Mammal Temporal range: 225–0 Ma (Kemp) or 167–0 Ma (Rowe) See discussion of dates in text PreꞒ Ꞓ O S D C P T J K Pg N
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Amniota
Clade:	Synapsida
Clade:	Mammaliaformes
Class:	Mammalia Linnaeus, 1758
Subgroups
Subclass Yinotheria Infraclass Australosphenida Order Monotremata †Order Eutriconodonta Subclass Theriiformes Infraclass Holotheria Superlegion Trechnotheria Legion Cladotheria Supercohort Theria Cohort Marsupialia Cohort Placentalia

Mammals (class Mammalia /məˈmeɪli.ə/ from Latin mamma "breast") are a clade of endothermic amniotes distinguished from reptiles and birds by the possession of a neocortex (a region of the brain), hair, three middle ear bones and mammary glands.

Mammals include the largest animals on the planet, the great whales, as well as some of the most intelligent, such as elephants, primates and cetaceans. The basic body type is a terrestrial quadruped, but some mammals are adapted for life at sea, in the air, in trees, underground or on two legs. The largest group of mammals, the placentals, have a placenta, which enables the feeding of the fetus during gestation.

Mammals range in size from the 30–40 mm (1.2–1.6 in) bumblebee bat to the 33-meter (108 ft) blue whale. With the exception of the five species of monotreme (egg-laying mammals), all modern 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 number of species are Rodentia: mice, rats, porcupines, beavers, capybaras and other gnawing mammals; Chiroptera: bats; and Soricomorpha: shrews, moles and solenodons. The next three biggest orders, depending on the biological classification scheme used, are the Primates including the great apes and monkeys; the Cetartiodactyla including whales and even-toed ungulates; and the Carnivora which includes cats, dogs, weasels, bears and seals.

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, 5,416 species were known in 2006. These were grouped in 1,229 genera, 153 families and 29 orders. In 2008 the International Union for Conservation of Nature (IUCN) completed a five-year, 1,700-scientist Global Mammal Assessment for its IUCN Red List, which counted 5,488 species. In some classifications, extant mammals are divided into two subclasses: the Prototheria, that is, the order Monotremata; and the Theria, or the infraclasses Metatheria and Eutheria. The marsupials constitute the crown group of the Metatheria, and include all living metatherians as well as many extinct ones; the placentals are the crown group of the Eutheria. While mammal classification at the family level has been relatively stable, several contending classifications regarding the higher levels—subclass, infraclass and order, especially of the marsupials—appear in contemporaneous literature. Much of the changes reflect the advances of cladistic analysis and molecular genetics. Findings from molecular genetics, for example, have prompted adopting new groups, such as the Afrotheria, and abandoning traditional groups, such as the Insectivora.

The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that produced the non-mammalian Dimetrodon. At the end of the Carboniferous period, this group diverged from the sauropsid line that led to today's reptiles and birds. The line following the stem group Sphenacodontia split-off several diverse groups of non-mammalian synapsids—sometimes referred to as mammal-like reptiles—before giving rise to the proto-mammals (Therapsida) in the early Mesozoic era. The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of non-avian dinosaurs, and have been among the dominant terrestrial animal groups from 66 million years ago to the present.

Classification

Mammal classification has been through several iterations since Carl Linnaeus initially defined the class. No classification system is universally accepted; McKenna & Bell (1997) and Wilson & Reader (2005) provide useful recent compendiums.^[1] George Gaylord Simpson's "Principles of Classification and a Classification of Mammals" (AMNH Bulletin v. 85, 1945) provides systematics of mammal origins and relationships that were 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.^[2]

Definitions

The word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma ("teat, pap"). In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of 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] Since this ancestor lived in the Jurassic period, Rowe's definition excludes all animals from the earlier Triassic, despite the fact that Triassic fossils in the Haramiyida have been referred to the Mammalia since the mid-19th century.^[4] If Mammalia is considered as the crown group, its origin can be roughly dated as the first known appearance of animals more closely related to some extant mammals than to others. Ambondro is more closely related to monotremes than to therian mammals while Amphilestes and Amphitherium are more closely related to the therians; as fossils of all three genera are dated about 167 million years ago in the Middle Jurassic, this is a reasonable estimate for the appearance of the crown group.^[5]

T. S. Kemp has provided a more traditional definition: "synapsids that possess a dentary–squamosal jaw articulation and occlusion between upper and lower molars with a transverse component to the movement" or, equivalently in Kemp's view, the clade originating with the last common ancestor of Sinoconodon and living mammals.^[6] The earliest known synapsid satisfying Kemp's definitions is Tikitherium, dated 225 Ma, so the appearance of mammals in this broader sense can be given this Late Triassic date.^[7][8]

McKenna/Bell classification

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,^[9] is a comprehensive work on the systematics, relationships and occurrences of all mammal taxa, living and extinct, down through the rank of genus, though 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.^[2]

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 †Eutriconodonta: eutriconodonts
  • Infraclass Holotheria: modern live-bearing mammals and their prehistoric relatives
    • Superlegion †Kuehneotheria
    • Supercohort Theria: live-bearing mammals
      • Cohort Marsupialia: marsupials
        • Magnorder Australidelphia: Australian marsupials and the monito del monte
        • Magnorder Ameridelphia: New World marsupials. Now considered paraphyletic, with shrew opossums being closer to australidelphians.^[10][11]
      • Cohort Placentalia: placentals
        • Magnorder Xenarthra: xenarthrans
        • Magnorder Epitheria: epitheres
          • Superorder Anagalida: lagomorphs, rodents and elephant shrews
          • Superorder Ferae: carnivorans, pangolins, †creodonts and relatives
          • Superorder Lipotyphla: insectivorans
          • Superorder Archonta: bats, primates, colugos and treeshrews
          • Superorder 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

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.^[12] Classification systems based on molecular studies reveal three major groups or lineages of placental mammals- Afrotheria, Xenarthra and Boreoeutheria- which diverged in the Cretaceous. The relationships between these three lineages is contentious, and all three possible different hypotheses have been proposed with respect to which group is basal. These hypotheses are Atlantogenata (basal Boreoeutheria), Epitheria (basal Xenarthra) and Exafroplacentalia (basal Afrotheria).^[13] Boreoeutheria in turn contains two major lineages- Euarchontoglires and Laurasiatheria.

Estimates for the divergence times between these three placental groups range from 105 to 120 million years ago, depending on type of DNA (such as nuclear or mitochondrial)^[14] and varying interpretations of paleogeographic data.^[13]

Cladogram based on Tarver et al. (2016)^[15]

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 and extinct relatives (Americas)

Group III: Boreoeutheria

• Clade: Euarchontoglires (Supraprimates)
  • Superorder Euarchonta
    • Order Scandentia: treeshrews (Southeast Asia).
    • Order Dermoptera: flying lemurs or colugos (Southeast Asia)
    • Order Primates: lemurs, bushbabies, monkeys, apes, humans (cosmopolitan)
  • Superorder Glires
    • Order Lagomorpha: pikas, rabbits, hares (Eurasia, Africa, Americas)
    • Order Rodentia: rodents (cosmopolitan)
• Clade Laurasiatheria
  • Order Eulipotyphla: shrews, hedgehogs, moles, 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 (Note that Artiodactyla is paraphyletic to Cetacea)
    • 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

Statistics

Most mammals, including the six most species-rich orders, belong to the placental group. The three largest orders in numbers of species are Rodentia: mice, rats, porcupines, beavers, capybaras and other gnawing mammals; Chiroptera: bats; and Soricomorpha: shrews, moles and solenodons. The next three biggest orders, depending on the biological classification scheme used, are the Primates including the great apes and monkeys; the Cetartiodactyla including whales and even-toed ungulates; and the Carnivora which includes cats, dogs, weasels, bears and seals.^[16] According to Mammal Species of the World, 5,416 species were known in 2006. These were grouped in 1,229 genera, 153 families and 29 orders.^[16] In 2008 the International Union for Conservation of Nature (IUCN) completed a five-year, 1,700-scientist Global Mammal Assessment for its IUCN Red List, which counted 5,488 species.^[17]

Evolutionary history

Life timeline
−4500 — – — – −4000 — – — – −3500 — – — – −3000 — – — – −2500 — – — – −2000 — – — – −1500 — – — – −1000 — – — – −500 — – — – 0 —	Water   Single-celled life   Photosynthesis   Eukaryotes   Multicellular life   P l a n t s   Arthropods Molluscs Flowers Dinosaurs   Mammals Birds Primates H a d e a n A r c h e a n P r o t e r o z o i c P h a n e r o z o i c	← Earth formed ← Earliest water ← LUCA ← Earliest fossils ← LHB meteorites ← Earliest oxygen ← Pongola glaciation* ← Atmospheric oxygen ← Huronian glaciation* ← Sexual reproduction ← Earliest multicellular life ← Earliest fungi ← Earliest plants ← Earliest animals ← Cryogenian ice age* ← Ediacaran biota ← Cambrian explosion ← Andean glaciation* ← Earliest tetrapods ← Karoo ice age* ← Earliest apes / humans ← Quaternary ice age*
(million years ago) *Ice Ages

Synapsida, a clade that contains mammals and their extinct relatives, originated during the Pennsylvanian subperiod, when they split from reptilian and avian lineages. Crown group mammals evolved from earlier mammaliaforms during the Early Jurassic.

The cladogram following takes Mammalia to be the crown group.^[18]

Mammaliaformes classification

†Adelobasilus cromptoni Lucas & Hunt 1990 †Sinoconodon rigneyi Patterson & Olson 1961 †Morganucodonta †Docodonta †Hadrocodium wui Luo, Crompton & Sun 2001 †Kuehneotheriida Mammalia Yinotheria †Shuotheriidae Australosphenida †Henosferidae †Ambondro mahabo †Ausktribosphenidae Monotremata †Fruitafossor windscheffeli Luo & Wible 2005 †Volaticotherium antiquum Meng et al. 2006 emend. Meng et al. 2007 Theriimorpha †Eutriconodonta †Allotheria Holotheria †Chronoperates paradoxus Fox, Youzwyshyn & Krause 1992 Trechnotheria †Amphiodontidae Gobiconodontidae Klameliidae Amphilestidae Spalacotheriida Symmetrodonta Triconodontidae Zhangheotherium quinquecuspidens Hu et al. 1997 Cladotheria Dryolestida Zatheria Mammaliaformes †Tricuspes von Huene 1933 [? non-mammaliaform chiniquodontoid] †Repenomamus Li et al. 2001 {Repenomamidae Li et al. 2001} [?gobiconodont] Mammalia Linnaeus 1758 sensu stricto Rowe 1988 (true mammals) †Reigitherium bunodontum Bonaparte 1990 {Reigitheriidae Bonaparte 1990} †Tikitherium copei Datta 2005 Trechnotheria †Argentoconodon fariasorum Rougier et al. 2007 †Austrotriconodon mckennai Bonaparte 1986 {Austrotriconodontidae Bonaparte 1990} †Dyskritodon Sigogneau-Russell 1995 †Hallautherium schalchi Clemens 1980 †Liaotherium gracile Zhou, Cheng & Wang 1991 †Jeholodentidae Luo et al. 2007 Paraphyletic Symmetrodonta Simpson 1925 †Thereuodon Sigogneau-Russell 1989 [Dryolestida, or Zatheria indet., milk tooth (Averianov, 2002)] †Atlasodon Sigogneau-Russell 1991 [Dryolestida, or Zatheria indet., milk tooth (Averianov, 2002)] †Eurylambda aequicrurius †Peralestes longirostris †Tribactonodon bonfieldi Sigogneau-Russell, Hooker & Ensom 2001 A cladogram compiled by Mikko Haaramo based on individual cladograms of After Rowe 1988; Luo, Crompton & Sun 2001; Luo, Cifelli & Kielan-Jaworowska 2001, Luo, Kielan-Jaworowska & Cifelli 2002, Kielan-Jaworowska, Cifelli & Luo 2004 and Luo & Wible 2005.[19]

Evolution from amniotes

The first fully terrestrial vertebrates were amniotes. Like their amphibious tetrapod predecessors, they had lungs and limbs. Amniotic eggs, however, have internal membranes that 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 amphibious tetrapods,^[20] which lived on land that was already inhabited by insects and other invertebrates as well as ferns, mosses and other plants. Within a few million years, two important amniote lineages became distinct: the synapsids, which would later include the common ancestor of the mammals; and the sauropsids, which now include turtles, lizards, snakes, crocodilians, dinosaurs and birds.^[21] 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.^[22] Nonmammalian synapsids are sometimes called "mammal-like reptiles".^[23][24]

Therapsids descended from pelycosaurs in the Middle Permian, about 265 million years ago, and became the dominant land vertebrates.^[23] They differ from basal eupelycosaurs in several features of the skull and jaws, including: larger skulls and incisors which are equal in size in therapsids, but not for eupelycosaurs.^[23] The therapsid lineage leading to mammals went through a series of stages, beginning with animals that were very similar to their pelycosaur ancestors and ending with probainognathian cynodonts, some of which could easily be mistaken for mammals. Those stages were characterized by:^[25]

• The gradual development of a bony secondary palate.
• Progression 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 which, by 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).

First mammals

The Permian–Triassic extinction event, which was a prolonged event due to the accumulation of several extinction pulses, ended the dominance of carnivores therapsids.^[26] In the early Triassic, most medium to large land carnivore niches were taken over by archosaurs^[27] which, over an extended period of time (35 million years), came to include the crocodylomorphs,^[28] the pterosaurs and the dinosaurs;^[29] however, large cynodonts like Trucidocynodon and traversodontids still occupied large sized carnivorous and herbivorous niches respectively. By the Jurassic, the dinosaurs had come to dominate the large terrestrial herbivore niches as well.^[30]

The first mammals (in Kemp's sense) appeared in the Late Triassic epoch (about 225 million years ago), 40 million years after the first therapsids. They expanded out of their nocturnal insectivore niche from the mid-Jurassic onwards;^[31] The Jurassic Castorocauda, for example, had adaptations for swimming, digging and catching fish.^[32] Most, if not all, are thought to have remained nocturnal (the Nocturnal bottleneck), accounting for much of the typical mammalian traits.^[33] The majority of the mammal species that existed in the Mesozoic Era were multituberculates, eutriconodonts and spalacotheriids.^[34] 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.^[35]

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 late Jurassic.^[36] A later eutherian, Eomaia, dated to 125 million years ago in the early 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.^[37] In particular, the epipubic bones extend forwards from the pelvis. These are not found in any modern placental, but they are found in marsupials, monotremes, nontherian mammals and Ukhaatherium, an early Cretaceous animal in the eutherian order Asioryctitheria. This also applies to the multituberculates.^[38] They are apparently an ancestral feature, which subsequently disappeared in the placental lineage. These epipubic bones seem to function by stiffening the muscles 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.

The earliest known monotreme was Teinolophos, which lived about 120 million years ago in Australia.^[39] Monotremes have some features which may be inherited from the original amniotes such as the same orifice to urinate, defecate and reproduce (cloaca) – as lizards and birds also do –^[40] and they lay eggs which are leathery and uncalcified.^[41]

Earliest appearances of features

Hadrocodium, whose fossils date from approximately 195 million years ago, in the early 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.^[42]

The earliest clear evidence of hair or fur is in fossils of Castorocauda, from 164 million years ago in the mid-Jurassic. In the 1950s, it was suggested that the foramina (passages) in the maxillae and premaxillae (bones in the front of the upper jaw) of cynodonts were channels which supplied blood vessels and nerves to vibrissae (whiskers) and so were evidence of hair or fur;^[43][44] it was soon pointed out, however, that foramina do not necessarily show that an animal had vibrissae, as the modern lizard Tupinambis has foramina that are almost identical to those found in the nonmammalian cynodont Thrinaxodon.^[24][45] Popular sources, nevertheless, continue to attribute whiskers to Thrinaxodon.^[46] Studies on Permian coprolites suggest that non-mammalian synapsids of the epoch already had fur, setting the evolution of hairs possibly as far back as dicynodonts.^[47]

When endothermy first appeared in the evolution of mammals is uncertain, though it is generally agreed to have first evolved in non-mammalian therapsids.^[47][48] Modern monotremes have lower body temperatures and more variable metabolic rates than marsupials and placentals,^[49] 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.^[50] Likewise, some modern therians like afrotheres and xenarthrans have secondarily developed lower body temperatures.^[51]

The evolution of erect limbs in mammals is incomplete — living and fossil monotremes have sprawling limbs. The parasagittal (nonsprawling) limb posture appeared sometime in the late Jurassic or early Cretaceous; it is found in the eutherian Eomaia and the metatherian Sinodelphys, both dated to 125 million years ago.^[52] Epipubic bones, a feature that strongly influenced the reproduction of most mammal clades, are first found in Tritylodontidae, suggesting that it is a synapomorphy between them and mammaliformes. They are omnipresent in non-placental mammaliformes, though Megazostrodon and Erythrotherium appear to have lacked them.^[53]

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, the egg-laying mammals.^[54][55]

Rise of the mammals

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 non-avian dinosaurs and other groups of reptiles.^[56] Then mammals diversified very quickly; both birds and mammals show an exponential rise in diversity.^[56] For example, the earliest known bat dates from about 50 million years ago, only 16 million years after the extinction of the dinosaurs.^[57]

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.^[58] However, no placental fossils have been found from before the end of the Cretaceous.^[59] The earliest undisputed fossils of placentals comes from the early Paleocene, after the extinction of the dinosaurs.^[59] In particular, scientists have identified an early Paleocene animal named Protungulatum donnae as one of the first placental mammals.^[60] The earliest known ancestor of primates is Archicebus achilles^[61] from around 55 million years ago.^[61] This tiny primate weighed 20–30 grams (0.7–1.1 ounce) and could fit within a human palm.^[61]

Anatomy and morphology

Distinguishing features

Living mammal species can be identified by the presence of sweat glands, including those that are specialized to produce milk to nourish their young.^[62] In classifying fossils, however, other features must be used, since soft tissue glands and many other features are not visible in fossils.^[63]

Many traits shared by all living mammals appeared among the earliest members of the group:

• Jaw joint - The dentary (the lower jaw bone, which carries the teeth) and the squamosal (a small cranial 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 quadrate (a small bone at the back of the upper jaw).^[42]
• Middle ear - In crown-group mammals, 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.^[64]
• Tooth replacement - Teeth are replaced once or (as in toothed whales and murid rodents) not at all, rather than being replaced continually throughout life.^[65]
• 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.^[66]
• Occipital condyles - Two knobs at the base of the skull fit into the topmost neck vertebra; most other tetrapods, in contrast, have only one such knob.^[67]

For the most part, these characteristics were not present in the Triassic ancestors of the mammals.^[68] Nearly all mammal groups possess an epipubic bone, the exception being modern placentals.^[69]

Biological systems

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 just six, and the three-toed sloth which has nine cervical vertebrae.^[70] All mammalian brains possess a neocortex, a brain region unique to mammals.^[71] Placental mammals have a corpus callosum, unlike monotremes and marsupials.^[72]

The lungs of mammals are spongy and honeycombed. Breathing is mainly achieved with the diaphragm, which divides the thorax from the abdominal cavity, forming a dome convex to the thorax. Contraction of the diaphragm flattens the dome, increasing the volume of the lung cavity. Air enters through the oral and nasal cavities, and travels through the larynx, trachea and bronchi, and expands the alveoli. Relaxing the diaphragm has the opposite effect, decreasing the volume of the lung cavity, causing air to be pushed out of the lungs. During exercise, the abdominal wall contracts, increasing pressure on the diaphragm, which forces air out quicker and more forcefully. The rib cage is able to expand and contract the chest cavity through the action of other respiratory muscles. Consequently, air is sucked into or expelled out of the lungs, always moving down its pressure gradient.^[73][74] This type of lung is known as a bellows lung due to its resemblance to blacksmith bellows.^[74]

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.^[75]: 97 Although other animals have features such as whiskers, feathers, 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.^[75]: 61

Herbivores have developed a diverse range of physical structures to facilitate the consumption of plant material. To break up intact plant tissues, mammals have developed teeth structures that reflect their feeding preferences. For instance, frugivores (animals that feed primarily on fruit) and herbivores that feed on soft foliage have low-crowned teeth specialized for grinding foliage and seeds. Grazing animals that tend to eat hard, silica-rich grasses, have high-crowned teeth, which are capable of grinding tough plant tissues and do not wear down as quickly as low-crowned teeth.^[76] Most carnivorous mammals have carnassialiforme teeth (of varying length depending on diet), long canines and similar tooth replacement patterns.^[77]

The stomach of Artiodactyls is divided into four sections: the rumen, the reticulum, the omasum and the abomasum (only ruminants have a rumen). After the plant material is consumed, it is mixed with saliva in the rumen and reticulum and separates into solid and liquid material. The solids lump together to form a bolus (or cud), and is regurgitated. When the bolus enters the mouth, the fluid is squeezed out with the tongue and swallowed again. Ingested food passes to the rumen and reticulum where cellulytic microbes (bacteria, protozoa and fungi) produce cellulase, which is needed to break down the cellulose in plants.^[78] Perissodactyls, in contrast to the ruminants, store digested food that has left the stomach in an enlarged cecum, where it is fermented by bacteria.^[79] Carnivora have a simple stomach adapted to digest primarily meat, as compared to the elaborate digestive systems of herbivorous animals, which are necessary to break down tough, complex plant fibers. The caecum is either absent or short and simple, and the large intestine is not sacculated or much wider than the small intestine.^[80]

The mammalian heart has four chambers, two upper atria, the receiving chambers, and two lower ventricles, the discharging chambers.^[81] The heart has four valves, which separate its chambers and ensures blood flows in the correct direction through the heart (preventing backflow). After gas exchange in the pulmonary capillaries (blood vessels in the lungs), oxygen-rich blood returns to the left atrium via one of the four pulmonary veins. Blood flows nearly continuously back into the atrium, which acts as the receiving chamber, and from here through an opening into the left ventricle. Most blood flows passively into the heart while both the atria and ventricles are relaxed, but toward the end of the ventricular relaxation period, the left atrium will contract, pumping blood into the ventricle. The heart also requires nutrients and oxygen found in blood like other muscles, and is supplied via coronary arteries.^[82]

Color variation

Mammalian hair, also known as pelage, can vary in color between populations, organisms within a population, and even on the individual organism. Light-dark color variation is common in the mammalian taxa. Sometimes, this color variation is determined by age variation, however, in other cases, it is determined by other factors. Selective pressures, such as ecological interactions with other populations or environmental conditions, often lead to the variation in mammalian coloration.^[83] These selective pressures favor certain colors in order to increase survival. Camouflage is thought to be a major selection pressure shaping coloration in mammals, although there is also evidence that sexual selection, communication and physiological processes may influence its evolution as well.^[84] Camouflage is the most predominant mechanism for color variation, as it aids in the concealment of the organisms from predators or from their prey. Sloths sometimes appear to have green fur and blend into their green jungle environment, but this color is caused by algal growths.^[85]

Coat color can also be for intraspecies communication such as warning members of their species about predators, indicating health for reproductive purposes, communicating between mother and young and intimidating predators.^[84] Studies have shown that in some cases, differences in female and male coat color could indicate information nutrition and hormone levels, which are important in the mate selection process.^[83] For example, some primates and marsupials have shades of violet, green, or blue skin on parts of their bodies, which indicates some distinct advantage in their largely arboreal habitat due to convergent evolution.^[86]

Another mechanism for coat color variation is physiological response purposes, such as temperature regulation in tropical or arctic environments.^[84] Although much has been observed about color variation, much of the genetic that link coat color to genes is still unknown. The genetic sites where pigmentation genes are found are known to affect phenotype by altering the spatial distribution of pigmentation of the hairs, and altering the density and distribution of the hairs.^[87] Although the genetic sites are known, it is largely unknown how these genes are expressed.^[83]

Reproductive system

Most mammals are viviparous, giving birth to live young. However, the five species of monotreme, the platypus and the four species of echidna, lay eggs. The monotremes have a sex determination system different from that of most other mammals.^[88] In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian mammal.^[89]

The mammary glands of mammals are specialized to produce milk, the primary source of nutrition for newborns. 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.^[90]

Viviparous mammals are in the subclass Theria; those living today are in the marsupial and placental infraclasses. Marsupials have 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. This is the plesiomorphic condition among viviparous mammals; the presence of epipubic bones in all non-placental mammals prevents the expansion of the torso needed for full pregnancy.^[69] Even non-placental eutherians probably reproduced this way.^[91] The placentals are unusual among mammals in giving birth to complete and fully developed young, usually after long gestation periods.^[92]

Endothermy

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.^[93] 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.^[94] Birds are also endothermic, so endothermy is not unique to mammals.^[95]

Behavior

Feeding

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. An herbivorous diet includes subtypes such as granivory (seed eating), folivory (leaf eating), frugivory (fruit eating), nectivory (nectar eating), gummivory (gum eating) and mycophagy (fungus eating). Some mammals may be coprophagous, and consume feces, usually to consume more nutrients.^{[75]: 131–137} 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.^[97]

The size of an animal is also a factor in determining diet type (Allen's rule). 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 an 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).^[98] 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).^[99]

Intelligence

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.^[99]

Tool use by animals may indicate different levels of learning and cognition. The sea otter uses rocks as essential and regular parts of its foraging behaviour (smashing abalone off of rocks or breaking open shells), with some populations spending 21% of their time making tools.^[100] Other tool use, such as chimpanzees using twigs to "fish" for termites, may be developed by watching others use tools and may even be a true example of animal teaching.^[101] Tools may even be used in solving puzzles in which the animal appears to experience a "Eureka moment".^[102] Other mammals that do not use tools, such as dogs, can also experience a Eureka moment.^[103]

Brain size was previously considered a major indicator of the intelligence of an animal. Since most of the brain is used for maintaining bodily functions, greater ratios of brain to body mass may increase the amount of brain mass available for more complex cognitive tasks. Allometric analysis indicates that mammalian brain size scales at approximately the ⅔ or ¾ exponent of the body mass. Comparison of a particular animal's brain size with the expected brain size based on such allometric analysis provides an encephalisation quotient that can be used as another indication of animal intelligence.^[104] Sperm whales have the largest brain mass of any animal on earth, averaging 8,000 cubic centimetres (490 in³) and 7.8 kilograms (17 lb)^* in mature males.^[105]

Self-awareness appears to be a sign of abstract thinking. Self-awareness, although not well-defined, is believed to be a precursor to more advanced processes such as metacognitive reasoning. The traditional method for measuring this is the mirror test, which determines if an animals possesses the ability of self-recognition.^[106] Mammals that have 'passed' the mirror test are:

• Asian elephants, however not all subjects have passed. Three female elephants were tested, but only one passed, and two other elephants tested in another study also failed to pass.^[107]
• Chimpanzees, but mirror tests with a juvenile (11 months old) male chimpanzee failed to reveal self-recognition.^[108]
• Bonobos ^[109]
• Bornean orangutan ^[110]
• Sumatran orangutan ^[110]
• Humans, which show signs of self-recognition at 18 months (mirror stage)^[111]
• Bottlenose dolphins, since they don't have arms and can't touch the marked areas, decreased latency to approach the mirror, repetitious head circling and close viewing of the marked areas were considered signs of self-recognition ^[112]
• Killer whales ^[113]
• False killer whales ^[113]

Social structure

Eusociality is the highest level of social organization. These societies have an overlap of adult generations, the division of reproductive labor and cooperative caring of young. Usually insects, such as bees, ants and termites, have eusocial behavior, but it is demonstrated in two rodent species: the naked mole-rat^[114] and the Damaraland mole-rat.^[115]

Presociality is when animals exhibit more than just sexual interactions with members of the same species, but fall short of qualifying as eusocial. That is, presocial animals can display communal living, cooperative care of young, or primitive division of reproductive labor, but they do not display all of the three essential traits of eusocial animals. Humans and some species of Callitrichidae are unique among primates in their degree of cooperative care of young.^[116] Harry Harlow set up an experiment with rhesus monkeys, presocial primates, in 1958; the results from this study showed that social encounters are necessary in order for the young monkeys to develop both mentally and sexually.^[117]

A fission-fusion society are societies that change frequently in their size and composition, making up a permanent social group called the "parent group". Permanent social networks consist of all individual members of a community and often varies to track changes in their environment. In a fission–fusion society, the main parent group can fracture (fission) into smaller stable subgroups or individuals to adapt to environmental or social circumstances. For example, a number of males may break off from the main group in order to hunt or forage for food during the day, but at night they may return to join (fusion) the primary group to share food and partake in other activities. Many mammals exhibit this, such as primates (for example orangutans and spider monkeys),^[118] elephants,^[119] hyenas,^[120] lions,^[121] and dolphins.^[122]

Solitary animals defend a territory and avoid social interactions with the members of its species, except during breeding season. This is to avoid resource competition, as two individuals of the same species would occupy the same niche, and prevent depletion of food.^[123] A solitary animal, while foraging, can also be less conspicuous to predators or prey.^[124]

In a hierarchy, individuals are either dominant or submissive. A despotic hierarchy is where one individual is dominant while the others are submissive, as in wolves and lemurs,^[125] and a pecking order which is a linear ranking of individuals where there is a top individual and a bottom individual. Pecking orders may also be ranked by sex, where the lowest individual of a sex has a higher ranking than the top individual of the other sex, as in hyenas.^[126] Dominant individuals, or alphas, have a high chance of reproductive success, especially in harems where one or a few males (resident males) have exclusive breeding rights to females in a group.^[127] Non-resident males can also be accepted in harems, but some species may be more strict.^[128]

When two animals mate, they both share an interest in the success of the offspring, though often to different extremes. Unless the male and female are perfectly monogamous, meaning that they mate for life and take no other partners, even after the original mate’s death, as with wolves, Eurasian beavers, and otters. The amount of parental care will vary.^[129][130] There are three types of polygamy: either one or multiple dominant males have with breeding rights (polygyny), multiple males that females mate with (polyandry), or multiple males have exclusive relations with multiple females (polygynandry). It is much more common for polygynous mating to happen, which, excluding leks, are estimated to occur in up to 90% of mammals.^[131] Lek mating occurs in harems, wherein one or a few males protect their harem of females from other males who would otherwise mate with the females, as in elephant seals;^[132] or males congregate around females and try to attract them with various courtship displays and vocalizations, as in harbor seals.^[133]

Locomotion

Terrestrial

Most vertebrates—the amphibians, the reptiles and some mammals such as humans and bears—are plantigrade, walking on the whole of the underside of the foot. Many mammals, such as cats and dogs are digitigrade, walking on their toes, the greater stride length allowing more speed. Digitigrade mammals are also often adept at quiet movement.^[134] Some animals such as horses are unguligrade, walking on the tips of their toes. This even further increases their stride length and thus their speed.^[135] A few mammals, namely the great apes, are also known to walk on their knuckles, at least for their front legs. Giant anteaters^[136] and platypuses^[137] are also knuckle-walkers.

Animals will use different gaits for different speeds, terrain and situations. For example, horses show four natural gaits, the slowest horse gait is the walk, then there are three faster gaits which, from slowest to fastest, are the trot, the canter and the gallop. Animals may also have unusual gaits that are used occasionally, such as for moving sideways or backwards. For example, the main human gaits are bipedal walking and running, but they employ many other gaits occasionally, including a four-legged crawl in tight spaces.^[138] Mammals show a vast range of gaits, the order that they place and lift their appendages in locomotion. Gaits can be grouped into categories according to their patterns of support sequence. For quadrupeds, there are three main categories: walking gaits, running gaits and leaping gaits.^[139] Walking is the most common gait, where some feet are on the ground at any given time, and found in almost all legged animals. Running is considered to occur when at some points in the stride all feet are off the ground in a moment of suspension.^[138]

Arboreal

Arboreal animals frequently have elongated limbs that help them cross gaps, reach fruit or other resources, test the firmness of support ahead and, in some cases, to brachiate.^[140] Many arboreal species, such as tree porcupines, Silky Anteaters, spider monkeys and possums, use prehensile tails to grasp branches. In the spider monkey, the tip of the tail has either a bare patch or adhesive pad, which provides increased friction. Claws can be used to interact with rough substrates and re-orient the direction of forces the animal applies. This is what allows squirrels to climb tree trunks that are so large to be essentially flat from the perspective of such a small animal. However, claws can interfere with an animal's ability to grasp very small branches, as they may wrap too far around and prick the animal's own paw. Frictional gripping is used by primates, relying upon hairless fingertips. Squeezing the branch between the fingertips generates frictional force that holds the animal's hand to the branch. However, this type of grip depends upon the angle of the frictional force, thus upon the diameter of the branch, with larger branches resulting in reduced gripping ability. To control descent, especially down large diameter branches, some arboreal animals such as squirrels have evolved highly mobile ankle joints that permit rotating the foot into a 'reversed' posture. This allows the claws to hook into the rough surface of the bark, opposing the force of gravity. Small size provides many advantages to arboreal species: such as increasing the relative size of branches to the animal, lower center of mass, increased stability, lower mass (allowing movement on smaller branches) and the ability to move through more cluttered habitat.^[140] Size relating to weight affects gliding animals such as the sugar glider.^[141] Some species of primate, bat and all species of sloth achieve passive stability by hanging beneath the branch. Both pitching and tipping become irrelevant, as the only method of failure would be losing their grip.^[140]

Aerial

Bats are the only mammals that can truly fly. They fly through the air at a constant speed by moving their wings up and down (usually with some fore-aft movement as well). Because the animal is in motion, there is some airflow relative to its body which, combined with the velocity of the wings, generates a faster airflow moving over the wing. This will generate a lift force vector pointing forwards and upwards, and a drag force vector pointing rearwards and upwards. The upwards components of these counteract gravity, keeping the body in the air, while the forward component provides thrust to counteract both the drag from the wing and from the body as a whole.^[142]

The wings of bats are much thinner and consist of more bones than that of birds, allowing bats to maneuver more accurately and fly with more lift and less drag.^[143][144] By folding the wings inwards towards their body on the upstroke, they use 35% less energy during flight than birds.^[145] The membranes are delicate, ripping easily; however, the tissue of the bat's membrane is able to regrow, such that small tears can heal quickly.^[146] The surface of their wings is equipped with touch-sensitive receptors on small bumps called Merkel cells, also found on human fingertips. These sensitive areas are different in bats, as each bump has a tiny hair in the center, making it even more sensitive and allowing the bat to detect and collect information about the air flowing over its wings, and to fly more efficiently by changing the shape of its wings in response.^[147]

Fossorial

Fossorial creatures live in subterranean environments. Many fossorial mammals were classified under the, now obsolete, order Insectivora, such as shrews, hedgehogs and moles. Fossorial mammals have a fusiform body, thickest at the shoulders and tapering off at the tail and nose. Unable to see in the dark burrows, most have degenerated eyes, but degeneration varies between species; pocket gophers, for example, are only semi-fossorial and have very small yet functional eyes, in the fully fossorial marsupial mole the eyes are degenerated and useless, talpa moles have vestigial eyes and the cape golden mole has a layer of skin covering the eyes. External ears flaps are also very small or absent. Truly-fossorial mammals have short, stout legs as strength is more important than speed to a burrowing mammal, but semi-fossorial mammals have cursorial legs. The front paws are broad and have strong claws to help in loosening dirt while excavating burrows, and the back paws have webbing, as well as claws, which aids in throwing loosened dirt backwards. Most have large incisors to prevent dirt from flying into their mouth.^[148]

Aquatic

Fully aquatic mammals, the cetaceans and sirenians, have lost their legs and have a tail fin to propel themselves through the water. Flipper movement is continuous. Whales swim by moving their tail fin and lower body up and down, propelling themselves through vertical movement, while their flippers are mainly used for steering. Their skeletal anatomy allows them to be fast swimmers. Most species have a dorsal fin to prevent themselves from turning upside-down in the water.^[149][150] The flukes of sirenians are raised up and down in long strokes to move the animal forward, and can be twisted to turn. The forelimbs are paddle-like flippers which aid in turning and slowing.^[151]

Semi-aquatic mammals, like pinnipeds, have two pairs of flippers on the front and back, the fore-flippers and hind-flippers. The elbows and ankles are enclosed within the body.^[152][153] Pinnipeds have several adaptions for reducing drag. In addition to their streamlined bodies, they have smooth networks of muscle bundles in their skin that may increase laminar flow and make it easier for them to slip through water. They also lack arrector pili, so their fur can be streamlined as they swim.^[154] They rely on their fore-flippers for locomotion in a wing-like manner similar to penguins and sea turtles.^[155] Fore-flipper movement is not continuous, and the animal glides between each stroke.^[153] Compared to terrestrial carnivorans, the fore-limbs are reduced in length, which gives the locomotor muscles at the shoulder and elbow joints greater mechanical advantage;^[152] the hind-flippers serve as stabilizers.^[154] Other semi-aquatic mammals include beavers, hippopotamuses, otters and platypuses.^[156] Hippos are very large semi-aquatic mammals, and their barrel-shaped bodies have graviportal skeletal structures,^[157] adapted to carrying their enormous weight, and their specific gravity allows them to sink and move along the bottom of a river.^[158]

Hybrids

Hybrids are offspring resulting from the breeding of two genetically distinct individuals, which usually will result in a high degree of heterozygosity, though hybrid and heterozygous are not synonymous. The deliberate or accidental hybridizing of two or more species of closely related animals through captive breeding is a human activity which has been in existence for millennia and has grown for economic purposes (Domestication syndrome).^[159] Hybrids between different subspecies within a species (such as between the Bengal tiger and Siberian tiger) are known as intra-specific hybrids. Hybrids between different species within the same genus (such as between lions and tigers) are known as interspecific hybrids or crosses. Hybrids between different genera (such as between sheep and goats) are known as intergeneric hybrids.^[160] Natural hybrids will occur in hybrid zones, where two populations of species within the same genera or species living in the same or adjacent areas will interbreed with each other. Some hybrids have been recognized as species, such as the red wolf (though this is controversial).^[161]

Artificial selection, the deliberate selective breeding of domestic animals, is being used to breed back recently extinct animals in an attempt to achieve an animal breed with a phenotype that resembles that extinct wildtype ancestor. A breeding-back (intraspecific) hybrid may be very similar to the extinct wild type in phenotype, ecological niche and to some extent genetics, but the initial gene pool of that wild type is lost forever with its extinction. As a result, some breeds, like Heck cattle, are vague look-alikes of the extinct wildtype aurochs.^[162]

See also

References

Further reading

• Brown W.M. (2001). "Natural selection of mammalian brain components". Trends in Ecology and Evolution. 16 (9): 471–473. doi:10.1016/S0169-5347(01)02246-7.
• Jaffa, Khalaf-von; Taher, Norman Ali Bassam Ali (2006). "Mammalia Palaestina: The Mammals of Palestine". The Palestinian Biological Bulletin (55): 1–46.
• McKenna, Malcolm C.; Bell, Susan K. (1997). Classification of Mammals Above the Species Level. New York: Columbia University Press. ISBN 978-0-231-11013-6. OCLC 37345734.
• Nowak, Ronald M. (1999). Walker's mammals of the world (6 ed.). Baltimore: Johns Hopkins University Press. ISBN 978-0-8018-5789-8. OCLC 937619124.
• Simpson, George Gaylord (1945). "The principles of classification and a classification of mammals". Bulletin of the American Museum of Natural History. 85: 1–350.
• Murphy, William J.; Eizirik, Eduardo; O'Brien, Stephen J.; Madsen, Ole; Scally, Mark; Douady, Christophe J.; Teeling, Emma; Ryder, Oliver A.; Stanhope, Michael J.; de Jong, Wilfried W.; Springer, Mark S. (2001). "Resolution of the Early Placental Mammal Radiation Using Bayesian Phylogenetics". Science. 294 (5550): 2348–2351. doi:10.1126/science.1067179.
• Springer, Mark S.; Stanhope, Michael J.; Madsen, Ole; de Jong, Wilfried W. (2004). "Molecules consolidate the placental mammal tree" (PDF). Trends in Ecology and Evolution. 19 (8): 430–438. doi:10.1016/j.tree.2004.05.006. PMID 16701301.
• Vaughan, Terry A.; Ryan, James M.; Capzaplewski, Nicholas J. (2000). Mammalogy (4 ed.). Fort Worth, Texas: Saunders College Publishing. ISBN 978-0-03-025034-7. OCLC 42285340.
• Ole Kriegs, Jan; Churakov, Gennady; Kiefmann, Martin; Jordan, Ursula; Brosius, Juergen; Schmitz, Juergen (2006). "Retroposed Elements as Archives for the Evolutionary History of Placental Mammals". PLoS Biol. 4 (4): e91. doi:10.1371/journal.pbio.0040091. PMC 1395351. PMID 16515367.{{cite journal}}: CS1 maint: unflagged free DOI (link)
• MacDonald, David W.; Norris, Sasha (2006). The Encyclopedia of Mammals (3 ed.). London: Brown Reference Group. ISBN 978-0-681-45659-4. OCLC 74900519.

External links

• BBC Wildlife Finder – video clips from the BBC's natural history archive
• Biodiversitymapping.org – All mammal orders in the world with distribution maps
• Paleocene Mammals, a site covering the rise of the mammals, paleocene-mammals.de
• Evolution of Mammals, a brief introduction to early mammals, enchantedlearning.com
• Mammal Species, collection of information sheets about various mammal species, learnanimals.com
• European Mammal Atlas EMMA from Societas Europaea Mammalogica, European-mammals.org
• Marine Mammals of the World—An overview of all marine mammals, including descriptions, both fully aquatic and semi-aquatic, noaa.gov
• 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