MAMMALS

MAMMALS. 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.

See also Aversion to Food; Cattle; Christianity; Dairy Products; Disgust; FAO (Food and Agriculture Organization); Food Safety; Goat; Government Agencies; High-Technology Farming; Horse; Hunting and Gathering; Inspection; Judaism; Mammals, Sea; Meat; Organic Agriculture; Pig; Prehistoric Societies; Sheep; Vegetarianism .

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

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

Wason, Betty. The Language of Cookery: An Informal Dictionary. New York: World, 1968.

Willett, Walter, with P. J. Skerrett, Edward L. Giovanucci, and Maureen Callahan. Eat Drink and Be Healthy: The Harvard Medical School Guide to Healthy Eating. New York: Simon and Schuster, 2001.

Robin Kline

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.

mammals Mammals are not a diverse group, and yet they have achieved a dominant position in many ecosystems. There are estimated to be 4000 species of mammals alive today, a low number compared to the insects or flowering plants, and indeed only two-thirds of the current diversity of reptiles. The success of the mammals is measured rather in their wide adaptability, and by the sheer abundance of some species, such as humans and rats.

Class Mammalia is divided into three unequal groups, the Subclasses Monotremata, Marsupialia (or Metatheria), and Placentalia (or Eutheria). The monotremes, the duck-billed platypus and the echidnas of Australasia, are characterized by the fact that they lay eggs, the retention of a primitive reptilian character. Marsupials, such as the opossums of the Americas and the kangaroos, koalas, and wombats of Australasia, generally give birth to tiny immature young which then complete their developments in a pouch. The placental mammals, by far the largest group, and consisting of all other mammals from mice to elephants, aardvarks to zebras, and bats to whales, give birth to well-developed young which have been nourished by means of a placenta in the maternal womb.

The success of the mammals: chewing and homeostasis

It is impossible to point to a single feature of mammals that can explain their great success. Mammals are distinguished from their reptilian forebears by a number of anatomical and physiological characters, and some of these appear to have led to the great adaptability of the group.

Perhaps mammals owe their success to their teeth. Unlike reptiles, amphibians, and fishes, which have rather uniform teeth, mammals have differentiated teeth, snipping incisors at the front, pointed piercing canines to the sides, and broad crushing and shearing cheek teeth (premolars and molars) behind (Fig. 1). Tooth differentiation allows mammals to process their food efficiently: less is wasted and it is cut up or ground into fragments before it is swallowed. The jaws and teeth are modified in other ways.

Unlike reptiles, mammals generally have only two sets of teeth during their lives, a milk set and an adult set. Reptiles retain the primitive style in which teeth are inserted continuously as old ones wear out. This might seem to be a better system than ours; indeed, continuous tooth replacement means no visits to the dentist. However, mammals are forced to minimize the amount of tooth replacement because of the precise occlusion (fitting) between the upper and lower cheek teeth. The benefits of close interlocking between the crests (cusps) and troughs of the premolars and molars, the fact that this arrangement allows mammals to chew and comminute their food, thereby aiding digestion, far outweigh the advantages of being able to shed worn-out teeth.

Reptiles cannot chew; their jaws generally merely open and shut in a single orientation, rather like a simple hinge. Mammals have complex rotatory and sliding jaw joints, and the mandibles (lower jaw) may move sideways and backwards and forwards over considerable distances. Parents and nannies used to urge children to chew their food a hundred times before swallowing; this injunction was intended to instil good manners and to prevent indigestion, but chewing means that the digestive processes begin even while the food is in a mammal's mouth. Reptiles and birds gulp their food down and hence lose much nutritive value.

The mobile jaw joint and differentiated teeth of mammals have been cited as a major reason for the success of the group as a whole, and as an explanation for their wide diversification. Mammals have a broad range of diets, including fish, shellfish, plankton, flying insects, ants, worms, snails, other land vertebrates, grass, leaves, roots, wood, fruit, seeds, and nectar. The teeth are highly variable and specifically adapted for the diet: needle-like piercers in insectivores, filtering curtains in plankton-eating whales, and broad ever-growing crushers in grass-eaters (Fig. 1). Some anteaters even have no teeth.

The second major adaptive complex of mammals concerns their control of body temperature. Mammals, independently of birds, evolved endothermy, commonly called warm-bloodedness. In other words, mammals produce a great deal of heat within their bodies by physiological means: indeed, it is estimated that fully nine-tenths of what we eat is devoted to temperature control (which is why a crocodile consumes one-tenth of what a human does). The heat production is closely monitored to maintain a precisely constant body temperature. In other words, mammals are endothermic (‘internal heat’) homeotherms (‘constant temperature’). Different species of mammals maintain different temperature set points, but these are generally in the range 35–40 °C, and small deviations may be fatal. The adaptive advantage of such constancy of body temperature is in terms of biochemical functioning; mammalian enzymes have evolved to act at peak efficiency at the normal set-point temperature, whereas reptilian enzymes must be able to cope with wide fluctuations in body temperature, and they can rarely function ideally. The maintenance of constant body temperatures involves a complex array of homeostatic (feedback) mechanisms: when you are too hot, you sweat, you turn red (blood flows in the skin layers), and you seek a cool place; when you are cold, you seek a warm place, you shiver, your peripheral blood vessels close off, and your body generates internal heat as fast as it can.

The advantages of endothermic homeothermy are seen in the wide range of habitats occupied by mammals, wider than for any other group of animals of similar diversity. Mammals uniquely live from the Equator to the poles, in the depths of the oceans, underground, and in the treetops and skies. Mammals operate at night and during the day.

The third major reason for the success of the mammals is clearly their relatively large brains. In proportion to body size, mammals have brains about ten times the size of reptilian brains. Large parts of the brain are, admittedly, devoted to the homeostatic control of body temperature, but the bulk of the new brain volume is concerned with intelligent functioning. Many mammals live complex lives, for example in trees, and they must learn how to move around and find a diverse array of food. Most mammals are more able to solve problems than reptiles.

The large brain and intelligence of mammals requires a learning period after birth. Reptiles generally abandon their nests after the eggs have been laid, but mammals devote long periods to feeding and caring for their young. Indeed, reptiles typically produce large numbers of eggs, perhaps 20–50, and their evolutionary strategy is to invest in quantity rather than quality (r-selection strategy). Mammals do the opposite, producing few young, typically 1–8, and devoting effort to protecting them while they grow to adulthood (K-selection strategy). The large brain and need to learn require parental care. Parental care includes feeding, and a characteristic feature of mammals is that the mothers feed their young on milk produced by mammary glands (mammae, hence, mammal, and indeed ‘mama’).

The origin of the mammals

Teeth, warm-bloodness, and brains: which came first? All three adaptive complexes seem to have evolved in parallel in the ancestors of the mammals, but warm-bloodness may have come last. The ancestry of mammals can be traced back to the mid-Carboniferous, when the great vertebrate group, the Amniota arose. Amniotes are the fully land-living vertebrates, including reptiles, birds, and mammals, that broke away from the dependence on water shown by the amphibians. The Amniota divided early on into three lineages, the Anapsida (leading to turtles), the Diapsida (leading to lizards, crocodiles, and birds), and the Synapsida (leading to mammals).

The pre-mammalian synapsids, often called mammal-like reptiles, showed hints of mammalian characters from the start. Tooth differentiation was well advanced by the Late Permian. In addition, brain expansion was also under way. Reptilian skulls are typically in two parts, a large outer portion that supports the jaws and sensory organs, and a small braincase fitted fairly loosely inside at the back. The best analogy is a shoebox with a cigarette packet placed inside at one end. Permian and Triassic mammal-like reptiles showed various stages in a process of brain enlargement. The bones surrounding the brain moved outwards, and fused with the outer skull bones. In the end, in mammals, the brain dominates the back half or more of the skull, and the sense organs are concentrated in front.

Mammals arose from within a major clade of synapsids termed the Cynodontia. Cynodonts included a number of lineages in the Triassic, many of which probably looked very mammal-like (Fig. 2). The teeth were fully differentiated, the braincase bulged at the back of the head, and many of them walked in an upright manner, instead of the primitive sprawling gait of other reptiles. Later cynodonts may have had hair, providing an insulating layer, which is a necessary part of warm-bloodedness. These animals still laid eggs, and indeed all Mesozoic mammals probably did. The oldest mammal is Adelobasileus from the Late Triassic of Texas, and the transition from mammal-like reptile to mammal may seem rather subtle.

The key feature that is traditionally taken to define the Class Mammalia is the full expression of the mammalian jaw joint (Fig. 2). In reptiles, the jaw hinges between the articular bone at the back of the lower jaw, and the quadrate. In mammals, the jaw joint has shifted bodily to the dentary in the lower jaw and the squamosal. This extraordinary shift did not occur in one dramatic step: some later cynodonts had both joints close together and in operation at the same time. The old articular–quadrate joint of the reptiles did not disappear, but it changed function and became part of the chain of three ear ossicles (‘little bones’) of mammals. Reptiles have a single ear ossicle, the stapes or stirrup bone. Mammals have three, the stapes, the malleus (hammer), and incus (anvil), the last two being the old reptilian jaw joint still conserved deep within our ear. Ears and jaw joints are still closely associated: you can ‘hear’ yourself chewing.

Mammalian evolution

The oldest mammals (Fig. 2) originated at the same time as the dinosaurs. However, the dinosaurs rose to dominate the Earth for the remainder of the Mesozoic, a span of some 155 million years. During this time, the mammals radiated and evolved, but they could not make the breakthrough to becoming large or to diversifying their modes of like. Most were rat- or mouse-sized, and they filled nocturnal niches, mainly as insectivores. One group, the multituberculates (‘many cusps’), were highly successful gnawing herbivores, and they outlasted the Mesozoic, surviving until the Oligocene. The multituberculates had gnawing incisors, a deep cutting premolar, and broad multi-cusped molars.

The monotremes, marsupials, and placentals arose in the Early Cretaceous. These modern groups were relatively rare during the Cretaceous; the early monotremes are known only by isolated specimens from Australia and South America, and the marsupials and placentals from North and South America, with some superbly preserved placentals from Central Asia. Even these mammals of modern aspect, although diversifying to some extent, did not exceed cat size, and they would have been unimpressive when seen beside the larger and more abundant dinosaurs with which they shared their habitats.

As is well known, the dinosaurs died out 65 million years ago, at a time of numerous other extinctions at the Cretaceous– Tertiary (K–T) boundary. Several groups of Mesozoic mammals also disappeared, and the marsupials nearly died out. Little is known of the evolution of monotremes, beyond the fact that they appear to have existed always at low diversity, and are essentially restricted to Australasia.

The marsupials that survived the K–T event radiated in North and South America. They diversified particularly successfully in South America, giving rise in time to a wide diversity of carnivores, from opossums to marsupial sabre-toothed cats and dogs. At some time in the Eocene marsupials traversed Antarctica to reach Australasia. As Gondwana broke up, the link was broken, and the Australasian marsupials evolved in isolation, giving rise to the modern groups of koalas, bandicoots, wombats, and kangaroos. From North America, marsupials migrated to Europe, and from there to Central Asia and Africa. Those radiations were short-lived, and marsupials died out all over the northern hemisphere by Mid-Tertiary times. The South American marsupials radiated back into Central and North America when the Panamanian land bridge was formed 3 million years ago, but many of the more dramatic South American forms eventually died out at the end of the Pleistocene.

The placentals seemingly diversified through the K–T mass extinction, and the modern orders virtually all originated within the first 10 million years of the Tertiary, essentially during the Palaeocene and Early Eocene. The oldest bats, whales, proboscideans (elephants), perissodactyls (horses, rhinos), artiodactyls (cattle, pigs, camels), primates (monkeys, humans), insectivores (shrews, hedgehogs), rodents (rats, squirrels), and carnivores (cats dogs) are known from this early phase of radiation. In addition, there were numerous other major groups of Palaeocene and Eocene mammals, some plant-eaters, others meat-eaters, that radiated, some successfully, but died out during mid-Tertiary times, as the modern orders became established.

The radiation of the placental mammals (Fig. 3) is often chosen as a classic example of an adaptive radiation, in other words, a rapid diversification of a clade that is apparently driven by, or facilitated by, some adaptation or adaptations. To a large extent, the radiation was opportunistic: the extinction of the dinosaurs vacated large volumes of ecospace on land, and at first it was not clear that the mammals would prevail. In certain parts of the world, huge hunting birds or fast-running crocodiles dominated the Palaeogene scene. The radiation can be dissected into several phases: (1) the opening of ecospace; (2) the initial diversification when new species seem to have arisen rapidly and with minimal competition from others; (3) a later phase when the ecospace was filling up, and more stable food chains and competitive interactions became established; and (4) a final phase when many groups died out and modern ecosystems became established.

Modern terrestrial mammalian ecosystems perhaps owe their shape to two further major environmental changes that occurred in the Mid- to Late Cenozoic. During the Oligocene and Miocene, climates worldwide became more arid, and areas that had formerly been covered by lush tropical and subtropical forests opened up and became grasslands. Great savannahs spread over much of the Americas, Europe, Asia and Africa. Mammals that had formerly lived secretive lives in the forests either died out or adapted. Horses adapted: they had previously been small, terrier-sized animals that fed on leaves, and they evolved into taller, long-legged animals that fed on grass.

The second major event was the Pleistocene ice ages. In the northern hemisphere, ice sheets several times advanced over much of North America, as well as northern Eurasia. Pluvial and arid climatic changes affected the whole world. Land mammals underwent numerous large-scale migrations as climates changed, and some groups died out. Other ice-adapted species evolved, such as mammoths and woolly rhinos, and perhaps Neanderthal humans. At the end of the Pleistocene, some 11 000 years ago, large mammals died out in many parts of the world, perhaps as a result of environmental changes, perhaps partly as a result of the rapid worldwide spread of human hunters.

Current human activities are threatening the futures of some endemic mammalian species, for example certain primates that occur in small populations in Madagascar, the African apes, pandas, elephants, and other specialists. The majority of mammalian species, however, seem to be highly adaptable and rather immune to such dramatic environmental modification.

Mammals can never rival insects or microbes in terms of abundance or diversity, but their large body size and adaptability have made them highly successful over the past 65 million years.

M. J. Benton

Bibliography

Benton, M. J. (1991) The rise of the mammals. Apple Press, London: Crescent Books, New York.
Benton, M. J. (1997) Vertebrate palaeontology (2nd edn). Blackwell Science, Oxford.
Carroll, R. L. (1987) Vertebrate palaeontology and evolution. W. H. Freeman, San Francisco.
Gingerich, P. D. (1984) Primate evolution. In Gingerich, P. D. and Badgley, C. E. (eds) Mammals. Notes for a short course, pp. 167–81. University of Tennessee, Knoxville.
Kemp, T. S. (1982) Mammal-like reptiles and the origin of mammals. Academic Press, London.
Pough, F. H.,, Janis, C. M.,, and and Heiser, J. B. (1989) Vertebrate life (5th edn). Prentice Hall, New Jersey.
Savage, R. J. G. and and Long, M. R. (1986) Mammal evolution. British Museum (Natural History), London.

MAMMALOGY

MAMMALOGY is a subdivision of vertebrate zoology, and its practitioners specialize in the scientific study of the biology of those species included in the class mammalia. Mammalogy as a discipline grew out of the study of the natural history of terrestrial vertebrates; a renewed interest in natural sciences began in Europe during the sixteenth century. In America the scientific study of living and fossil organisms was fostered by President Thomas Jefferson. His description of fossil mammal remains led him to hope that living representatives survived in the interior of North America. The expeditions of Lewis and Clark (1804–1806) and Zebulon Pike (1805–1807) were charged with recording data on the flora and fauna encountered during their travels. Subsequent expeditions sponsored by the U.S. government were staffed in part by scientists, who made important collections of birds and mammals. During the 1820s Thomas Say accompanied Major Stephan Long to the Rocky Mountains; William Keating was the naturalist on Long's Mississippi expeditions. The work of the U.S. Topological Survey led by John C. Fremont in the 1840s and 1850s resulted in many important discoveries of new vertebrate species.

The first major work on North American mammals, written by John James Audubon and J. Bachman, was published between 1846 and 1854. This work was followed by the scholarly work of Spencer Baird in 1859. The scientific study of mammals involved the description of new species and the preservation of reference specimens. The need for a repository for biological materials, such as a museum, became obvious; a reference library to house scientific literature published worldwide was also necessary. The first public museum in what was to become the United States was founded at Charleston, South Carolina, in 1773, and Charles Willson Peale founded his private museum in Philadelphia in 1785, but the national collection of artifacts and specimens began with the establishment of the Smithsonian Institution in 1846. Within the Smithsonian, Baird organized the U.S. National Museum in 1879. Under the guidance of C. Hart Merriman, the Bureau of Biological Survey was organized in 1886 within the U.S. Department of Agriculture. The collections resulting from the survey were housed at the Smithsonian. The efforts of Merriam and his coworkers greatly advanced knowledge of mammal species and their distributions.

To increase communication among North American mammalogists, the American Society of Mammalogists was founded in 1919 under the leadership of Hartley H. T. Jackson of the Biological Survey. The Society initiated The Journal of Mammalogy, which has published articles dealing in a broad manner with studies on the biology of mammals. The society has also fostered research and the publication of results of research continuously since its founding, and it has served, through its annual meetings, as a forum for reviewing research results and addressing national and international issues.

Taxonomic Advances

The description and naming of species and their classification is termed "taxonomy." The collections of the U.S. Biological Survey and other museums' collections have contributed greatly to this endeavor. In 1959 E. Raymond Hall and Keith Kelson published The Mammals of North America, a two-volume synthesis of taxonomic and biogeographical information. The volumes offered range maps of the species and subspecies, thus providing an empirical basis for advances in biogeographical theory. This work also laid the groundwork for the 1999 natural history synthesis of the mammalian fauna of North America, The Smithsonian Book of North American Mammals.

Studies of the physiological and anatomical structure of mammals have contributed much information to our understanding of how mammalian bodies function. The applications of such knowledge to human medicine is incalculable. The standard laboratory mammals, the mouse, rat, and guinea pig, have made possible many medical advances and breakthroughs. The contribution of mammalian studies to genetics and cytogenetics is also vast. Studies of mammal populations have led to a significant understanding of how ecological communities are organized and function. The study of life history and reproduction has been essential to the formulation of plans for the management and conservation of wildlife populations. The bacteria, viruses, and parasites of non-human mammals are often capable of infecting human hosts, thus causing serious outbreaks of disease. The study of mammalian populations and their diseases has been a focus of public health studies for decades. Rabies, tularemia, plague, Lyme disease, and hemorrhagic fevers are carried by a variety of mammalian hosts, and the study of mammalian populations is necessary to control disease outbreaks.

The fossil mammals of North America have long been studied. The richness of the fauna and the history of episodic extinctions have supplied data and fueled speculation about events leading to a faunal collapse. Extinctions in the last 12,000 years have been linked to human occupation of North America. The ancient mammal fauna of North America was summarized in Evolution of Tertiary Mammals (1998), edited by C. M. Janis, K. Scott, and L. L. Jacobs.

Conservation

Conservation concerns in the late nineteenth century focused on North American species: the American bison, pronghorned antelope, beaver, big horned sheep, and grizzly bear. The New York Zoological Society and the National Zoological Park were involved early on in the propagation of endangered species. World concern with vanishing wildlife led to the publication of two important volumes—Extinct and Vanishing Mammals of the Western Hemisphere (1942) by G. M. Allen and Extinct and Vanishing Mammals of the Old World (1945) by F. Harper—assessing the status of wild mammal populations: Work on endangered species continues to be a priority for conservation efforts. The scientific study of mammals touches on many related disciplines in biology. Mammalogy is taught as a college-level discipline at most major universities. Collections for study are supported at most major state and university museums.

BIBLIOGRAPHY

Allen, Glover Morrill. Extinct and Vanishing Mammals of the Western Hemisphere. New York: New York Zoological Park, 1942.

Audubon, John J., and John Bachman. The Viviparous Quadrupeds of North America. 3 vols. New York: J. J. Audubon, 1846– 1854.

Baird, Spencer Fullerton. Mammals of North America. Philadelphia: J. B. Lippincott, 1859.

Birney, Elmer C., and Jerry R. Choate, eds. Seventy-Five Years ofMammalogy. Provo, Utah: American Society of Mammalogists, 1994.

Hall, E. Raymond. The Mammals of North America. 2d ed. 2 vols. New York: Wiley, 1981.

Janis, Christine M., Kathleen Scott, and Louis L. Jacobs, eds. Evolution of Tertiary Mammals in North America. New York: Cambridge University Press, 1998.

Martin, Paul S., and Richard G. Klein, eds. Quaternary Extinctions. Tucson: University of Arizona Press, 1984.

Sterling, Keir B. Last of the Naturalists: The Career of C. HartMerriam. New York: Arno Press, 1977.

Wilson, D. E., and John F. Eisenberg. "Origin and Applications of Mammalogy in North America." In Current Mammalogy, edited by Hugh Genoways. New York: Plenum, 1990.

John F.Eisenberg

See alsoEndangered Species ; Zoological Parks .

Mammals

More than 4,000 species of living mammals belong to the vertebrate class Mammalia. This diverse group of animals has certain common features: all have four legs, bodies covered by hair, a high and constant body temperature, a muscular diaphragm used in breathing, a lower jaw consisting of a single bone, and three bones in the middle ear. In addition, all female mammals have milk-producing glands. There are three living subclasses of mammals: the Monotremata (egg-laying mammals), the Marsupialia (pouched mammals), and the Placentalia (placental mammals).

Mammals range in size from bats, some of which weigh less than an ounce, to the blue whale, which weighs more than 200,000 pounds. Mammals are found in arctic climates, in hot deserts, and in every terrain in between. Marine mammals, such as whales and seals, spend most of their time in the ocean. Mammals are not as numerous and diverse as other classes of animals, such as birds or insects. Nonetheless, mammals have a tremendous impact on the environment, particularly because of the activities of one species of mammal: humans.

Adaptations

Species of mammals have developed a variety of adaptations in response to the different environments in which they live. Mammals in cold climates have insulating layers consisting of a thick coat of fur or a thick layer of fat (blubber). This layer helps retain body heat and keeps the animal's body temperature constant. Some mammals that live in deserts survive by special adaptations in their kidneys and sweat glands that allow them to survive when only very small amounts of water are available to them. Other adaptations for survival in extreme climates include hibernation (a state of winter dormancy) or estivation (summer dormancy). These responses make it possible for the animal to conserve energy when food supplies become scarce.

Care and development of the young

The care of the young animals is notable among mammals. Born at an average of 10 percent of its mother's weight, mammalian young grow rapidly. The protection the young receive from one or both parents during the early stages of their lives enables mammals to maintain a strong survival rate in the animal kingdom.

The subclass Placentalia contains the majority of living mammals. The embryo of placentals develops in the mother's uterus (womb), is nourished by blood from the placenta (an organ in the uterus), and is retained until it reaches an advanced state of development. The young of the Marsupialia, by contrast, develop inside the uterus of the mother, usually with a placenta connected to a yolk sac. Young marsupials are born in a very undeveloped state and are sheltered in a pouch that contains the nipples of the milk glands. Kangaroos, wallabies, and most Australian mammals are marsupials, as is the opossum of the Americas.

The Monotremata have hair and secrete milk like other mammals, but they lay eggs. Monotremes in Australia include the duck-billed platypus and two species of spiny anteaters.

Mammal

Mammals are taxonomically separated from other animals at the class level (kingdom Animalia, phylum Chordata, subphylum Vertebrata, class Mammalia). Modern mammals are readily differentiated from other animals by the following characteristics: hair; a four-chambered heart with the aorta descending on the left; red blood cells that lack a nucleus (allowing for increased surface area for oxygen transport); a muscular diaphragm separating the abdominal and thoracic cavities that aids in breathing; descent of the testes into a scrotum to achieve a temperature environment amenable to sperm development; a variety of skin glands including sebaceous, sweat, and milk or mammary (the characteristic giving mammals their name); and elaborate dermal musculature (controlling the skin), particularly in the face (associated with suckling in young). All of these characters relate to the high metabolic rate of mammals. Mammals and birds are the only vertebrates that maintain a consistent body temperature through physiological (endothermy) rather than behavioral means (ectothermy).

None of these characters are readily apparent in fossils. There are, however, a number of skeletal and dental traits that are unique to mammals. The characters most useful in tracing the origin of mammals are: a bony secondary palate in the skull; a jaw joint between the dentary (jaw bone) and the squamosal bone of the skull (other terrestrial vertebrates have a quadrate-articular jaw joint); three bony ossicles (malleus, incus, and stapes) in the middle ear for sound transport rather than just one (stapes); teeth that are specialized for a variety of functions, including stabbing, nipping, shearing, and grinding; and a limb skeleton that can passively support the body off the ground (versus the reptilian posture of legs to the side). Most of these traits also relate to the high metabolic demands of endotherms. The reptile-to-mammal transition is one of the best documented in the fossil record. The first mammals appeared over 200 million years ago, about the same time as the first dinosaurs.

see also Body Cavities; Evolution, Evidence for; Reptile; Skin

William P. Wall

Bibliography

Martin, R. E., R. H. Pine, and A. F. DeBlase. A Manual of Mammalogy with Keys to Families of the World. New York: McGraw-Hill, 2001.

Nowak, R. M. Walker's Mammals of the World. Baltimore, MD: Johns Hopkins University Press, 1999.

Vaughan, T. A., J. M. Ryan, and N. J. Czaplewski. Mammalogy. Fort Worth, TX: Saunders College Publishing, 2001.

Wilson, D. E., and F. R. Cole. Common Names of Mammals of the World. Washington, DC: Smithsonian Institution Press, 2000.

Wilson, D. E., and D. Reeder. Mammal Species of the World: A Taxonomic and Geographic Reference. Washington, DC: Smithsonian Institution Press, 1993.

mammal an animal of the highest class of vertebrates, the Mammalia. The female has mammary glands, which secrete milk for the nourishment of the young after birth. In the majority of mammals the body is partially or wholly covered with hair; the heart has four chambers, and only the left aortic arch is present; and a muscular diaphragm separates the chest from the abdominal cavity. Mammals are warm-blooded; that is, they have a relatively constant body temperature independent of the temperature of their surroundings. The mature red blood cells (erythrocytes) usually lack a nucleus. Except for the egg-laying monotremes (the platypus and the echidna, or spiny anteater), mammals give birth to live young. A marsupial is born in a more undeveloped state than the young of other mammals, although all are relatively helpless at birth. In some marsupials and in higher mammals the young receive prenatal nourishment through a placenta . The order Carnivora, or flesh-eating animals, includes terrestrial families such as the cat, dog, and bear as well as the aquatic seal, sea lion, and walrus. Other aquatic mammals are the whale, porpoise, and dolphin of the order Cetacea and the manatee and dugong of the order Sirenia. Unusual adaptations are also found in the bat (order Chiroptera); in the elephant (order Proboscidea); in the sloth, armadillo, and anteater (order Edentata); and in the beaver, woodchuck, porcupine, and squirrel (order Rodentia). The order Soricomorpha includes the shrew and the mole, and the spiny and hairy hedgehogs form Erinaceomorpha; both orders were formerly classed as Insectivora. There are two groups of ungulates, or hoofed mammals: most members of the order Perissodactyla, including the horse and the rhinoceros, are odd-toed, with the third digit the largest; those of the order Artiodactyla, including the deer, antelope, camel, pig, and cow, are even-toed, with the third and fourth digits symmetrical and functional. Humans, monkeys, apes, and lemurs belong to the order Primates. Some remains of mammals are identified as from the Jurassic period of the Mesozoic era, but mammals remained small creatures during the Mesozoic. The group became diversified relatively rapidly in geological terms in the Tertiary period of the Cenozoic era after the dinosaurs had become extinct.

Bibliography: See E. P. Walker et al., Mammals of the World (2 vol., rev. ed. 1968); S. Anderson, ed., Simon & Schuster's Guide to Mammals (1984); G. B. Corbett and J. E. Hill, World List of Mammalian Species (1986); H. H. Genoways, ed., Current Mammalogy (2 vol., 1987–89).

Mammalia (phylum Chordata, superclass Gnathostomata) A class of homoiothermic animals in which the head is supported by a flexible neck, typically with seven vertebrae, articulating through two occipital condyles, the side wall of the skull is formed by the alisphenoid bone, the lower jaw is formed from the dentary bone and articulates with the squamosal, the quadrate and articular bones form auditory ossicles, and the angular bone forms the tympanic bone. Typically teeth are present and are thecodont, heterodont, and diphyodont; the mouth cavity is separated by a hard palate from the nasal cavity. Epiphyses are present on many of the bones. The thorax and abdomen are separated by a diaphragm, the heart is four-chambered, the right aortic arch is absent, except in Monotremata the egg is small and develops in the uterus, and the young are fed milk secreted by mammae (which give the class its name). The skin has at least a few hairs. Many mammalian features were present in therapsids (mammal-like reptiles) during the Triassic. Mammals are believed to have appeared first toward the end of the Triassic and to have diversified rapidly from the end of the Mesozoic, 100 Ma later, following the mass extinction which marks the Mesozoic-Tertiary boundary 65 Ma ago.

mammal A member of a class (Mammalia) of homoiothermic, chordate animals in which the young are fed milk secreted by mammae (which give the class its name). The head is supported by a flexible neck typically with seven vertebrae, articulating through two occipital condyles, the side wall of the skull is formed by the alisphenoid bone, the lower jaw is formed from the dentary bone and articulates with the squamosal, the quadrate and articular bones form auditory ossicles, and the angular bone forms the tympanic bone. Typically teeth are present and the mouth cavity is separated by a hard palate from the nasal cavity. The thorax and abdomen are separated by a diaphragm, the heart is four-chambered, the right aortic arch is absent, except in Monotremata (platypus and echidnas), the egg is small and develops in the uterus, and the skin has at least a few hairs. Many mammalian features were present in therapsids (mammal-like reptiles) during the Triassic. Mammals are believed to have appeared first towards the end of the Triassic and to have diversified rapidly from the end of the Mesozoic, 100 Ma later, following the mass extinction which marks the Mesozoic-Tertiary boundary 65 Ma ago.

Mammalia (mammals; phylum Chordata) Class of homoiothermic animals in which the head is supported by a flexible neck, typically with seven vertebrae, articulating through two occipital condyles. The lower jaw is composed of one bone only, the dentary, which articulates directly with the skull; the middle ear contains three small bones, two of which are derived from bones in the lower jaw of reptiles. Typically, mammalian teeth are of differing forms (heterodont), they are set in sockets (the-codont), and milk teeth are shed and replaced by a second set. A hard palate separates the nasal cavity from the mouth. Except in monotremes, the egg is small and develops in the uterus; and the young are fed milk secreted by mammae (which give the class its name). The skin has at least a few hairs. Many mammalian features were present in therapsids (mammal-like reptiles) during the Triassic. Mammals are believed to have appeared first toward the end of the Triassic and to have diversified rapidly from the end of the Mesozoic, 100 Ma later, following the mass extinction which marks the Mesozoic—Tertiary boundary 65 Ma ago.

Mammalia A class of vertebrates containing some 4250 species. Mammals are warm-blooded animals (see homoiothermy), typically having sweat glands whose secretion cools the skin and an insulating body covering of hair. All female mammals have mammary glands, which secrete milk to nourish the young. Mammalian teeth are differentiated into incisors, canines, premolars, and molars and the middle ear contains three sound-conducting ear ossicles. The four-chambered heart enables complete separation of oxygenated and deoxygenated blood and a muscular diaphragm takes part in breathing movements, both of which ensure that the tissues are well supplied with oxygen. This, together with well-developed sense organs and brain, have enabled mammals to pursue an active life and to colonize a wide variety of habitats.

Mammals evolved from carnivorous reptiles in the Triassic period about 225 million years ago. There are two subclasses: the primitive egg-laying Prototheria (monotremes) and the Theria, which includes all other mammals and consists of the infraclasses Metatheria (marsupials) and Eutheria (placental mammals).

mammal Any member of the class Mammalia of vertebrate animals characterized by mammary glands in the female and full, partial or vestigial hair covering. Mammals are warm-blooded. They have a four-chambered heart with circulation to the lungs separate from the rest of the body. As a group, mammals are active, alert, and intelligent. They usually bear fewer young than other animals, and give them longer and better parental care. Most mammals before birth grow inside the mother's body and are nourished from her by means of a placenta. When born, they continue to feed on milk from the mother's mammary glands. There is a wide range of features, shapes, and sizes among mammals. Mammals include 17 orders of placentals, one marsupial order – all live-bearing – and an order of egg-laying monotremes. They probably evolved c.180 million years ago from a group of warm-blooded reptiles. Today, mammals range in size from shrews weighing a few grams to the blue whale, which can weigh as much as 150 tonnes.

mam·mal / ˈmaməl/ • n. a warm-blooded vertebrate animal of a class that is distinguished by the possession of hair or fur, the secretion of milk by females for the nourishment of the young, and (typically) the birth of live young. DERIVATIVES: mam·ma·li·an / məˈmālēən/ adj.

mammal regions A biogeographic scheme that has been proposed by the biogeographer Charles H. Smith on the basis of statistical analyses. Four regions are recognized: Holarctica (the only region consistent with traditional faunal regions); Afro-Tethyan; Island; and Latin American.

mammal regions A biogeographic scheme that has been proposed by Charles H. Smith on the basis of statistical analyses. Four regions are recognized: Holarctica (the only region consistent with traditional faunal regions); Afro-Tethyan; Island; and Latin American.

mammal XIX; animal of the class mammalia (XVIII), who suckle their young. modL., n. pl. of L. mammālis, f. mamma; see prec., -AL1.

mammal •sewellel •camel, enamel, entrammel, mammal, trammel •miasmal, phantasmal •Carmel •abysmal, baptismal, catechismal, dismal, paroxysmal •animal • minimal • lachrymal •maximal •decimal, infinitesimal •septimal • optimal • primal • Rommel •abnormal, conformal, formal, normal, paranormal, subnormal •chromosomal • Kümmel •Brummell, pommel, pummel •epidermal, geothermal, isothermal, pachydermal, taxidermal, thermal