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Rocky Mountain Goat (Oreamnos americanus)
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertabrata
Class: Mammalia
Linnaeus, 1758

Mammals are the class of vertebrate animals characterized by the presence of hair (or fur) and mammary glands, which in females produce milk for the nourishment of young. The other extant (living) classes of vertebrates (animals with backbones) include fish (with a few recognized classes), amphibians, reptiles, and birds.

Like birds, mammals are endothermic or "warm-blooded," and have four-chambered hearts. Mammals also have a diaphragm, a muscle below the rib cage that aids breathing. Some other vertebrates have a diaphragm, but mammals are the only vertebrates with a prehepatic diaphragm, that is, in front of the liver. Mammals are also the only vertebrates with a single bone in the lower jaw.

The choice of the word mammal to describe the class—rather than the presence of hair or a single bone in the lower jaw—is an interesting choice. In the eighteenth century, some scientists did refer to this group as hairy things, or "Pilosa" (now a designation for a group of placental mammals, including anteaters and sloths) (Marks 1997). But Carolus Linneaus provided the term Mammalia. Some authorities trace Linneaus choice to his advocacy of mothers' breastfeeding their own children, and indeed Linneaus authored a book on the issue (Marks 1997). Whatever the reason, this terminology ties mammals to a feature that is connected to a key mammalian characteristic: parental behavior. Although caring for young is evident in many animals, including crocodiles, it reaches a zenith in birds and mammals. Among the primates, human mothers spend more time with their infants than any other species.

There are three major subdivisions of mammals: monotremes, marsupials, and placentals. Monotremes are mammals that lay eggs, and include the platypuses and echidnas (spiny anteaters). Marsupials are generally characterized by the female having a pouch in which it rears its young through early infancy, as well as various reproductive traits that distinguish them from other mammals. Opossums, wombats, kangaroos, and wallabies are examples of marsupials. Placentals generally can be distinguished from other mammals in that the fetus is nourished during gestation via a placenta, although bandicoots (marsupial omnivores) are a conspicuous exception to this rule. About 5,500 living species of mammals have been identified.

Phylogenetically (classification based on evolutionary relatedness), Class Mammalia is defined as all descendants of the most recent common ancestor of monotremes and the therian mammals (Theria is a taxon that includes the closely affiliated marsupials and placentals).



Although all mammals are endothermic, this is not a defining feature of mammals, since this trait is shared with birds. However, the presence of hair is a unique mammalian characteristic. This filamentous outgrowth of the skin projects from the epidermis, though it grows from follicles deep in the dermis. In non-human species, hair is commonly referred to as fur. The presence of hair has helped mammals to maintain a stable core body temperature. Hair and endothermy has aided mammals in inhabiting a wide diversity of environments, from deserts to polar environments, and be active daytime and nighttime.

The amount of hair reflects the environment to which the animal is adapted. Polar bears have thick, water-repellent fur with hollow hairs that trap heat well. Whales have very limited hair in isolated areas, thus reducing drag in the water. Instead, they maintain internal temperatures with a thick layer of blubber (vascularized fat).

No mammals have hair that is naturally blue or green in color. Some cetaceans (whales, dolphins and porpoises), along with the mandrills, appear to have shades of blue skin. Many mammals are indicated as having blue hair or fur, but in all cases it will be found to be a shade of gray. The two-toed sloth can seem to have green fur, but this color is caused by algal growths.

Although monotremes are endothermic, their metabolic rates and body temperature are lower than that of most other mammals. Monotremes maintain an average temperature of around 32°C (90°F) compared to about 35°C (95°F) for marsupials and 38°C (100°F) for most placentals.

Mammals have integumentary systems made up of three layers: the outermost epidermis, the dermis, and the hypodermis. This characteristic is not unique to mammals, but is found in all vertebrates. The epidermis is typically ten to thirty cells thick, its main function being 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 fifteen to forty 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.

Along with hair, the presence of mammary glands, for feeding milk to their young, is another defining feature of mammals. The monotremes do not have nipples, but they do have mammary glands. The milk is secreted via the skin onto a surface, where it can be sucked or lapped up by the newborn.

Most mammals give birth to live young, but the monotremes lay eggs. Live birth also occurs in some non-mammalian species, such as guppies and hammerhead sharks; thus, it is not a distinguishing characteristic of mammals.

Mammals have three bones in each ear and one (the dentary) on each side of the lower jaw; all other vertebrates with ears have one bone (the stapes) in the ear and at least three on each side of the jaw. A group of therapsids called cynodonts had three bones in the jaw, but the main jaw joint was the dentary and the other bones conducted sound. The extra jawbones of other vertebrates are thought to be homologous with the malleus and incus of the mammal ear.

Northern right whale dolphin (L. borealis)

All mammalian brains possess a neocortex (or neopallium) that is involved in higher functions, such as sensory perception, generation of motor commands, spatial reasoning, and in humans, language and conscious thought. This brain region is unique to mammals (as is a single bone in the lower jaw, and the prehepatic diaphragm, mentioned above).

Most mammals are terrestrial, but some are aquatic, including sirenia (manatees and dugongs) and the cetaceans. Whales are the largest of all animals. There are semi-aquatic species, such as seals, which come to land to breed but spend most of the time in water. Most cetaceans live in salt water, but there are some dolphin species that live in fresh water, such as the Amazon River Dolphin (Inia geoffrensis) that lives in the Amazon and Orinoco River basins.

True flight has evolved only once in mammals, the bats; mammals such as flying squirrels and flying lemurs are actually gliding animals.


Mammals represent a "class" of vertebrates, Mammalia. There are about 5,500 known species of mammals, which are distributed into various taxa, such as orders, families, genera, and so forth (see taxonomy). However, the division of mammals into taxa below the level of class varies considerably according to the classification scheme used. McKenna and Bell (1997) recognize 46 orders, 425 families, and 5,000 genera. Other systems recognize considerably less orders, families, and genera of mammals.

In general, class Mammalia can be divided into two main branches, the monotremes (sometimes labeled as the subclass, "Prototheria") and the "live-bearing mammals" (sometimes placed in the subclass "Theria"). The live-bearing mammals can be divided into two further taxa, the marsupials (sometimes labeled as infraclass Metatheria) and the placentals (infraclass Eutheria).

George Gaylord Simpson's Principles of Classification and a Classification of Mammals (1945) was an original authoritative source for the taxonomy of mammals. 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 fieldwork gradually made Simpson's classification outdated, for some time it remained the closest thing to an official classification of mammals.

The basic orders listed in the box at the top of the article, and in the subsection below, are based on the work of Simpson, and serve as a traditional way of organizing the orders of mammals.

Traditional classification of living orders

  • Subclass Monotremata
  • Subclass Marsupialia
    • Dasyuromorphia:  quolls, dunnarts, Numbat, Tasmanian Devil
    • Didelphimorphia:  opossums
    • Diprotodontia:  kangaroos, wallabies, possums, Koala, wombats
    • Microbiotheria:  Monito del Monte or Colocolo
    • Notoryctemorphia:  marsupial moles
    • Paucituberculata:  shrew opossums
    • Peramelemorphia:  bandicoots and bilbies
  • Subclass Eutheria/Placentalia
    • Afrosoricida:  golden moles of southern Africa and the tenrecs of Madagascar
    • Artiodactyla:  even-toed ungulates: hippos, camels, llamas, deer, giraffes, cattle, sheep, goats
    • Carnivora:  cat-like mammals (cats, mongooses, hyenas); dog-like mammals (dogs, skunks, weasels, seals, raccoons, bears)
    • Cetacea:  whales, porpoises, dolphins
    • Chiroptera:  bats
    • Dermoptera:  colugos
    • Hyracoidea:  hyraxes
    • Insectivora:  hedgehogs, shrews, moles
    • Lagomorpha:  rabbits, hares, pika
    • Macroscelidea:  elephant shrews or jumping shrews
    • Perissodactyla:  odd-toed ungulates (tapirs, rhinoceroses, horses)
    • Pholidota:  pangolins
    • Primates:  monkeys, lemurs, gibbon, orangutan, gorilla, human
    • Proboscidea:  elephants
    • Rodentia:  squirrels, woodchucks, mice, rats, muskrats
    • Scandentia:  treeshrews
    • Sirenia:  manatees and dugongs
    • Tubulidentata:  Aardvark
    • Xenarthra:  anteaters, sloths, and armadillos (originally with aardvark and pangolins in Order Edentata)

Order Rodentia has the most number of species, with more than one-third of known species. The bats (order Chiroptera) are the next most populous in terms of species, followed by Carnivora, Insectivora, and Primates. New mammal species continue to be found, although this taxon is fairly well known, relative to many other taxa, such as insects or fish.

A highly successful order, rodents are distributed worldwide, including terrestrial, tree dwelling, semi-aquatic, and underground habitats. Squirrels and their relatives (beavers, chipmunks, gophers, etc.) are generally placed in the suborder Sciuromorpha, and mouse-like rodents (rats, lemmings, mice, hamsters, etc.) are placed in the suborder Myomorphia.

Hoofed mammals are placed into two orders. The Artiodactyla are even-toed mammals and include pigs, camels, cattle, elk, deer, and the American bison, among others. The Perissodactyla are odd-toed mammals, including rhinoceroses, horses, zebras, and tapirs. Horses and zebras are placed together in the Family Equidae.

The members of the Order Insectivora—shrews, moles, and hedgehogs—tend to be most active at night, and feed on insects, as well as worms and spiders.

A pacific walrus

The Order Carnivora includes such well-known families as Canidae (dogs, wolves, coyotes, jackels), Felidae (tigers, lions, cats, jaguars), and Ursidae (brown bears, black bears, giant panda, sloth bear). Carnivora also includes skunks, weasels, badgers, otters, walruses, sea lions, seals, and racoons.

Seals, sea lions, and walruses are called pinnipeds ("fin feet") and are sometimes placed in their own order, Pinnipedia, rather than in the Order Carnivora with Pinnipedia as a suborder. Yet other taxonomists place pinnipeds in the suborder Caniformia, made up of the families Odobenidae (walruses), Otariidae (sea lions, eared seals, and fur seals), and Phocidae (true seals). In this case, Pinnipedia is considered as a superfamily.

In modern classification, the primates are generally divided into two suborders: Strepsirrhini, the "wet-nosed" primates, and Haplorrhini, the "dry-nosed" primates. The Strepsirrhini include the prosimians (except for the tarsiers), consisting of the lemur families, as well as the lorises, Aye-ayes, galagos, and the pottos. The suborder Haplorrhini include the prosimian tarsiers, as well as the New World monkeys (marmosets, spider monkeys, howler monkeys, capuchins) and the Old World primates. The Old World primates include the Old World monkeys (baboons, colobus monkeys, macaques), as well as the apes, gibbons, and humans.

McKenna/Bell classification

Biological classification generally seeks to reflect evolutionary relationships between organisms. For such reasons, it tends to be a dynamic rather than a static science, as taxonomies are continually changing as more information is obtained.

In 1997, the mammals were comprehensively revised by Malcolm McKenna and Susan Bell, which has resulted in the "McKenna/Bell classification."

McKenna and Bell's Classification of Mammals: Above the Species Level (1997) is one of the more comprehensive works to date on the systematics, relationships, and occurrences of all mammal taxa, living and extinct, down through the rank of genus. Many paleontologists quickly accepted this new system. McKenna inherited the project from Simpson and, with Bell, constructed a completely updated hierarchical system, one that reflects the assumed historical genealogy of Mammalia.

Below is a synopsis of part of the McKenna/Bell classification. Extinct groups are represented by a dagger (†).

Class Mammalia

  • Subclass Prototheria: monotremes: platypuses and echidnas
  • 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

Molecular classification of placentals

Molecular studies by molecular systematists, based on DNA analysis, have suggested new relationships among mammal taxa. One classification based on molecular studies points to four groups or lineages of placental mammals that diverged from early common ancestors in the Cretaceous.

Lar gibbon

In this classification, the first divergence was that of the Afrotheria 110–100 million years ago. 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. The Boreoeutheria split into the Laurasiatheria and Euarchontoglires between 95 and 85 million years ago; 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, and the formation of the Isthmus of Panama linked South America and North America, facilitating the distribution of mammals seen today. With the exception of bats and murine rodents, no placental land mammals reached Australasia until the first human settlers arrived approximately 50,000 years ago.

However, it should be noted that these molecular results are still controversial, mainly because they are not reflected by morphological data and thus not accepted by many systematists. It is also important to note that fossil taxa are not, and in most cases cannot be, included. Although there are instances of DNA being recovered from prehistoric mammals, such as the ground sloth Mylodon and Neanderthal humans, Homo neanderthalensis, fossils can generally only be used in morphological analyses.

The following taxonomy only includes living placentals (infraclass Eutheria):

  • Group I: Afrotheria
    • Clade Afro-insectiphillia
      • 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 (cosmopolitcan tropical)
  • Group II: Xenarthra
    • Order Xenarthra: sloths and anteaters (Neotropical) and armadillos (Neotropical and Nearctic)
  • 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 (cosmopolitan).
      • Superorder Glires
        • Order Lagomorpha: pikas, rabbits, hares (Eurasia, Africa, Americas).
        • Order Rodentia: rodents (cosmopolitan)
    • Group IV: Laurasiatheria
      • Order Insectivora: moles, hedgehogs, shrews, solenodons
      • Order Chiroptera: bats (cosmopolitan)
      • Order Cetartiodactyla: cosmopolitan; includes former orders Cetacea (whales, dolphins and porpoises) and Artiodactyla (even-toed ungulates, including pigs, hippopotamus, camels, giraffe, deer, antelope, cattle, sheep, goats).
      • Order Perissodactyla: odd-toed ungulates
      • Clade Fera]
        • Order Pholidota: pangolins or scaly anteaters (Africa, South Asia).
        • Order Carnivora: carnivores (cosmopolitan)

The Cetartiodactyla is a hypothesized order to which whales (including dolphins) and even-toed ungulates are proposed to belong. The term was coined by merging the name for the two orders, Cetacea and Artiodactyla, into a single word. The term Cetartiodactyla is most commonly used to describe the hypothesis that whales evolved from within the artiodactyls. Under this definition, their closest living land relative is thought to be the hippopotamus. Alternatively, the term Cetartiodactlya is used to denote a clade where Cetacea and Artiodactyla are sister groups, but where Cetacea did not actually evolve from within the Artiodactyla. Under this definition, all artiodactyls, including hippos, are more closely related to one another than any are to the whales.


Evidence from fossils and comparative anatomy suggest that mammals evolved from therapsid reptiles during the Triassic period (approximately 200-250 million years ago). The therapsids, which first appeared in the fossil record near the end of the Paleozoic era, about 280 million years ago, had both reptilian and mammalian characteristics. The oldest mammalian fossil is from about 200 million years ago, in the Mesozoic era at the end of the Triassic period (see geologic time scale).

Mammals belong among the amniotes (vertebrates that have membranous sacs that surround and protect the embryo) and in particular to a sub-group called the synapsids. Synapsids are distinguished by the shape of their skulls, in particular the presence of a single hole on each side where jaw muscles attach, called temporal fenestra. In comparison, dinosaurs, birds, and most reptiles are diapsids, with two temporal fenestrae on each side of the skull. Turtles, with no temporal fenestra, are anapsids. Synapsid therapsids, the assumed ancestors of mammals, became common during the Permian period at the end of the Paleozoic era. (Sauropsids is the name given to the sub-group of amniotes that are not synapsids—dinosaurs, birds, modern reptiles, and most extinct reptiles, exluding those that are synapsids.)

Pre-mammalian ears began evolving in the late Permian to early Triassic to their current state, as three tiny bones (incus, malleus, and stapes) inside the skull; accompanied by the transformation of the lower jaw into a single bone. Other animals, including reptiles and pre-mammalian synapsids and therapsids, have several bones in the lower jaw. This transition is evidence of mammalian evolution from reptilian beginnings: from a single ear bone, and several lower jaw bones (for example the sailback pelycosaur, Dimetrodon) to progressively smaller "hearing jaw bones" (for example the cynodont, Probainognathus), and finally (possibly with Morganucodon, but definitely with Hadrocodium), true mammals with three ear bones in the skull and a single lower jaw bone. Hence pelycosaurs and cynodonts are sometimes called "mammal-like reptiles," but this is strictly incorrect as these two are not reptiles but synapsids.

During the Mesozoic Period, mammals appeared to diversify into four main groups: multituberculates (Allotherium), monotremes, marsupials, and placentals. Multituberculates went extinct during the Oligocene, about 30 million years ago, but the three other mammal groups are all represented today.

The designations Prototheria, Metatheria, and Eutheria reflect the theory that Placentalia were descendants of Marsupialia, which were in turn descendants of Monotremata. However, this theory is now is disfavor. Nonetheless, the terms Eutheria and Metatheria remain in common use in paleontology, especially with regards to mammals of the Mesozoic.

Repenomamus, the largest mammal known from the Cretaceous period of the Mesozoic, and the only one with evidence that it ate dinosaurs.

Based on the fossil record, most early mammals appear to have remained small and shrew-like throughout the Mesozoic, but rapidly developed into larger, more diverse forms following the Cretaceous-Tertiary extinction event 65 million years ago, at the time of the extinction of the dinosaurs.

Although early mammals are commonly portrayed as small animals that fed on insects and lived in the shadow of dinosaurs, Hu et. al. (2005) report the finding of a fossil of a Mesozoic mammal (Repenomanusu) that is more than one meter in length, with the remains of a baby dinosaur in its stomach. They suggest that larger mammals may have competed with the dinosaurs for food and territory.

During the next eight million years, in the Paleocene period (64–58 million years ago), the fossil record suggests that mammals exploded into the ecological niches left by the extinction of the dinosaurs. Small rodent-like mammals still dominated, but medium and larger-sized mammals also appeared.

A possible mammal evolutionary progression is:

  • Reptiliomorpha: Early Carboniferous period
  • Pelycosaurs (synapsids, or "mammal-like reptiles"): late Carboniferous period to very early Triassic period
  • Therapsids: Early Permian-Triassic includes dicynodonts, dinocephalia, etc.)
  • Cynodonts: Permian-Cretaceous Paleocene?
  • Mammaliformes: Mid-Triassic to Early Oligocene (includes non-therian mammals)
  • Mammals: mid-Jurassic period to today (Monotremes, Metatheria, and Eutheria)


  • Bergsten, J. 2005. “A review of long-branch attraction.” Cladistics 21: 163–193.
  • Hu, Y., J. Meng, Y. Wang, and C. Li. 2005. “Large Mesozoic mammals fed on young dinosaurs.” Nature 433 (7022): 149-152.
  • Kriegs, J. O., G. Churakov, M. Kiefmann, U. Jordan, J. Brosius, and J. Schmitz. 2006. “Retroposed elements as archives for the evolutionary history of placental mammals.” PLoS Biol 4 (4): 91.
  • Marks, J. 1997. Scientific and folk ideas about heredity. For: The Human Genome Project: Reaching Minority Communities in Maryland.
  • McKenna, M. C., and S. K. Bell. 1997. Classification of Mammals Above the Species Level. New York, NY: Columbia University Press.
  • Murphy, W. J., E. Eizirik, M. S. Springer, et al. 2001. “Resolution of the early placental mammal radiation using Bayesian phylogenetics.” Science 294 (5550): 2348-2351.
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  • Springer, M. S., M. J. Stanhope, O. Madsen, and W. W. de Jong. 2004. “Molecules consolidate the placental mammal tree.” Trends in Ecology and Evolution, 19: 430–438.
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  • Wilson, D. E., and D. M. Reeder (eds). 1993. Mammal Species of the World, Second Edition. Smithsonian Institution Press.


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