Temporal range: 225–0 Ma (Kemp) or 167–0 Ma (Rowe) See discussion of dates in text
|Examples of various mammalian orders. Click to see originals.
Row 1: common vampire bat, Virginia opossum, eastern grey kangaroo.
Mammals (class Mammalia //) are a clade of endothermic amniotes distinguished from reptiles and birds by the possession of hair, three middle ear bones, mammary glands, and a neocortex (a region of the brain). The mammalian brain regulates body temperature and the circulatory system, including the four-chambered heart. The mammals include the largest animals on the planet, the rorquals and some other whales, as well as some of the most intelligent, such as elephants, some primates and some cetaceans. The basic body type is a four-legged land-borne animal, but some mammals are adapted for life at sea, in the air, in the trees, or on two legs. The largest group of mammals, the placentals, have a placenta which feeds the offspring during pregnancy. Mammals range in size from the 30–40 mm (1.2–1.6 in) bumblebee bat to the 33-meter (108 ft) blue whale.
The word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma ("teat, pap"). All female mammals nurse their young with milk, which is secreted from special glands, the mammary glands. According to Mammal Species of the World, 5,416 species were known in 2006. These were grouped in 1,229 genera, 153 families and 29 orders. In 2008 the IUCN completed a five-year, 1,700-scientist Global Mammal Assessment for its IUCN Red List, which counted 5,488 accepted species at the end of that period. In some classifications, the mammals are divided into two subclasses (not counting fossils): the Prototheria (order of Monotremata) and the Theria, the latter composed of the infraclasses Metatheria and Eutheria. The marsupials constitute the crown group of the Metatheria and therefore include all living metatherians as well as many extinct ones; the placentals likewise constitute the crown group of the Eutheria.
Except for the five species of monotremes (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 descending order, are Rodentia (mice, rats, porcupines, beavers, capybaras, and other gnawing mammals), Chiroptera (bats), and Soricomorpha (shrews, moles and solenodons). The next three largest orders, depending on the classification scheme used, are the primates, to which the human species belongs, the Cetartiodactyla (including the even-toed hoofed mammals and the whales), and the Carnivora (cats, dogs, weasels, bears, seals, and their relatives). While the classification of mammals at the family level has been relatively stable, different treatments at higher levels—subclass, infraclass, and order—appear in contemporaneous literature, especially for the marsupials. Much recent change has reflected the results of cladistic analysis and molecular genetics. Results from molecular genetics, for example, have led to the adoption of new groups such as the Afrotheria and the abandonment of traditional groups such as the Insectivora.
The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that also included Dimetrodon. At the end of the Carboniferous period, this group diverged from the sauropsid line that led to today's reptiles and birds. Preceded by many diverse groups of non-mammalian synapsids (sometimes referred to as mammal-like reptiles), the first mammals appeared in the early Mesozoic era. The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of the dinosaurs 66 million years ago.
- 1 Varying definitions, varying dates
- 2 Distinguishing features
- 3 Classification
- 4 Evolutionary history
- 5 Anatomy and morphology
- 6 Physiology
- 7 Hybrid mammals
- 8 See also
- 9 References
- 10 Further reading
- 11 External links
Varying definitions, varying dates 
In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group mammals, the clade consisting of the most recent common ancestor of living monotremes (echidnas and platypuses) and therian mammals (marsupials and placentals) and all descendants of that ancestor. 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.
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.
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 in the Middle Jurassic, this is a reasonable estimate for the appearance of the crown group. The earliest known synapsid satisfying Kemp's definitions is Tikitherium, dated , so the appearance of mammals in this broader sense can be given this Late Triassic date. In any case, the temporal range of the group extends to the present day.
Living mammal species can be identified by the presence of sweat glands, including those that are specialized to produce milk to nourish their young. In classifying fossils, however, other features must be used, since soft tissue glands and many other features are not visible in fossils.
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 the quadrate (a small bone at the back of the upper jaw).
- 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.
- Tooth replacement - Teeth are replaced once or (as in toothed whales and murid rodents) not at all, rather than being replaced continually throughout life.
- Prismatic enamel - The enamel coating on the surface of a tooth consists of prisms, solid, rod-like structures extending from the dentin to the tooth's surface.
- Occipital condyles - Two knobs at the base of the skull fit into the topmost neck vertebra; most tetrapods, in contrast, have only one such knob.
For the most part, these characteristics were not present in the Triassic ancestors of the mammals.
For palaeontologists who define Mammalia phylogenetically, no limit can be set on the features used to distinguish the group. Any feature may be relevant to a fossil's phylogenetic position. Palaeontologists defining Mammalia in terms of traits, on the other hand, need only consider those features that appear in the definition. The dentary-squamosal jaw joint is generally included.
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.
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, is the most comprehensive work to date on the systematics, relationships, and occurrences of all mammal taxa, living and extinct, down through the rank of genus, though recent molecular genetic data challenge several of the higher level groupings. The authors worked together as paleontologists at the American Museum of Natural History, New York. McKenna inherited the project from Simpson and, with Bell, constructed a completely updated hierarchical system, covering living and extinct taxa that reflects the historical genealogy of Mammalia.
The McKenna/Bell hierarchical listing of many terms used for mammal groups above the species includes extinct mammals, as well as modern groups, and introduces some fine distinctions such as legions and sublegions (ranks which fall between classes and orders) that are likely to be glossed over by the nonprofessionals.
- Subclass Prototheria: monotremes: echidnas and the platypus
- Subclass Theriiformes: live-bearing mammals and their prehistoric relatives
- Infraclass †Allotheria: multituberculates
- Infraclass †Triconodonta: triconodonts
- Infraclass Holotheria: modern live-bearing mammals and their prehistoric relatives
- Superlegion †Kuehneotheria
- Supercohort Theria: live-bearing mammals
- Cohort Marsupialia: marsupials
- 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
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. Classification systems based on molecular studies reveal three major groups or lineages of placental mammals- Afrotheria, Xenarthra, and Boreoeutheria- which diverged from early common ancestors in the Cretaceous. The relationships between these three lineages is contentious, and three different hypotheses have been proposed with respect to which group is basal with respect to other placentals. These hypotheses are Atlantogenata (basal Boreoeutheria), Epitheria (basal Xenarthra), and Exafroplacentalia (basal Afrotheria). 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 (e.g. nuclear or mitochondrial) and varying interpretations of paleogeographic data.
Group I: Afrotheria
- Clade Afroinsectiphilia
- Clade Paenungulata
Group II: Xenarthra
- Order Pilosa: sloths and anteaters (neotropical)
- Order Cingulata: armadillos and extinct relatives (Americas)
Group III: Boreoeutheria
- Clade: Euarchontoglires (Supraprimates)
- Superorder Euarchonta
- Superorder Glires
- Clade Laurasiatheria
- Order Erinaceomorpha: hedgehogs
- Order Soricomorpha: moles, shrews, solenodons
- Clade Ferungulata
- Clade Cetartiodactyla
- Clade Pegasoferae
- Order Chiroptera: bats (cosmopolitan)
- Clade Zooamata
Synapsida, the group which contains mammals and their extinct relatives, originated during the Pennsylvanian subperiod, when they split from the lineage that led to reptiles and birds. Crown group mammals evolved from earlier mammaliaforms during the Early Jurassic.
Cladogram following, which takes Mammalia to be the crown group.
A cladogram compiled by Mikko Haaramo and 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.
Evolution from amniotes in the Paleozoic
The first fully terrestrial vertebrates were amniotes. Like their amphibian predecessors, they have lungs and limbs. Amniotes' eggs, however, have internal membranes which allow the developing embryo to breathe but keep water in. Hence, amniotes can lay eggs on dry land, while amphibians generally need to lay their eggs in water.
The first amniotes apparently arose in the Late Carboniferous. They descended from earlier reptiliomorph amphibians, which lived on land that was already inhabited by insects and other invertebrates as well as by ferns, mosses and other plants. Within a few million years, two important amniote lineages became distinct: the synapsids, which include mammals; and the sauropsids, which include turtles, lizards, snakes, crocodilians, dinosaurs and birds. Synapsids have a single hole (temporal fenestra) low on each side of the skull.
Therapsids descended from pelycosaurs in the Middle Permian, about 265 million 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 therapsid lineage leading to mammals went through a series of stages, beginning with animals that were very like their pelycosaur ancestors and ending with probainognathian cynodonts, some of which could easily be mistaken for mammals. Those stages were characterized by:
- gradual development of a bony secondary palate.
- Progress was made towards an erect limb posture, which would increase the animals' stamina by avoiding Carrier's constraint. But this process was slow and erratic: for example, all herbivorous nonmammaliaform therapsids retained sprawling limbs (some late forms may have had semierect hind limbs); Permian carnivorous therapsids had sprawling forelimbs, and some late Permian ones also had semisprawling hindlimbs. In fact, modern monotremes still have semisprawling limbs.
- The dentary gradually became the main bone of the lower jaw and, in the Triassic, progressed towards the fully mammalian jaw (the lower consisting only of the dentary) and middle ear (which is constructed by the bones that were previously used to construct the jaws of reptiles).
The mammals appear
The Permian–Triassic extinction event, which was a prolonged event due to the accumulation of several extinction pulses, ended the dominance of the carnivores among the therapsids. In the early Triassic, all the medium to large land carnivore niches were taken over by archosaurs which, over an extended period of time (35 million years), came to include the crocodylomorphs, the pterosaurs, and the dinosaurs. By the Jurassic, the dinosaurs had come to dominate the large terrestrial herbivore niches as well.
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; Castorocauda, for example, had adaptations for swimming, digging and catching fish.
The earliest known monotreme is Teinolophos, which lived about 123 million years ago in Australia. Monotremes have some features which may be inherited from the original amniotes:
- They use the same orifice to urinate, defecate and reproduce ("monotreme" means "one hole") – as lizards and birds also do.
- They lay eggs which are leathery and uncalcified, like those of lizards, turtles and crocodilians.
Unlike other mammals, female monotremes do not have nipples and feed their young by "sweating" milk from patches on their bellies.
The earliest known metatherian is Sinodelphys, found in 125 million-year-old Early Cretaceous shale in China's northeastern Liaoning Province. The fossil is nearly complete and includes tufts of fur and imprints of soft tissues.
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. 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. In particular:
- Epipubic bones extend forwards from the pelvis. These are not found in any modern placental, but they are found in marsupials, monotremes, and nontherian mammals like the multituberculates as well as in Ukhaatherium, an Early Cretaceous animal in the eutherian order Asioryctitheria. They are apparently an ancestral feature which subsequently disappeared in the placental lineage. These epipubic bones seem to function by stiffening the muscles of these animals during locomotion, reducing the amount of space being presented, which placentals require to contain their fetus during gestation periods.
- A narrow pelvic outlet indicates that the young were very small at birth and therefore pregnancy was short, as in modern marsupials. This suggests that the placenta was a later development.
Rise to dominance in the Cenozoic
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 groups of reptiles that were now absent. Then mammals diversified very quickly; both birds and mammals show an exponential rise in diversity. For example, the earliest known bat dates from about 50 million years ago, only 16 million years after the extinction of the dinosaurs.
Recent molecular phylogenetic studies suggest that most placental orders diverged about 100 to 85 million years ago and that modern families appeared in the period from the late Eocene through the Miocene. But paleontologists object that no placental fossils have been found from before the end of the Cretaceous. The earliest undisputed fossils of placentals come from the early Paleocene, after the extinction of the dinosaurs. In particular, scientists have recently identified an early Paleocene animal named Protungulatum donnae as one of the first placental mammals. The earliest known ancestor of primates is Archicebus achilles from around 55 million years ago. This tiny primate weighed 20–30 grams (0.7–1.1 ounce) and could fit within a human palm.
During the Cenozoic, several groups of mammals appeared which were much larger than their nearest modern equivalents, but none was even close to the size of the largest dinosaurs with similar feeding habits.
Earliest appearances of features
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.
The earliest clear evidence of hair or fur is in fossils of Castorocauda, from 164 million years ago in the Middle 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; it was soon pointed out, however, that foramina do not necessarily show that an animal had vibrissae, as the modern lizard Tupinambis has foramina which are almost identical to those found in the nonmammalian cynodont Thrinaxodon. Popular sources, nevertheless, continue to attribute whiskers to Thrinaxodon.
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 Early Cretaceous or latest Jurassic; it is found in the eutherian Eomaia and the metatherian Sinodelphys, both dated 125 million years ago.
When endothermy first appeared in the evolution of mammals is uncertain. Modern monotremes have lower body temperatures and more variable metabolic rates than marsupials and placentals, 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. Some of the evidence found so far suggests that Triassic cynodonts had fairly high metabolic rates, but it is not conclusive. For small animals, an insulative covering like fur is necessary for the maintenance of a high and stable body temperature.
Anatomy and morphology
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||This article may be expanded with text translated from the corresponding article in the Italian Wikipedia. (February 2013)|
The majority of mammals have seven cervical vertebrae (bones in the neck), including bats, giraffes, whales, and humans. The exceptions are the manatee and the two-toed sloth, which have only six cervical vertebrae, and the three-toed sloth with nine cervical vertebrae.
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.
All mammalian brains possess a neocortex, a brain region unique to mammals. Placental mammals have a corpus callosum, unlike monotremes and marsupials. The size and number of cortical areas (Brodmann's areas) is least in monotremes (about 8-10) and most in placentals (up to 50).
The epidermis is typically 10 to 30 cells thick; its main function is to provide a waterproof layer. Its outermost cells are constantly lost; its bottommost cells are constantly dividing and pushing upward. The middle layer, the dermis, is 15 to 40 times thicker than the epidermis. The dermis is made up of many components, such as bony structures and blood vessels. The hypodermis is made up of adipose tissue. Its job is to store lipids, and to provide cushioning and insulation. The thickness of this layer varies widely from species to species.
Although other animals have features such as 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.
Some primates and marsupials have shades of violet, green, or blue skin on parts of their bodies. The two-toed sloth and the polar bear sometimes appear to have green fur, but this color is caused by algal growths.
Most mammals are viviparous, giving birth to live young. However, the five species of monotreme, the platypuses and the echidnas, lay eggs. The monotremes have a sex determination system different from that of most other mammals. In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian mammal.
The mammary glands of mammals are specialized to produce milk, a liquid used by newborns as their primary source of nutrition. The monotremes branched early from other mammals and do not have the nipples seen in most mammals, but they do have mammary glands. The young lick the milk from a mammary patch on the mother's belly.
Viviparous mammals are in the subclass Theria; those living today are in the marsupial and placental infraclasses. A marsupial has a short gestation period, typically shorter than its estrous cycle, and gives birth to an undeveloped newborn that then undergoes further development; in many species, this takes place within a pouch-like sac, the marsupium, located in the front of the mother's abdomen. The placentals give birth to complete and fully developed young, usually after long gestation periods.
Nearly all mammals are endothermic ("warm-blooded"). Most mammals also have hair to help keep them warm. Like birds, mammals can forage or hunt in weather and climates too cold for nonavian reptiles and large insects.
Endothermy requires plenty of food energy, so mammals eat more food per unit of body weight than most reptiles. Small insectivorous mammals eat prodigious amounts for their size.
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.
Mammals evolved from four-legged ancestors. They use their limbs to walk, climb, swim, or fly. Some land mammals have toes that produce claws for climbing or hooves for running. Aquatic mammals like whales and dolphins have flippers which evolved from legs.
Whales and dolphins propel themselves through the water by moving their tail flukes up and down, adjusting the angle of the flukes as needed. The more massive front of the body contributes stability.
To maintain a high constant body temperature is energy expensive – mammals therefore need a nutritious and plentiful diet. While the earliest mammals were probably predators, different species have since adapted to meet their dietary requirements in a variety of ways. Some eat other animals – this is a carnivorous diet (and includes insectivorous diets). Other mammals, called herbivores, eat plants. A herbivorous diet includes subtypes such as fruit-eating and grass-eating. An omnivore eats both prey and plants. Carnivorous mammals have a simple digestive tract, because the proteins, lipids, and minerals found in meat require little in the way of specialized digestion. Plants, on the other hand, contain complex carbohydrates, such as cellulose. The digestive tract of an herbivore is therefore host to bacteria that ferment these substances, and make them available for digestion. The bacteria are either housed in the multichambered stomach or in a large cecum. The size of an animal is also a factor in determining diet type. Since small mammals have a high ratio of heat-losing surface area to heat-generating volume, they tend to have high energy requirements and a high metabolic rate. Mammals that weigh less than about 18 oz (500 g) are mostly insectivorous because they cannot tolerate the slow, complex digestive process of a herbivore. Larger animals, on the other hand, generate more heat and less of this heat is lost. They can therefore tolerate either a slower collection process (those that prey on larger vertebrates) or a slower digestive process (herbivores). Furthermore, mammals that weigh more than 18 oz (500 g) usually cannot collect enough insects during their waking hours to sustain themselves. The only large insectivorous mammals are those that feed on huge colonies of insects (ants or termites).
Specializations in herbivory include: Granivory "seed eating", folivory "leaf eating", frugivory "fruit eating", nectivory "nectar eating", gummivory "gum eating", and mycophagy "fungus eating".
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The deliberate or accidental hybridising 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 in recent times for economic purposes. The number of successful interspecific mammalian hybrids is relatively small, although it has come to be known that there is a significant number of naturally occurring hybrids between forms or regional varieties of a single species. These may form zones of gradation known as clines. Indeed the distinction between some hitherto distinct species can become clouded once it can be shown that they may not only breed but produce fertile offspring. Some hybrid animals exhibit greater strength and resilience than either parent. This is known as hybrid vigor. The existence of the mule (donkey sire; horse dam) being used widely as a hardy draught animal throughout ancient and modern history is testament to this. Other well known examples are the lion/tiger hybrid, the liger, which is by far the largest big cat and sometimes used in circuses; and cattle hybrids such as between European and Indian domestic cattle or between domestic cattle and American bison, which are used in the meat industry and marketed as Beefalo. There is some speculation that the donkey itself may be the result of an ancient hybridisation between two wild ass species or sub-species. Hybrid animals are normally infertile partly because their parents usually have slightly different numbers of chromosomes, resulting in unpaired chromosomes in their cells, which prevents division of sex cells and the gonads from operating correctly, particularly in males. There are exceptions to this rule, especially if the speciation process was relatively recent or incomplete as is the case with many cattle and dog species. Normally behavior traits, natural hostility, natural ranges and breeding cycle differences maintain the separateness of closely related species and prevent natural hybridisation. However the widespread disturbances to natural animal behaviours and range caused by human activity, cities, dumping grounds with food, agriculture, fencing, roads and so on do force animals together which would not normally breed. Clear examples exist between the various sub-species of grey wolf, coyote and domestic dog in North America. As many birds and mammals imprint on their mother and immediate family from infancy, a practice used by animal hybridizers is to foster a planned parent in a hybridization program with the same species as the one with which they are planned to mate.
- Mammal classification
- Evolution of mammals
- List of mammal genera- living mammals
- List of extinct mammals- extinctions during recorded human history
- Prehistoric mammals
- Lists of mammals by population size
- Lists of mammals by region
- Mammals discovered in the 2000s
- List of threatened mammals of the United States
- List of mammalogists
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- 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
- High-Resolution Images of various Mammalian Brains, brainmaps.org
- Mammal Species, collection of information sheets about various mammal species, learnanimals.com
- 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