Mammals (formally
Mammalia ) are members of a
class of air-breathing
vertebrate animals characterized by the possession of
hair, three
middle ear bones, and
mammary glands functional in mothers with young. Most mammals also possess
sweat glands and specialized
teeth, and 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, including a four-chambered
heart. Mammals range in size from the 30–40 millimetre (1- to 1.5-inch)
Bumblebee Bat to the 33-metre (108-foot)
Blue Whale.
The number of species accepted by the zoological community on any given calendar date depends on the classification scheme, and the figure changes continually as new species are discovered and the classification of others is revised. According to a major reference work, Mammal Species of the World, which is updated through periodic editions, 5676 species were known in 2005, distributed in 1,229 genera, 153 families and 29 orders. In 2008 the IUCN completed a 5-year, 17,000 scientist Global Mammal Assessment for its IUCN Red List, which counted 5488 accepted species at the end of that period. The class is divided into two subclasses (not counting fossils): the Prototheria (order of Monotremata) and the Theria, the latter containing the infraclasses Metatheria (including marsupials) and Eutheria (the placentals). The classification of mammals between the relatively stable class and family levels having changed often, different treatments of subclass, infraclass and order appear in contemporaneous literature, especially for Marsupialia.
Except for the five species of monotremes (which lay eggs), all living mammal species 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 include 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). This means that some extinct groups of "mammals" are not members of the crown group Mammalia, even though most of them have all the characteristics that traditionally would have classified them as mammals. These "mammals" are now usually placed in the unranked clade Mammaliaformes.
Distinguishing features
Living mammal species can be identified by the presence of sweat glands, including
those that are specialized to
produce milk. However, other features are required when classifying
fossils, since soft tissue glands and some other features are not visible in fossils.
Paleontologists use a distinguishing feature that is shared by all living mammals (including
monotremes), but is not present in any of the early
Triassic synapsids: mammals use two bones for hearing that were used for eating by their ancestors. The earliest synapsids had a jaw joint composed 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). Most reptiles including
lizards,
crocodilians,
dinosaurs (and their descendants the birds) use this system, as did non-mammalian synapsids such as
therapsids. Mammals have a different jaw joint, however, composed only of the
dentary (the lower jaw bone which carries the teeth) and the
squamosal (another small skull bone). In mammals the quadrate and articular bones have become the
incus and
malleus bones in the
middle ear.
Mammals also have a double occipital condyle: they have two knobs at the base of the skull which fit into the topmost neck vertebra, and other vertebrates have a single occipital condyle. Paleontologists use only the jaw joint and middle ear as criteria for identifying fossil mammals, since it would be confusing if they found a fossil that had one feature, but not the other.
Classification
ia (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.
Standardized textbook classification
A somewhat standardized classification system has been adopted by most current
mammalogy classroom textbooks. The following taxonomy of extant and recently extinct mammals is from
Vaughan et al. (2000).
Class Mammalia
Subclass Prototheria: monotremes: platypuses and echidnas
Subclass Theria: live-bearing mammals
*Infraclass Metatheria: marsupials
*Infraclass Eutheria: placentals
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, 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
lizards,
snakes,
crocodilians,
dinosaurs and
birds. Synapsids have a single hole (
temporal fenestra) low on each side of the skull.
One synapsid group, the pelycosaurs, were the most common land vertebrates of the early Permian and included the largest land animals of the time.
Therapsids descended from pelycosaurs in the middle Permian, about 260M years ago, and took over their position as the dominant land vertebrates. They differ from pelycosaurs in several features of the skull and jaws, including: larger temporal fenestrae and incisors which are equal in size. The therapsids went through a series of stages, beginning with animals which were very like their pelycosaur ancestors and ending with the Triassic cynodonts, some of which could easily be mistaken for mammals. Those stages were characterized by:
gradual development of a bony secondary palate.
progress 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 non-mammaliaform therapsids retained sprawling limbs (some late forms may have had semi-erect hind limbs); Permian carnivorous therapsids had sprawling forelimbs, and some late Permian ones also had semi-sprawling hindlimbs. In fact modern monotremes still have semi-sprawling limbs.
the dentary gradually becoming the main bone of the lower jaw; and in the Triassic, progress 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 possible evidence of hair in Triassic therapsids, but none for Permian therapsids.
some scientists have argued that some Triassic therapsids show signs of lactation.
True mammals evolve in the Triassic
The
Permian–Triassic extinction event, which was a prolonged event due to the accumulation of several extinction pulses ended the dominance of the therapsids. In the Early Triassic all the medium to large land animal niches were taken over by early
archosaurs, which over an extended period of time (30 million years) evolved into
crocodilians,
pterosaurs,
dinosaurs and
birds.
The first true mammals appeared in the Late Triassic (ca. 200 million years ago), over 70 million years after the first therapsids and approximately 30 million years after the first mammaliaformes. Hadrocodium appears to be in the middle of the transition to true mammal status — it had a mammalian jaw joint (formed by the dentary and squamosal bones), but there is some debate about whether its middle ear was fully mammalian. The majority of the mammal species that existed in the Mesozoic Era were characterized by Multituberculates.
The earliest known monotreme is Teinolophos, which lived about 123M 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 oldest known marsupial is Sinodelphys, found in 125M-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.
jeffersonii, Iowa Museum of Natural History, University of Iowa.]]
The living Eutheria ("true beasts") are all placentals. But the earliest known eutherian, Eomaia, found in China and dated to 125M years ago, obtained some features which are more like those of marsupials, which suggested it was perhaps a transitional fossil that eventually give rise to the placental lineage (the surviving metatherians):
Epipubic bones extending forwards from the pelvis, which are not found in any modern placental, but are found in marsupials, monotremes and mammaliformes such as multituberculates. In other words, they appear to be 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, which 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.
It is not certain when true placental mammals evolved – the earliest undisputed fossils of placentals come from the early
Paleocene, after the extinction of the dinosaurs.
Mammals and near-mammals expanded out of their nocturnal insectivore niche from the mid Jurassic onwards – for example Castorocauda had adaptations for swimming, digging and catching fish.
Rise to dominance in the Cenozoic
Mammals took over the medium- to large-sized ecological niches in the
Cenozoic, after the
Cretaceous–Tertiary extinction event emptied ecological space once filled by reptiles. Then mammals diversified very quickly, both birds and mammals show an exponential rise in diversity.
Recent molecular phylogenetic studies suggest that most placental orders diverged about 100M to 85M years ago, but that modern families first appeared in the late Eocene and early Miocene But paleontologists object that no placental fossils have been found from before the end of the Cretaceous.
The earliest clear evidence of hair or fur is in fossils of Castorocauda, from 164M years ago in the mid Jurassic. From 1955 onwards some scientists have 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. But foramina do not necessarily show that an animal had vibrissae – for example the modern lizard Tupinambis has foramina which are almost identical to those found in the non-mammalian cynodont Thrinaxodon.
was one of the abundant Pleistocene megafauna, a wide variety of very large mammals that lived during the Pleistocene and went extinct about 10,000 years ago.]]
The evolution of erect limbs in mammals is incomplete — living and fossil monotremes have sprawling limbs. In fact some scientists think that the parasagittal (non-sprawling) limb posture is a synapomorphy (distinguishing characteristic) of the Boreosphenida, a group which contains the Theria and therefore includes the last common ancestor of modern marsupial and placentals – and therefore that all earlier mammals had sprawling limbs. Sinodelphys (the earliest known marsupial) and Eomaia (the earliest known eutherian) lived about 125M years ago, so erect limbs must have evolved before then.
It is currently very difficult to be confident when endothermy first appeared in the evolution of mammals. Modern monotremes have a lower body temperature and more variable metabolic rate than marsupials and placentals. So the main question is when a monotreme-like metabolism evolved in mammals. The evidence found so far suggests Triassic cynodonts may have had fairly high metabolic rates, but is not conclusive. In particular it is difficult to see how small animals can maintain a high and stable body temperature without fur.
Anatomy and morphology
Skeletal system
The majority of mammals have seven
cervical vertebrae (bones in the
neck); this includes
bats,
giraffes,
whales, and humans. The few exceptions include the
manatee and the
two-toed sloth, which have only six cervical
vertebrae, and the
three-toed sloth with nine cervical vertebrae.
Respiratory system
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
All mammalian brains possess a
neocortex, a brain region that is 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
The
integumentary system is made up of three layers: the outermost
epidermis, the
dermis, and the
hypodermis.
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.
Although mammals and other animals have cilia that superficially may resemble it, no other animals except mammals have hair. It is a definitive characteristic of the class. Some mammals have very little, but nonetheless, careful examination reveals the characteristic, often in obscure parts of their bodies. None are known to have hair that naturally is blue or green in color although some cetaceans, along with the mandrills appear to have shades of blue skin. Many mammals are indicated as having blue hair or fur, but in all known cases, it has been found to be a shade of gray. The two-toed sloth and the polar bear may seem to have green fur, but this color is caused by algae growths.
Reproductive system
kids will stay with their mother until they are weaned]]
Most mammals give birth to live young (
vivipary), but a few, namely the
monotremes, lay
eggs. The
platypus and the
echidna present a
particular sex determination system that is different from other vertebrates.
Certain glands of mammals known as mammary glands 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.
Viviparous mammals are classified into the subclass Theria and are divided into two infraclasses: Metatheria (of which only the Marsupialia survive), and Eutheria. Marsupialia, or marsupials, have short gestation periods and give birth to undeveloped young which are contained within a pouch-like sac (marsupium) located in front of the mothers' abdomen. Eutherians, commonly known as placentals, are mammals that give birth to complete and fully developed young. This is usually characterized by long gestation periods. The majority of mammal species are classified as eutherians.
Physiology
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 cold weather and climates where non-avian reptiles and large insects cannot.
Endothermy requires plenty of food energy, so pound for pound mammals eat more food 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
and
gorilla in the MIAT museum - front view,
Gent,
Belgium]]
can reach 120 kmh (75 mph)]]
]]
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.
Social structure
Locomotion
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 such as whales and dolphins have flippers which evolved from legs.
Terrestrial
Arboreal
Sloths travel slowly along branches rather than swinging energetically like monkeys.
Aquatic
Buoyed by their aquatic environment,
whales have evolved into the largest mammals and indeed the largest animals ever.
Aerial
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. A herbivorous diet includes sub-types 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 multi-chambered
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).
Specializations in herbivory include:
Granivory "seed eating", folivory "leaf eating", fruivory "fruit eating", nectivory "nectar eating", gumivory "gum eating", and mycophagy "fungus eating".
See also
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
References
Bibliography
Bergsten, Johannes. February 2005. "A review of long-branch attraction". Cladistics 21:163–193. (pdf version)
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
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.
David MacDonald, Sasha Norris. 2006. The Encyclopedia of Mammals, 3rd edition. Printed in China, 930 pp. ISBN 0-681-45659-0.
External links
BBC Wildlife Finder – video clips from the BBC's natural history archive
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
Mammals
