptiles and large insects.

Endothermy requires plenty of food energy, so mammals eat more food per unit of body weight than most reptiles. Small insectivorous mammals eat prodigious amounts for their size.

A rare exception, the naked mole rat, produces little metabolic heat, so it is considered an operational poikilotherm. Birds are also endothermic, so endothermy is not a defining mammalian feature.

Intelligence[link]

In intelligent mammals, such as primates, the cerebrum is larger relative to the rest of the brain. Intelligence itself is not easy to define, but indications of intelligence include the ability to learn, matched with behavioral flexibility. Rats, for example, are considered to be highly intelligent, as they can learn and perform new tasks, an ability that may be important when they first colonize a fresh habitat. In some mammals, food gathering appears to be related to intelligence: a deer feeding on plants has a brain smaller than a cat, which must think to outwit its prey.^[39]

Social structure[link]

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Locomotion[link]

Mammals evolved from four-legged ancestors. They use their limbs to walk, climb, swim, and fly. Some land mammals have toes that produce claws and hooves for climbing and running. Aquatic mammals like whales and dolphins have flippers which evolved from legs.

Terrestrial[link]

Arboreal[link]

Aquatic[link]

Whales and dolphins propel themselves through the water by moving their tail flukes up and down, adjusting the angle of the flukes as needed. The more massive front of the body contributes stability.^[40][41]

Aerial[link]

Feeding[link]

To maintain a high constant body temperature is energy expensive – mammals therefore need a nutritious and plentiful diet. While the earliest mammals were probably predators, different species have since adapted to meet their dietary requirements in a variety of ways. Some eat other animals – this is a carnivorous diet (and includes insectivorous diets). Other mammals, called herbivores, eat plants. A herbivorous diet includes subtypes such as fruit-eating and grass-eating. An omnivore eats both prey and plants. Carnivorous mammals have a simple digestive tract, because the proteins, lipids, and minerals found in meat require little in the way of specialized digestion. Plants, on the other hand, contain complex carbohydrates, such as cellulose. The digestive tract of an herbivore is therefore host to bacteria that ferment these substances, and make them available for digestion. The bacteria are either housed in the multichambered stomach or in a large cecum. The size of an animal is also a factor in determining diet type. Since small mammals have a high ratio of heat-losing surface area to heat-generating volume, they tend to have high energy requirements and a high metabolic rate. Mammals that weigh less than about 18 oz (500 g) are mostly insectivorous because they cannot tolerate the slow, complex digestive process of a herbivore. Larger animals, on the other hand, generate more heat and less of this heat is lost. They can therefore tolerate either a slower collection process (those that prey on larger vertebrates) or a slower digestive process (herbivores). Furthermore, mammals that weigh more than 18 oz (500 g) usually cannot collect enough insects during their waking hours to sustain themselves. The only large insectivorous mammals are those that feed on huge colonies of insects (ants or termites).^[39]

Specializations in herbivory include: Granivory "seed eating", folivory "leaf eating", frugivory "fruit eating", nectivory "nectar eating", gumivory "gum eating", and mycophagy "fungus eating".

See also[link]

• List of African mammals
• List of extinct mammals
• List of Indian mammals
• List of mammalogists
• List of mammal genera
• List of mammals
• Lists of mammals by region
• List of prehistoric mammals
• Mammal classification
• Mammals discovered in the 2000s
• Prehistoric mammals

Mammals portal

References[link]

Further reading[link]

• Bergsten, Johannes. February 2005. "A review of long-branch attraction". Cladistics 21:163–193. (pdf version)
• Brown W.M. (2001). "Natural selection of mammalian brain components" (PDF). Trends in Ecology and Evolution 16 (9): 471–473. DOI:10.1016/S0169-5347(01)02246-7. http://www.rci.rutgers.edu/~wmbrown/BROWN.TREE.pdf. 
• Khalaf-von Jaffa, Norman Ali Bassam Ali Taher (2006). Mammalia Palaestina: The Mammals of Palestine. Gazelle: The Palestinian Biological Bulletin. Number 55, July 2006. pp. 1–46.
• McKenna, Malcolm C., and Bell, Susan K. 1997. Classification of Mammals Above the Species Level. Columbia University Press, New York, 631 pp. ISBN 0-231-11013-8
• Nowak, Ronald M. 1999. Walker's Mammals of the World, 6th edition. Johns Hopkins University Press, 1936 pp. ISBN 0-8018-5789-9
• Simpson, George Gaylord (1945). "The principles of classification and a classification of mammals". Bulletin of the American Museum of Natural History 85: 1–350. 
• William J. Murphy, Eduardo Eizirik, Mark S. Springer et al., Resolution of the Early Placental Mammal Radiation Using Bayesian Phylogenetics,Science, Vol 294, Issue 5550, 2348–2351, 14 December 2001.
• Springer, Mark S., Michael J. Stanhope, Ole Madsen, and Wilfried W. de Jong. 2004. "Molecules consolidate the placental mammal tree". Trends in Ecology and Evolution, 19:430–438. (PDF version)
• Vaughan, Terry A., James M. Ryan, and Nicholas J. Capzaplewski. 2000. Mammalogy: Fourth Edition. Saunders College Publishing, 565 pp. ISBN 0-03-025034-X (Brooks Cole, 1999)
• Jan Ole Kriegs, Gennady Churakov, Martin Kiefmann, Ursula Jordan, Juergen Brosius, Juergen Schmitz. (2006) Retroposed Elements as Archives for the Evolutionary History of Placental Mammals. PLoS Biol 4(4): e91."PLoS Biology – Retroposed Elements as Archives for the Evolutionary History of Placental Mammals". Biology.plosjournals.org. DOI:10.1371/journal.pbio.0040091. http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0040091. Retrieved 2009-03-08. 
• David MacDonald, Sasha Norris. 2006. The Encyclopedia of Mammals, 3rd edition. Printed in China, 930 pp. ISBN 0-681-45659-0.

External links[link]

Find more about Mammal on Wikipedia's sister projects:
	Definitions and translations from Wiktionary
	Images and media from Commons
	Learning resources from Wikiversity
	News stories from Wikinews
	Quotations from Wikiquote
	Source texts from Wikisource
	Textbooks from Wikibooks

Wikispecies has information related to: Mammalia

The Wikibook Dichotomous Key has a page on the topic of Mammalia

• BBC Wildlife Finder – video clips from the BBC's natural history archive
• Mammal Species Thousands of species, showing taxonomic classification, images, and other information, animalias.com
• GlobalTwitcher.com – All species in the world with distribution maps and images
• Paleocene Mammals, a site covering the rise of the mammals, paleocene-mammals.de
• Evolution of Mammals, a brief introduction to early mammals, enchantedlearning.com
• Tree of Life poster – Shows mammals' evolutionary relation to other organisms, tellapallet.com
• The Evolution of Mesozoic Mammals, a Rough Sketch, an informal introduction, home.arcor.de
• Carnegie Museum of Natural History, some discoveries of early mammal fossils, carnegiemnh.org
• High-Resolution Images of various Mammalian Brains, brainmaps.org
• Mammal Species, collection of information sheets about various mammal species, learnanimals.com
• Summary of molecular support for Epitheria, biology.plosjournals.org
• Mikko's Phylogeny Archive, fmnh.helsinki.fi
• European Mammal Atlas EMMA from Societas Europaea Mammalogica, European-mammals.org
• Marine Mammals of the World—An overview of all marine mammals, including descriptions, multimedia and a key, eti.uva.nl
• 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

lez:Некхъвадайбур

Peter Albert David Singer

Born	(1946-07-06) 6 July 1946 (age 65) Melbourne, Australia
Era	Contemporary philosophy
Region	Western philosophy
School	Analytic philosophy · Utilitarianism
Main interests	Ethics
Influenced by John Stuart Mill · Henry Sidgwick R.M. Hare · Jeremy Bentham
Influenced Peter Unger · Colin McGinn Roger Crisp · Dale Jamieson · Gregory Pence

Peter Albert David Singer (born 6 July 1946) is an Australian philosopher who is the Ira W. DeCamp Professor of Bioethics at Princeton University and Laureate Professor at the Centre for Applied Philosophy and Public Ethics at the University of Melbourne. He specialises in applied ethics and approaches ethical issues from a secular, preference utilitarian perspective.

He has served, on two occasions, as chair of philosophy at Monash University, where he founded its Centre for Human Bioethics. In 1996, he unsuccessfully stood as a Greens candidate for the Australian Senate. In 2004, he was recognised as the Australian Humanist of the Year by the Council of Australian Humanist Societies. He has been voted one of Australia's ten most influential public intellectuals.^[1] Singer serves on the Advisory Board of Incentives for Global Health, the NGO formed to develop the Health Impact Fund proposal.

Outside academic circles, Singer is best known for his book Animal Liberation, widely regarded as the touchstone of the animal liberation movement. Not all members of the animal liberation movement share this view, and Singer himself has said the media overstates his status. His views on that and other issues in bioethics have attracted attention and a degree of controversy.

Life and career[link]

Singer's parents were Viennese Jews who emigrated to Australia in 1938 from Vienna after Austria's annexation by Nazi Germany.^[2] They settled in Melbourne, where Singer was born. His grandparents were less fortunate: his paternal grandparents were taken by the Nazis to Łódź, and were never heard from again; his maternal grandfather died in the Theresienstadt concentration camp.^[3] He has a sister, Joan (now Joan Dwyer). Singer's father imported tea and coffee, while his mother practiced medicine. He attended Preshil^[4] and later Scotch College. After leaving school, Singer studied law, history and philosophy at the University of Melbourne, gaining his BA degree (hons) in 1967.^[5] He received an MA for a thesis entitled Why should I be moral? in 1969. He was awarded a scholarship to study at the University of Oxford, obtaining a B.Phil in 1971 with a thesis on civil disobedience, supervised by R. M. Hare, and subsequently published as a book in 1973.^[6] Singer names Hare and Australian philosopher H. D. McCloskey as his two most important mentors.^[7]

After spending two years as a Radcliffe lecturer at University College, Oxford, he was a visiting professor at New York University for 16 months. He returned to Melbourne in 1977, where he spent most of his career, aside from appointments as visiting faculty abroad, until his move to Princeton in 1999.^[8] In June 2011 it was announced he would join the professoriate of New College of the Humanities, a private college in London, in addition to his work at Princeton.^[9]

[edit] Animal Liberation

Published in 1975, Animal Liberation^[10] has been cited as a formative influence on leaders of the modern animal liberation movement.^[11] The central argument of the book is an expansion of the utilitarian idea that "the greatest good of the greatest number" is the only measure of good or ethical behaviour. Singer argues that there is no reason not to apply this to other animals. He popularized the term "speciesism", which was originally coined by Richard D. Ryder, to describe the practice of privileging humans over other animals.^[12]

Applied ethics[link]

Singer's most comprehensive work, Practical Ethics (1979),^[13] analyzes in detail why and how living beings' interests should be weighed. His principle of equal consideration of interests does not dictate equal treatment of all those with interests, since different interests warrant different treatment. All have an interest in avoiding pain, for instance, but relatively few have an interest in cultivating their abilities. Not only does his principle justify different treatment for different interests, but it allows different treatment for the same interest when diminishing marginal utility is a factor. For example, this approach would privilege a starving person's interest in food over the same interest of someone who is only slightly hungry.

Among the more important human interests are those in avoiding pain, in developing one's abilities, in satisfying basic needs for food and shelter, in enjoying warm personal relationships, in being free to pursue one's projects without interference, "and many others". The fundamental interest that entitles a being to equal consideration is the capacity for "suffering and/or enjoyment or happiness". He holds that a being's interests should always be weighed according to that being's concrete properties. He favors a 'journey' model of life, which measures the wrongness of taking a life by the degree to which doing so frustrates a life journey's goals. The journey model is tolerant of some frustrated desire and explains why persons who have embarked on their journeys are not replaceable. Only a personal interest in continuing to live brings the journey model into play. This model also explains the priority that Singer attaches to interests over trivial desires and pleasures.

Singer's ideas require the concept of an impartial standpoint from which to compare interests. He has wavered about whether the precise aim is the total amount of satisfied interests or the most satisfied interests among those beings who already exist prior to the decision one is making. The second edition of Practical Ethics disavows the first edition's suggestion that the total and prior-existence views should be combined. The second edition asserts that preference-satisfaction utilitarianism, incorporating the 'journey' model, applies without invoking the first edition's suggestion about the total view. But the details are fuzzy and Singer admits that he is "not entirely satisfied" with his treatment.^[14]

Ethical conduct is justifiable by reasons that go beyond prudence to "something bigger than the individual," addressing a larger audience. Singer thinks this going-beyond identifies moral reasons as "somehow universal", specifically in the injunction to 'love thy neighbor as thyself', interpreted by him as demanding that one give the same weight to the interests of others as one gives to one's own interests. This universalising step, which Singer traces from Kant to Hare,^[15] is crucial and sets him apart from those moral theorists from Hobbes to David Gauthier, who tie morality to prudence. Universalisation leads directly to utilitarianism, Singer argues, on the strength of the thought that one's own interests cannot count for more than the interests of others. Taking these into account, one must weigh them up and adopt the course of action that is most likely to maximise the interests of those affected; utilitarianism has been arrived at. Singer's universalising step applies to interests without reference to who has them, whereas a Kantian's applies to the judgments of rational agents (in Kant's kingdom of ends, or Rawls's Original Position, etc.). Singer regards Kantian universalization as unjust to animals.^[16] As for the Hobbesians, Singer attempts a response in the final chapter of Practical Ethics, arguing that self-interested reasons support adoption of the moral point of view, such as 'the paradox of hedonism', which counsels that happiness is best found by not looking for it, and the need most people feel to relate to something larger than their own concerns.

Practical Ethics includes a chapter arguing for the redistribution of wealth to ameliorate absolute poverty (Chapter 8, "Rich and Poor"), and another making a case for resettlement of refugees on a large scale in industrialised countries (Chapter 9, "Insiders and Outsiders").

Although the natural, non-sentient environment has no intrinsic value for a utilitarian like Singer, environmental degradation is a profound threat to sentient life, and for this reason environmentalists are right to speak of wilderness as a 'world heritage'.^[17]

Abortion, euthanasia and infanticide[link]

Consistent with his general ethical theory, Singer holds that the right to life is intrinsically tied to a being's capacity to hold preferences, which in turn is intrinsically tied to a being's capacity to feel pain and pleasure. In his view, the central argument against abortion may be stated as the following syllogism:

It is wrong to kill an innocent human being.
A human fetus is an innocent human being.
Therefore it is wrong to kill a human fetus.^[18]

In his book Rethinking Life and Death, as well as in Practical Ethics, Singer asserts that, if we take the premises at face value, the argument is deductively valid. Singer comments that defenders of abortion attack the second premise, suggesting that the fetus becomes a "human" or "alive" at some point after conception; however, Singer argues that human development is a gradual process, that it is nearly impossible to mark a particular moment in time as the moment at which human life begins.

Singer at MIT.