mammal

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Mammals—warm-blooded, milk-producing animals—have provided meat protein, milk protein, collagen, hides for leather and shelter, and bones and sinew for various tools since humans began to hunt. Mammals also have provided the power for transportation (still called horsepower) and for heavy lifting or pulling. They have often been regarded as companion animals. Indeed, the existence and progress of humanity have depended heavily on mammals. As human societies became more complex and some took up the settled practices of farming and animal husbandry, certain mammalian species were selected to provide sustainable supplies of meat protein. Bovine (cattle), porcine (swine), ovine (sheep), and caprine (goat) species became valued livestock. Domestic animals, whether raised for food, work, or companionship, were selectively bred by controlling the animals' breeding and food supply to ensure desired traits in the next generation.

Types of Mammals

The three main classes of mammals, based on food preference, are herbivores, omnivores, and carnivores. Herbivores are strict plant eaters (sheep, goats); omnivores are opportunistic meat and plant eaters (humans, pigs); carnivores are almost exclusively meat eaters (wolves, cats). Mammals, thus, are both prey and predator in any food chain, depending on their size and aggressive behavior.

Herbivores. Plant-eating mammals provide most of the world's protein. Virtually every culture around the world tends one of the grazing (herbivorous) species of mammals as a protein source. Dairy cattle, water buffalo, sheep and goats, camels, yaks, reindeer, and llamas and alpacas all provide dairy products such as yogurt, cheese, butter, and milk in various societies.

Cattle originated in northern Europe and were domesticated by the northern Germanic and Celtic tribes in approximately 4000 B.C.E. Romans then brought them into southern Europe in the first century B.C.E. From the upper reaches of the Nile to the plains of southern and eastern Africa, cattle herding was common. Cattle became the basis of wealth for warrior-dominated societies in southern Africa. During the Middle Ages in Europe, cattle represented real wealth as milk providers and as work animals, not as meat animals. Cattle were slaughtered only when they could no longer work. Beef was not widely eaten, as cattle and oxen (castrated dairy bulls) had tough, dry flesh.

Water buffalo, valuable for hauling, transportation, and other work, were also used for milk, and buffalo milk mozzarella is still enjoyed as a table cheese in Italy.

Sheep and goats, small ruminants, are kept for their fleece, hides, meat, and milk. Both are docile and socially inclined mammals, and were herded beginning in 8000 B.C.E. in southwest Asia. Camel, yak, and reindeer are herding animals that provide meat, milk, and hides for the nomadic tribes of Asia and the Arctic Circle, respectively. Reindeer herding developed in the northern latitudes even before herds were kept on the Eurasian steppes. Camel herding became common in Arabia and the Sudan of Africa, and camels were critical to the maintenance of trade routes that crossed the great deserts of Africa and Asia. The yak, a large, long-haired ox with a bushy tail, is native to the Tibetan plateau. It provides dairy products and is used for transport. Llamas and alpacas have provided the peoples of Peru and Bolivia with hides, fleece, meat, and milk since at least 3500 B.C.E.

The American bison, the largest land mammal of North America, is believed to have migrated from the steppes of Central Asia into what is now Alaska by crossing the narrow strip of land (Beringia) that existed during the last Ice Age. Native Americans revered bison for the wealth it provided in clothing, food, and tools made from sinew and bone.

Deer, along with their cousins—elk, moose, and caribou—are antlered, hoofed ruminants. These grazing animals supplied food and clothing to both Native Americans and, later, the European invaders of the North American continent. Antelope are the surviving members of an ancient family of grazing animals native to North America. Lewis and Clark, on their long exploratory trip across the continental United States, found large herds of antelope on the Great Plains. Gazelles and other wild grazing animals of Central Africa and Central Asia are hunted by native peoples for their meat.

Many species of small game have provided meat and fur when large game was not available. Wild hares and some rabbits, both native to Europe and the Americas, are hunted, while other breeds of rabbit are reared specifically for consumption. Muskrats, sometimes called "marsh rabbits," and squirrels are rodents found throughout North America; both have supplemented the human diet. Squirrels are still hunted today in many parts of the United States and are usually served in a stew. Guinea pigs are popular in many Peruvian dishes, especially in the Andes, where these herbivorous rodents (much larger than the guinea pigs kept as pets or laboratory animals) are raised in many households, like rabbits elsewhere. Rats and mice are rarely eaten, though both have provided meat for people in times of famine.

The beauty of its fur led to the beaver's being overhunted by British, French, and Russian trappers in the northern territories of the North American continent in the sixteenth and seventeenth centuries. Beaver pelts were in great demand in Europe, especially for men's top hats. The fatty tail of the beaver was also prized for food. In the Middle Ages, the tail was declared "fish" by the Catholic Church, since the animal lived in water, making it acceptable as a meal on meatless days. Because its meat is very strong, only farm-raised beavers are recommended for cooking.

Kangaroos and opossums, both marsupials, are not consumed widely, though in Australia a cottage industry has developed around the production of kangaroo meat. Opossums, though not farm-raised, are hunted in the southern states of the United States for their meat.

Omnivores. Pigs are descended from a distant ancestor in southern Asia. Domesticated pigs brought to North America by the Spanish occasionally escaped captivity and multiplied, increasing the populations of wild pigs in the southeastern United States. Other breeds subsequently brought to the United States also occasionally escaped and bred with feral pigs, further mongrelizing the pig population.

Peccaries, known also as javelinas, North America's native wild pig, are not related to domesticated pigs and wild boars. Peccaries belong to a separate genus indigenous only to North America. They favor a warm climate and are hunted in New Mexico, Arizona, and Texas.

Raccoons range widely throughout the United States. Although valued primarily for their fur, their meat was commonly eaten during colonial times, and raccoons are still hunted for their fur and meat in the southern states of the United States.

Archeological evidence suggests that bear meat was consumed by native peoples in North America following ritual hunts. Bear meat was prized by European colonists and Native Americans, mainly for its fat for cooking. Though not a widely popular meat, bear are culled from game reserves and the meat is sometimes available frozen.

Carnivores. The small Asiatic wolf, a social animal and meat eater—the ancestor of our canine companions—was reportedly domesticated as early as 11,000 B.C.E., probably because it was more useful for herding and hunting than as a source of food. This is not to say that the dog was not a source of meat. Dog meat has been eaten and enjoyed in Asian cultures, and is still commonly consumed in both China and Korea.

Domestication

Although the history of domestication of mammals by humans is not recorded, archeological evidence suggests that it occurred on all continents between 7000 and 10,000 B.C.E. Each human group chose local migrating herbivores for domestication on the basis of their availability and docility. The first mammals to live with people were likely wolves and small ruminants such as sheep and goats. By the end of the second millennium B.C.E., civilizations based on livestock domestication and agriculture had emerged in Asia, Europe, and Africa. Small grazing animals like deer and sheep, which could provide meat, milk, and fiber, were probably herded by humans as they roamed the broad landscapes of western Asia. No evidence exists that early humans domesticated the numerous grazing animals of Africa.

Goats and sheep. Besides being docile and adaptable, goats and sheep breed successfully in the company of humans, and in time each generation gradually lost more of its feral nature. It is widely believed that the goat was the first herding animal to be domesticated, due to its gregarious nature. As the Romans moved north through Europe during the first century B.C.E., sheep and goats accompanied them, becoming sources for the wool industry, and mutton became a readily available meat. Sheep store fat well and so are efficient animals to maintain.

Goats are browsers, able to digest not only grasses but also woody shrubs and less desirable plants. Goats are even more adaptive and less choosy about their diet than sheep and can graze in arid climates. Goats continue to be prized for their milk and the resulting fermented dairy products. Goat meat, particularly the tender and milder flavor of kid, was enjoyed throughout the Mediterranean and the Asian continent and is also eaten in some regions of the Americas.

Cattle. The ancestor of today's domestic cattle, the aurochs (Bos primigenius), is extinct. Members of the bovine genus inhabited most of the world's continents and were introduced into the Western Hemisphere during the European conquests of the late fifteenth and early sixteenth centuries.

Asian cattle, also known as humped back cattle (Bos indicus), have provided meat and motive power on the Asian subcontinent. Religious and cultural beliefs in India prevent cattle from being consumed as food, although the milk can be used. In Africa, cattle are probably descended from European and Indian breeds introduced by traders, probably in the first millennium B.C.E.

Veal, meat from castrated young dairy bulls, was a choice dish even in ancient times. Just-weaned calves produce veal, which still brings a handsome price, more per pound than beef. Veal is a light-colored meat because the animals are fed milk or milk-replacer diets and are never permitted to graze.

The distinction between beef and dairy cattle breeds began in eighteenth-century Europe. Breeds that were best for beef and those best for milk production were identified and cultivated. Among the dairy-consuming peoples of northern Europe, the dairy breeds of cattle were selected for the high butterfat content of their milk.

It is generally believed that cattle first came to the North American continent with the Spanish. Columbus carried cattle to Santo Domingo in 1493, and in 1519 Cortés brought long-horned Andalusian cattle to Mexico. In the early seventeenth century, Spanish missionaries were raising cattle throughout the southwest United States.

Pigs. The ancestors of domestic swine were dispersed throughout Europe, Asia, and North Africa. The nomadic lifestyle of early peoples precluded their domestication. They were probably first encountered as pillagers of crops and therefore hunted, but young pigs might have been taken into early settlements and raised for meat. The omnivorous habits of the pig meant that it could thrive on the scraps from humans combined with its own rooting and foraging.

Pigs have evolved gradually over a period of ten million years with a few minor variations. Early pigs were taller than six feet, with an elongated wedge-shaped head, lacking a modern pig's snout, and a body shape similar to that of the European boar. This ancestor of the pig ranged from Europe to Asia and became the ancestor of the European wild boar.

Columbus is credited with bringing the pig to the Americas in 1493. These hogs ran wild throughout the Spanish West Indies, and were later joined by a load of pigs that arrived in Mexico with Cortés in 1521. On his trek west to the Mississippi Delta in 1539, Hernando de Soto brought pigs from the West Indies to Florida.

Dogs. Evidence suggests that early canine-human interactions may have occurred over the kills of larger wild herbivores, leading dogs and humans to be wary competitors at first but ultimately to become allies. Bones of dogs are common in campsites of the late Stone Age from around 7000 to 6000 B.C.E. The Asian wolf was probably the first wild animal domesticated by humans, and it is believed to be the ancestor of all domestic dogs. Until the eighteenth or nineteenth centuries, most of the breeds of dog were described by their purpose (wolf-hound, sheepdog), and it was not until the nineteenth century that many breeds were developed.

Horses. The earliest fossil examples, Eohippus, are found in northwestern North America. This wild ancestor of the horse was not much larger than a cat and had four toes on its forefeet and three on its hind feet. It was probably very widely distributed across the globe. Around 4000 B.C.E. the horse was domesticated in eastern Europe, and played a significant role in transportation, draft power, and warfare. Mounted soldiers were important military weapons until the twentieth century. Modern horses were reintroduced to the Americas by the Spanish conquistadors and were quickly adopted by native peoples for transport.

Game mammals and hunting. Those mammals not domesticated were hunted. Hunting animals for food or sport, or to rid a locale of animals that are seen as pests, is a human activity that spans the centuries and the globe. As early as the Late Paleolithic period, successful hunts required methods to preserve meat after slaughter. Meat was dried, smoked, or frozen in pits dug in the earth, or carcasses were weighted down with stones and sunk in cold lakes that froze during the winter. Meat stored was eaten dry, boiled, or grilled.

Hunting still provides some animal protein for the human diet; amounts vary depending upon the culture and region. In developed countries, hunting is largely a sport, while in less developed countries it remains, with fishing, an important source of dietary protein.

Nutrition

Meat. Meat is a popular high-quality protein food that satisfies the appetite and taste of people around the world. With the exception of organ meats, which tend to have concentrated nutrients, all of the cuts of meat from an animal are equally nutritious, providing roughly equivalent amounts of protein, minerals, and vitamins. Nutrition experts recognize meat as a food that also contributes varying amounts of fat to the diet. Meat supplies complete protein (all essential amino acids), essential minerals such as iron and phosphorus, significant B-complex vitamins (for example, thiamin), and trace minerals such as zinc. The protein of meat is comparable to that of fish, poultry, eggs, and milk.

The consumption of organ meats is sometimes encouraged because of the extremely rich vitamin and mineral content contained in edible glands and organs, including the liver, heart, kidneys, brain, sweetbread (thymus gland), tongue, tripe (stomach), and testicles, as well as the lungs and spleen in some cultures.

Dairy. Dishes prepared with milk or cheese are sometimes called "meat alternates" because of the similarity of the nutrient profiles, particularly when it comes to complete protein. The most significant milk products are:

• Yogurt: A fermented milk product made from whole, low-fat, or skim milk, providing all the food value of the milk from which it was made.
• Cultured cream: A product similar to yogurt but made with cream and so higher in butterfat. Sour cream is used widely in eastern European cooking; crème fraîche is more popular in France.
• Butter: A concentrated milk fat that provides fat in the diet and fat-soluble vitamin A.
• Cheese: A concentrated form of milk, fermented and often aged, that loses some of its protein in the cheese-making process but remains a high-protein food.

Mammals and Human Societies

Mammals have long played an important role in human mythology, religion, and social customs. As an act of reverence, humans have sacrificed animals, drunk their blood, and eaten their flesh. There are also taboos against certain relationships between humans and some animals, from the kosher prohibitions on eating pork and certain cuts of other animals to sexual taboos concerning congress between man and beast. Animals have been believed to be the habitat of both evil spirits and the souls of deceased human beings. Superstitions abound about animals, from bad luck brought by a black cat crossing one's path to good luck brought by carrying a rabbit's foot.

Culture, religion, symbolism, tradition, and taboos. Animal worship figures in many cultures and religions, including the cow among Hindus and the cat in ancient Egypt, and involves the role of reincarnation in some Asian religions. In many cultures, the spirits of important food animals were appeased to ensure their continued fertility, or ceremonies were performed to propitiate predators that threatened human survival. Stone Age art, cave drawings dating from 20,000 to 40,000 B.C.E., shows the animals and activities most important to the peoples of those cultures. The archeological evidence strongly suggests that these early people hunted and killed wild animals. Anthropologists believe the caves in which these drawings are found were not dwellings but served a religious or ritual function because food animals and hunting scenes predominate.

The earliest records of meat consumption indicate that animals were ritually slaughtered and the meat distributed to members of the community on the basis of an individual's place in the social hierarchy. Such practices required settled groups engaged in crop and pasture production. With farming and the formation of population clusters came the division of labor necessary to support specific food practices—grain milling, baking, meat processing, leather tanning, and so on. In some societies, meat processing emerged as part of sacrificial offerings to the deities for atonement, appeasement, supplication, or thanksgiving.

Meat eating and religious practices. In ancient times, sacrifices to the gods and goddesses often consisted of roasted sheep, goats, and lambs. Homer, Virgil, and the authors of the Old Testament all give accounts of roasted meat being offered to please the gods or the Lord. The biblical Book of Leviticus stipulates that the sacrificial animal be perfect, without any physical flaws; thus, a castrated animal was forbidden as a sacrifice.

The story of Adam and Eve in the Book of Genesis suggests that humans were created essentially vegetarian. Meat eating followed Eve's transgression. Under the laws of Kashrut, which govern kosher practices, Jews are forbidden to eat pork and shellfish ("tref"). In addition, certain parts of an animal, such as the hindquarters (unless butchered in a special fashion) as well as some organ meats, are forbidden. Another dietary restriction is that meat and milk may not be eaten together. These limits have resulted in fewer choices when it comes to meat for Jews than for others.

Muslims also do not eat pork, and, like Jews, they slaughter their meat according to religious guidelines. Such meat is called halal, or lawful. The month-long fast of Ramadan, while strict, is more of a joyful occasion than the Christian Lent, a forty-day period of abstinence and penitence.

The Roman Catholic Church established many restrictions on eating meat on certain days during the year, particularly during Lent and on specified fast days. Until the reforms of Vatican II (1962), meat eating was traditionally forbidden on Fridays. For generations, fish on Fridays was the rule in Roman Catholic communities. Meat, broth, and fat from warm-blooded animals were forbidden, while meat from waterfowl and from cold-water fish was considered acceptable.

Given the Church calendar—abstaining from meat on Fridays, on the eve of certain feast days, and on other days as well—meat eating was forbidden almost every other day: 180 days a year. The Orthodox Church was even stricter. This refusal to eat meat and fat (including butter in some times and places) had an ascetic aspect as well as a penitential one in its denial of human desire. In India cattle are not consumed because of the religious proscriptions of the Hindu faith. Since pigs, goats, and sheep are raised for meat and milk, however, India is not entirely vegetarian. Butter from the milk of sacred Indian cows was made for religious ceremonies, and ghee, a kind of clarified butter, is used for cooking.

Meat eating and vegetarianism. Meat, whether from mammals, poultry, or fish, provides a concentrated, easily digestible source of protein and fat. Ruminants in particular are able to convert herbaceous material into muscle more efficiently than monogastric animals, such as pigs or poultry, and are therefore better suited as sources of meat protein.

A vegetarian diet—eschewing meat or any animal food products—is undertaken by individuals for many reasons: health reasons and concern for the environment, ecology, and world hunger issues. Vegetarians often also cite economic reasons and ethical considerations as reasons. For some, religious beliefs dictate following a diet that avoids animal products. In India, for example, many are vegetarians because they find the taking of life abhorrent; in addition, many believe in reincarnation and fear that a living soul could be inhabiting a living creature.

Significant scientific data suggest links between a vegetarian diet and reduced risk of developing several chronic degenerative diseases and conditions, including heart disease, high blood pressure, diabetes, obesity, and some types of cancer.

The eating patterns of vegetarians vary considerably. The lacto-ovo-vegetarian diet is based on grains, vegetables, fruits, legumes, seeds, nuts, dairy products, and eggs, and excludes meat, fish, and fowl. The vegan, or total vegetarian, eating pattern is similar with the additional exclusion of eggs, dairy, and other animal products, even honey. Even within these patterns, considerable variation exists in the extent to which animal products are avoided.

Human beings, however, have been omnivorous since before recorded history. It seems unlikely that they will turn en masse to vegetarianism. In fact, arguments from the 1968 Rome conferences of the Food and Agriculture Organization of the United Nations suggest that humans could not abandon the consumption of meat in favor of a solely vegetarian diet. There was not, nor is there now, sufficient arable land to produce adequate protein or calories for the world's population.

Global Issues

While some of the problems discussed here primarily reflect events and situations in Europe and the United States, their repercussions will almost certainly have global consequences as impoverished regions of the world struggle to provide a nutritious diet for their increasing populations. What began as animal husbandry in prehistory threatens worldwide disaster. As the human population has increased beyond the capacity of the planet to feed its numbers, the practice of high-intensity animal production has caused numerous environmental problems that endanger humans as well as the animals bred for food.

The risks and costs of high-intensity animal production. Since World War II, agricultural production has striven to produce more from less without, some critics say, thought of the consequences. With high-intensity animal production, because animals are kept in close quarters they are more susceptible to the various diseases and parasites afflicting livestock. To counter disease and parasitism, scientists developed inexpensive pharmaceuticals to protect and treat animals. Surprisingly, many of these drugs actually improved livestock feed conversion performance faster than breeding and breed selection. As a consequence, livestock producers adopted these products widely, and meat production operations grew and consolidated in rural areas near feed grain sources.

Feedlots and large poultry operations, however, though extraordinarily efficient, are smelly and environmentally risky as well. Also, starting in the early 1970s, mounting public concern about the residues of pharmaceutical products in meat used for human consumption entered the debate about the wisdom of intensive livestock production. The food supply seemed to be contaminated with unnecessary, and perhaps toxic, chemical substances, and the methods of raising animals that required their use became targets of public protests. One result of these concerns has been the increase in sustainable livestock production, sometimes called "natural" or "organic" production. In natural production the animals are raised without performance-enhancing chemicals or feed additives. Livestock living in herds are as susceptible to disease as those raised in close quarters, and the effects of disease are devastating to herds. However, ranchers claim that it is more expensive to raise pigs or cattle without the aid of drugs or additives and so justify the higher prices charged for such meat.

Organic livestock production is stricter still, involving the feeding of grains and oil seeds produced under National Organic Standards. As adopted by the U.S. Department of Agriculture (USDA), the National Organic Standards specify that livestock and poultry may not be treated with antibiotics or any medicine and must be fed grains and rations that derive from organic crop production.

Intensive livestock production systems are based on concentrating large numbers of animals (housed or not) on small parcels of land and feeding them high-energy diets that guarantee the fastest weight gain in the least time. While feed efficiency (pounds of gain per pounds of feed) is important to the owners of such systems, intensified livestock production also results in large-scale animal waste. The concentration of live animals in a total confinement unit rivals a small city in terms of the annual waste output. Cities of such size are required by law to maintain tertiary water treatment facilities to handle their wastewater outfall. No such provision has yet forced pig or cattle feeders to treat their production wastes in a similar manner.

Among mammals, pigs represent the biggest waste threat to the environment because of the very large confinement units used to raise them. The most efficient pig will convert two pounds of feed into one pound of additional body mass, not all of which is edible protein. In order to acquire that pound, the animal produces one pound of feces and urine. Cattle are even less efficient, converting twelve to eighteen pounds of feed to one pound of body weight during the last weeks of feeding. This waste presents a considerable disposal problem.

With the animals living in such limited space, the waste must be stored for later treatment or use. In the past, this meant applying the manure as fertilizer to agricultural land, but this method of handling manure is no longer sound. Lagoons that hold animal waste often leak or break, with disastrous consequences for local streams and lakes. The open pools of raw waste also fill the surrounding countryside with a prevailing stench. The recent history of such environmental disasters and resulting legal battles is a complex story about shifting the costs of production to others, including future generations. Moreover, the available solutions cost money, so are unacceptable to those watching the bottom line. Steel holding tanks or glass-lined tanks, for example, clearly better containment choices, are prohibitively expensive, usually more than the average pork or beef production operation can, or is willing to, pay. With the infusion of new capital into pork production in the late 1980s, more attention was given to waste management, but the disposal problem has not yet been solved.

Intensive livestock production poses other risks to the environment and human health, for example, pollution of surface and ground water by animal waste. Such spills contaminate water, cause loss of property values for residential land, and harm recreational areas. The frequent and periodic contamination of ground and surface water from manure spills has become a familiar headline, reminding the public that profit-driven production methods endanger their health and the welfare of future generations.

With the appearance in the 1990s of bovine spongiform encephalopathy (BSE; more familiar to the public as "mad cow disease") in England and France, and the deaths caused by its spread to humans who ate meat from diseased cows, vigilance with respect to safe meat production became even more critical. In spite of research demonstrating that the disease had been spread in herds that had eaten feed that contained meat products, some feed suppliers in the United States were found continuing the practice in 2001, and, without enough USDA inspectors to monitor meat production from start to finish, the public cannot be sure that the meat they eat does not come from cows infected with BSE.

Facing continual pressure from environmentalists, real estate developers, and non-farm landowners, livestock producers struggle with presenting a responsible image. This reality applies both to producers managing large, intensified operations and to those who pasture their livestock. In terms of the stocking capacity of open land, whether for cattle, pigs, or small ruminants, it is now being argued that small ruminants (sheep and goats) can provide as much meat per acre as cattle or pigs without the subsequent environmental risks. Raising dual-purpose sheep or goats (those that provide both food and fiber) can be a more efficient use of limited land resources than the typical practices of cattle ranching.

This issue will become more pressing in the future as residential suburbs push into traditionally rural areas. The resolution will need to be political because of the constituencies involved. Technological advances have made the cost of farming too expensive for family farmers. As they are forced to sell their land to the giants of agribusiness or go into bankruptcy, farmers are becoming a smaller and smaller percentage of the population, and their real voice in legislatures will continue to diminish. City dwellers will demand that a fairer burden of the cost of farming be placed on those who profit from it than has been the practice since the New Deal under Franklin D. Roosevelt's administration in 1932–1940.

Another aspect of the urban-rural confrontation involves the cropping practices needed to support the intensified meat-production industry. Of the more than 70 million acres of corn grown annually in the United States, more than 65 percent is used for animal feed, and the price of corn drives all other commodity prices. Federal farm policies during the twentieth century resulted in overproduction of corn and soy relative to world market demands, depressed world prices, and significant loss of farm income. Add to this the loss of agricultural diversity and soil productivity caused by producing the same crop or the same rotation of crops on the same land year in and year out. Such farming practices had forced farmers to use more and more chemical pesticides and fertilizers in order to achieve uniform yields. Biotech crops may be a solution, because they permit more intensified cultivation and higher yields. However, controversy remains within the scientific community about the sustainability of high yields from biotech seed crops. This concern is added to the ongoing problems of groundwater contaminated with fertilizer runoff and pesticides.

Bioengineering. Unlike plant biotechnology, which has quickly introduced numerous varieties of common plants genetically reengineered to include certain traits, such as resistance to common pests for corn, animal biotechnology has had little success in changing the basic properties of livestock or poultry. A few applications of genetic manipulation may eventually prove useful in producing meat protein for human consumption. Of these, cloning is the most obvious and most likely to succeed, if public opposition fails to halt such research. Cloning livestock requires the nuclear transfer from an animal with the most desired traits (for example, efficient feed conversion, muscling, and tenderness) to eggs from the same species. One application would be the cloning of highly desirable boar and sow lines to be used in creating market pigs with specific, repeatable characteristics.

The technology for cloning livestock at this time is prohibitively expensive compared to conventional breeding or artificial insemination. For this reason, cloning is not expected to make a significant contribution to meat production for years. Such genetic manipulation also arouses considerable controversy in public and scientific discourse regarding the ultimate safety of food derived from such genetically modified organisms.

As populations continue to expand and the food crisis intensifies, the twenty-first century will witness societies worldwide struggling with the multitude of social, environmental, economic, and health issues that surround the production of livestock.

Animal Rights

The animal rights movement is a loose-knit coalition of groups who oppose abusing, mutilating, or killing animals to serve human purposes, including inhumane "farming" methods to raise animals for high-status luxury items like fur and leather. Most visible in North America and Europe, the movement includes benign meat eaters and farmers who want to ensure that livestock are treated humanely to vegetarians to activists who smear blood on fur coats and urge supermarkets to remove their lobster tanks. The politically and ideologically motivated efforts have had an impact on mainstream economics, although those with a financial interest dismiss their efforts as romantic or as malicious and dangerous, especially if they still believe that humans are superior to other animals and, therefore, that they have the "right" to do whatever they wish to them in the name of some "grander" (human) purpose. Research has demonstrated that the humane treatment of animals actually improves production and meat quality. Some of the results of that research have been incorporated into animal raising practices. In addition, some major food companies have adopted policies for their meat suppliers that stipulate humane handling practices, and some retail food packages—for example, chicken sausage—bear labels declaring such policies. As people grasp the "radical" idea that animals feel pain and, like humans, have the right not to suffer, whatever the rationale, the animal rights movement grows.

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Schlosser, Eric. Fast Food Nation: The Dark Side of the All-American Meal. New York: Houghton Mifflin, 2001.

Simoons, Frederick J. Eat Not This Flesh: Food Avoidances in the Old World. Madison, Wis.: University of Wisconsin Press, 1961.

Sokolov, Raymond. Fading Feast. New York: Dutton, 1979.

Stevens, Patricia Bunning. Rare Bits: Unusual Originals of Popular Recipes. Athens, Ohio: Ohio University Press, 1998.

Swatland, H. J. Structure and Development of Meat Animals. Englewood Cliffs, N.J.: Prentice Hall, 1984.

Toussaint-Samat, Maguelonne. History of Food. (English translation by Anthea Bell.) Cambridge, Mass.: Blackwell, 1993.

"Vegetarian Diets." Position paper of the American Dietetic Association. Journal of the American Dietetic Association 97 (1997): 1317–1321.

Visser, Margaret. Much Depends on Dinner. New York: Collier, 1986.

Visser, Margaret. The Rituals of Dinner. New York: Grove, 1991.

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—Robin Kline

LearnThatWord.com is a free vocabulary and spelling program where you only pay for results!

A class of vertebrates characterized by the production of milk by the females and in most cases, by a hairy body covering. Most mammals give live birth to their young. Human beings are mammals.

Emanating from or pertaining to mammals.

• Mammals - mammal: member of a class of warm-blooded, usu. hairy vertebrates that nourish their young with milk secreted by female mammary glands

For other uses, see Mammal (disambiguation).

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

Mammals are members of class Mammalia ( /məˈmeɪli.ə/), air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young. Most mammals also possess sweat glands and specialised teeth. The largest group of mammals, the placentals, have a placenta which feeds the offspring during gestation. The mammalian brain, with its characteristic neocortex, regulates endothermic and circulatory systems, the latter featuring red blood cells lacking nuclei, and a four-chambered heart. Mammals range in size from the 30–40 millimeter (1- to 1.5-inch) bumblebee bat to the 33-meter (108-foot) blue whale.

The word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma ("teat, pap"). All female mammals nurse their young with milk, which is secreted from special glands, the mammary glands. According to Mammal Species of the World, which is updated through periodic editions, 5,676 species were known in 2005. These were distributed in 1,229 genera, 153 families and 29 orders.^[1] In 2008 the IUCN completed a five-year, 17,000-scientist Global Mammal Assessment for its IUCN Red List, which counted 5488 accepted species at the end of that period.^[2] In some classifications, the class is divided into two subclasses (not counting fossils): the Prototheria (order of Monotremata) and the Theria, the latter composed of the infraclasses Metatheria and Eutheria. The marsupials are the crown group of the Metatheria and therefore include all living metatherians as well as many extinct ones; the placentals are likewise the crown group of the Eutheria.

The classification of mammals between the relatively stable class and family levels has changed often; different treatments of subclass, infraclass and order appear in contemporaneous literature, especially for Marsupialia. Much recent change has reflected the results of cladistic analysis and molecular genetics. Results from molecular genetics, for example, have led to the adoption of new groups such as the Afrotheria and the abandonment of traditional groups such as the Insectivora.

Except for the five species of monotremes (which lay eggs), all living mammals give birth to live young. Most mammals, including the six most species-rich orders, belong to the placental group. The three largest orders, in descending order, are Rodentia (mice, rats, porcupines, beavers, capybaras, and other gnawing mammals), Chiroptera (bats), and Soricomorpha (shrews, moles and solenodons). The next three largest orders, depending on the classification scheme used, are the primates, to which the human species belongs, the Cetartiodactyla (including the even-toed hoofed mammals and the whales), and the Carnivora (dogs, cats, weasels, bears, seals, and their relatives).^[1]

The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that also included Dimetrodon. At the end of the Carboniferous period, this group diverged from the sauropsid line that led to today's reptiles and birds. Preceded by many diverse groups of non-mammalian synapsids (sometimes referred to as mammal-like reptiles), the first mammals appeared in the early Mesozoic era. The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era.

Contents 1 Varying definitions, varying dates 2 Distinguishing features 3 Classification 3.1 McKenna/Bell classification 3.2 Molecular classification of placentals 4 Evolutionary history 4.1 Evolution from amniotes in the Paleozoic 4.2 The mammals appear 4.3 Rise to dominance in the Cenozoic 4.4 Earliest appearances of features 5 Anatomy and morphology 5.1 Skeletal system 5.2 Respiratory system 5.3 Nervous system 5.4 Integumentary system 5.5 Reproductive system 6 Physiology 6.1 Endothermy 6.2 Intelligence 6.3 Social structure 6.4 Locomotion 6.4.1 Terrestrial 6.4.2 Arboreal 6.4.3 Aquatic 6.4.4 Aerial 6.5 Feeding 7 See also 8 References 9 Further reading 10 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.^[3] A broader phylogenetic definition was provided in a 2004 book by Kielan-Jaworowska, Cifelli, and Luo, who defined Mammalia as the clade originating with the most recent common ancestor, not only of the monotremes and the therians, but also of Sinoconodon, the morganucodonts, and the docodonts.^[4] The morganucodonts and the docodonts, included by Rowe in the unranked clade Mammaliaformes, had a widespread distribution in the northern continents and had many of the characteristics that traditionally would have classified them as mammals.^[5] In particular, some docodonts were furry.

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

Distinguishing features

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

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

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

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

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

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

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

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

Classification

Main article: Mammal classification

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

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

McKenna/Bell classification

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

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

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

Extinct groups are represented by a dagger (†).

Class Mammalia

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

Molecular classification of placentals

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

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

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

Evolutionary history

For more details on this topic, see Evolution of mammals.

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

	Tritylodontidae Adelobasileus Sinoconodon Mammaliaformes Morganucodontidae   Morganucodon Docodonta   Haldanon Mammalia Monotremata Ornithorhychus (Platypus) Tachyglossidae (Echidna) Theriiformes Triconodonts, Multituberculates, Marsupials, and Placentals

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

Evolution from amniotes in the Paleozoic

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

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

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

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

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

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

The mammals appear

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

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

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

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

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

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

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

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

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

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

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

Rise to dominance in the Cenozoic

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

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

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

Earliest appearances of features

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

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

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

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

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

Anatomy and morphology

Skeletal system

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

Respiratory system

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

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

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

Reproductive system

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

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

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

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

Physiology

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. 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

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

Social structure

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Locomotion

See Animal 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 like whales and dolphins have flippers which evolved from legs.

Terrestrial

See Terrestrial locomotion

Arboreal

See Arboreal locomotion

Aquatic

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

Aerial

See Aerial locomotion

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

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

See also

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

Mammals portal

References

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22. ^ ^{a b} Wible, J. R.; Rogier, G. W.; Novacek, M. J.; Asher, R. J. (2007). "Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary". Nature 447: 1003–06. doi:10.1038/nature05854. http://www.carnegiemnh.org/assets/science/mammals/Maelestes.pdf. 
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27. ^ Oftedal, O.T. (2002). "The origin of lactation as a water source for parchment-shelled eggs". Journal of Mammary Gland Biology and Neoplasia 7 (3): 253–266. doi:10.1023/A:1022848632125. PMID 12751890. 
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Further reading

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

External links

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Wikispecies has information related to: Mammalia

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

External identifiers for Mammalia
Encyclopedia of Life	1642
ITIS	179913
NCBI	40674
Also found in: Wikispecies, Arctos

• BBC Wildlife Finder – video clips from the BBC's natural history archive
• Mammal Species Thousands of species, showing taxonomic classification, images, and other information, animalias.com
• GlobalTwitcher.com – All species in the world with distribution maps and images
• Paleocene Mammals, a site covering the rise of the mammals, paleocene-mammals.de
• Evolution of Mammals, a brief introduction to early mammals, enchantedlearning.com
• Tree of Life poster – Shows mammals' evolutionary relation to other organisms, tellapallet.com
• The Evolution of Mesozoic Mammals, a Rough Sketch, an informal introduction, home.arcor.de
• Carnegie Museum of Natural History, some discoveries of early mammal fossils, carnegiemnh.org
• High-Resolution Images of various Mammalian Brains, brainmaps.org
• Mammal Species, collection of information sheets about various mammal species, learnanimals.com
• Summary of molecular support for Epitheria, biology.plosjournals.org
• Mikko's Phylogeny Archive, fmnh.helsinki.fi
• European Mammal Atlas EMMA from Societas Europaea Mammalogica, European-mammals.org
• Marine Mammals of the World—An overview of all marine mammals, including descriptions, multimedia and a key, eti.uva.nl
• Mammalogy.org The American Society of Mammalogists was established in 1919 for the purpose of promoting the study of mammals, and this website includes a mammal image library

v t e Extant Chordata classes by subphylum Kingdom Animalia Subkingdom Eumetazoa (unranked) Bilateria Superphylum Deuterostomia Urochordata (Tunicates) Ascidiacea (Ascidians) Thaliacea Appendicularia (Larvaceans) Sorberacea Cephalochordata (Lancelets) Leptocardii Craniata Myxini (Hagfish) Hyperoartia (Lampreys) Chondrichthyes (Cartilaginous fish) Actinopterygii (Ray-finned fish) Sarcopterygii (Lobe-finned fish) Amphibia (Amphibians) Sauropsida (Reptiles and Birds) Mammalia (Mammals)

v t e Extant mammal orders by infraclass Kingdom Animalia Phylum Chordata Subphylum Vertebrata (unranked) Amniota Australosphenida Monotremata (Platypus and echidnas) Metatheria (Marsupial inclusive) Ameridelphia Paucituberculata (Shrew opossums) Didelphimorphia (Opossums) Australidelphia Microbiotheria (Monito del Monte) Notoryctemorphia (Marsupial moles) Dasyuromorphia (Quolls and dunnarts) Peramelemorphia (Bilbies and bandicoots) Diprotodontia (Kangaroos and relatives) Eutheria (Placental inclusive) Xenarthra Cingulata (Armadillos) Pilosa (Anteaters and sloths) Afrotheria Afrosoricida (Tenrecs and golden moles) Macroscelidea (Elephant shrews) Tubulidentata (Aardvark) Hyracoidea (Hyraxes) Proboscidea (Elephants) Sirenia (Dugongs and manatees) Laurasiatheria Soricomorpha (Shrews and moles) Erinaceomorpha (Hedgehogs and relatives) Chiroptera (Bats) Pholidota (Pangolins) Carnivora Perissodactyla (Odd-toed ungulates) Artiodactyla (Even-toed ungulates) Cetacea (Whales and dolphins) Euarchontoglires Rodentia (Rodents) Lagomorpha (Rabbits and relatives) Scandentia (Treeshrews) Dermoptera (Colugos) Primates

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Common misspelling(s) of mammal

• mamal

Dansk (Danish)
n. - pattedyr

Nederlands (Dutch)
zoogdier

Français (French)
n. - mammifère

Deutsch (German)
n. - Säugetier, Säuger

Ελληνική (Greek)
n. - (ζωολ.) θηλαστικό, μαστοφόρο

Italiano (Italian)
mammifero

Português (Portuguese)
n. - mamífero (m)

Русский (Russian)
млекопитающее

Español (Spanish)
n. - mamífero

Svenska (Swedish)
n. - däggdjur

中文（简体）(Chinese (Simplified))
哺乳动物

中文（繁體）(Chinese (Traditional))
n. - 哺乳動物

한국어 (Korean)
n. - 포유동물

日本語 (Japanese)
n. - 哺乳動物

العربيه (Arabic)
‏(الاسم) حيوان من الثدييات‏

עברית (Hebrew)
n. - ‮יונק‬

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