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.

Mammalia

Mammals are a group of animals (of the class Mammalia) found throughout the world. Even in regions where the most extreme climatic conditions exist, there are likely to be mammals. Seals, walruses, and whales survive in the cold Arctic and Antarctic. Small pikas live near the tops of high mountain ranges. Arid deserts are home to a wide variety of rodents and their coyote predators. Camels and their amazing ability to survive for long periods without drinking water are an almost universal symbol of the desert. Mammals have successfully survived in so many environments that it is no wonder scientists call this the Age of Mammals.

Mammals, however, did not always exist over such a wide geographical range. They have an evolutionary history going back to the dinosaurs. The history of mammals may surprise people, because mammal fossils have been found in rock deposits similar in age to the fossils of dinosaurs. Fossils identified as belonging to mammals have been found in rocks of the Late Triassic, about 200 million years ago. Early mammal fossils have been discovered in Europe, Great Britain, southern Africa, the Turkestan Range of southern Russia, China, North America, and South America.

Although widespread, the fossils of early mammals are actually rare. Many of the fossils are incomplete skeletons and isolated teeth. By the Middle Jurassic era (208 to 144 million years ago), however, mammals were flourishing, and the numbers of various species and groups had increased dramatically. At the end of the Cretaceous era (144 to 66 million years ago), when the large dinosaurs faced their final extinction, mammals survived to become the group that would produce the largest animals remaining on Earth.

Characteristics of Mammals

What is a mammal? How is it recognized from other vertebrate animals? Some body shapes and features of mammals are shared with other groups of animals that have backbones. These shared characteristics are vertebrae, an internal skeleton, and a four-chambered heart.

On the other hand, several characteristics of mammals are unique and help distinguish a mammal from something else. The most familiar of the unique mammal characteristics is that their bodies are covered with hair or fur. Hair is a form of scales and indicates a shared ancestry with reptiles. In fact, scales are still found on some mammals, as on the tails of rats. In some mammals, such as whales, the amount of hair is greatly reduced. This is a secondary characteristic and is considered an evolutionary loss from an ancestor who had hair.

The skin of mammals is unique in that it contains sweat, scent, sebaceous oil, and mammary glands. The skin of all vertebrates is an important aspect of their health and survival, but few have developed such an elaborate variety of specialized glands as the mammals. Sweat glands help to cool the animals. Scent glands help species to recognize one another. Sebaceous glands provide a special type of oily substance for the maintenance of healthy hair and the prevention of bacterial infection. Mammary glands are unique to mammals and are, in fact, the characteristic for which the group is named. The mammary glands in females secrete the nourishing milk that helps the young to grow. There are very few other groups of animals in which so much maternal care is given as is the case with mammals.

The skeletal features of mammals are similar to those of other vertebrates—fish, amphibians, reptiles, and birds—but are easily distinguishable as mammalian. Some of the more obviously distinctive features of mammals are found in the skull. It is difficult to indicate a skeletal feature completely unique to mammals, especially because there are a variety of modifications.

However, the general structural pattern of mammals is easily identified. Their teeth, which many consider to be part of the skeletal system, are what most scientists rely upon to identify mammals. Individual molar teeth have many cusps, or points. Reptiles, amphibians, and fish usually have simple cone-shaped teeth that are often replaceable throughout the life of the animal. Birds have no teeth at all. Mammals have only one set of replacement teeth, and when the second set is worn out, the animal may starve.

The heart of mammals is very efficient because it is four-chambered. Only birds and possibly dinosaurs share this characteristic. There are two atria and two ventricles for increased circulation. The great efficiency of this type of structure is important for high-energy animals, who need a great deal of oxygen to support their high rate of metabolism. Reptiles have a three-chambered heart and, consequently, have less efficient bodies because of the mixing of unoxygenated and oxygenated blood. This increased circulation pressure in mammals is the primary reason they are considered warm-blooded, or homeothermic .

Perhaps the most significant characteristic of mammals is the placental uterus. While many animals keep their eggs inside their bodies until the young hatch, the young are still living off a yolk inside an enclosed egg. The egg may hatch inside the parent, but it receives no nourishment from the parent throughout its development. Mammals, on the other hand, are often simply described as placental animals because of the remarkable temporary organ called the placenta.

Once internal fertilization has occurred within the female, the egg attaches to the placenta, a blood-rich and nourishing lining of the uterus. The egg stays there and continues to develop into a fetus. The young mammal receives all its nourishment from the mother and is completely dependent on her even after it leaves the uterus.

Classifying Mammals

The classification of mammals is complicated and always changing. The natural world does not always fit neatly into schemes of classification. The monotremes are one example. The most famous monotreme is the duckbilled platypus. The spiny anteater, or echidna, is also a monotreme. The monotremes are the only group of mammals that are oviparous , which means they lay eggs that hatch outside the body. Their ancestry is not well-known and no monotreme fossils have been dated before the Pleistocene epoch (1.6 million years ago). Scientists continue to debate whether monotremes should be placed in the mammal group, but an agreement has not been reached.

Marsupials.

Yet there are numerous agreements about classification. For example, one of the many agreed-upon criteria for deciding which groups of mammals are more primitive is based on placental care. The most primitive mammals that are fully accepted into the group are marsupials. Marsupials all have an abdominal pouch, the marsupium, in which they raise their young. The young are initially nourished in the uterus, but only for a short time, and a placenta-like organ is not very well developed.

Perhaps these mammals represent the first type of true mammal. It is believed that the early fossil mammals were marsupials. Fossils with marsupial-like skulls are found in rocks over 100 million years old. Many of today's marsupials are small, rodent-looking mammals like the brown four-eyed opossum or the ashy-mouse opossum. It is easy to imagine that mammals similar to these small creatures lived in the debris of forest floor alongside the mighty dinosaurs. They would have been hard to see and too quick for predators. Insects were in abundance and would, as they do today, compose the major diet of these tiny primitive mammals.

It is believed that marsupials reigned in the mammal world for many millions of years. Fossils of marsupials, and even filmed footage of nowextinct marsupials, like the Tasmanian devil, show that, without competition from the large dinosaurs and reptiles, they grew to very large sizes. Many became large carnivorous marsupials, like the saber-toothed Thylacosmilus. This mighty predator was the size of a jaguar and very dangerous in appearance.

Marsupial-type fossils are found over most of the planet, with the exception of the oceans. Fossil opossums have even been found on Antarctica. They are believed to have survived on this vast expanse of land when the climate there was more comfortable for life. When South America split from the large supercontinent of Pangea, it became a type of continental raft on which species of marsupials evolved in isolation from their cousins in Africa and Europe.

During the Ice Age, when tremendous amounts of water were tied up in ice sheets, the Isthmus of Panama was exposed, enabling marsupials to travel north into North America. Although many have since become extinct, it is still possible to see frequently the hardy opossum, Didelphis, in most regions of Mexico and the United States. In South America many marsupials still live in the forests, although their numbers are dwindling.

The last stronghold for marsupials is the continent of Australia. Because of its isolation from the other continents after the split-up of Pangea, the more evolved and efficient placental mammals never reached Australia. It is as though time was frozen for the marsupials in Australia. Delightful animals, like the kangaroo, koala, wombat, and numbat, still exist in this remote continent. However, as in the rest of the world, they are under threat from the placental mammals. True mammals, like house cats and rabbits, are making it hard for the small marsupials to survive. Rabbits and other grazers are competing for grasslands. The eucalyptus forests necessary for the survival of the koala are threatened. However, many human steps are being taken to help preserve these mammals in the wild and there are still many marsupials found in Australia.

Placental mammals.

In every corner of the world, placental mammals thrive. They exist as two major types of animals, carnivores and herbivores. The story of the herbivores is as complicated as is that of the smaller groups of these mammals.

Rodents are often considered a nuisance, but their success at survival under the most extreme conditions is undisputed. They are hardy and reproduce quickly. Despite the efforts of many farmers, home dwellers, and urban developers, rodents have managed to live side by side with humans and even have secured an entire means of survival from the waste of humans, another highly successful placental mammal. Rodents are at the base of many food chains in the wild and in cities.

Around forty million years ago, an evolutionary adaptation of grasses provided the stimulus for a burst of evolution among the herbivores. Grasses developed a metabolic way to survive in more arid conditions. These new species are called the C4 grasses. These grasses, like crabgrass, are a type of grass that can survive in hostile environments. As these tougher species of grass became available in regions like flat plains and plateaus, the animals that ate the grass ventured into more open space. While this was good for herbivores because it expanded their food resources, it also became easier for predators to see them.

The animals that survived this evolutionary explosion were those with increased running ability. Grazing mammals evolved longer and narrower legs which they used to elude predators. Groups such as the gazelle, antelope, and horse became the fastest long-distance runners on Earth. They still exist and show no signs of slowing down.

In response, the predators also became faster. The large cats are unanimously considered the fleetest predator on land. The amazing speed of the cheetah has been recorded many times. It is heralded as the fastest land animal alive. Before their extermination by humans, many species of large cats roamed over most of the planet. The American lion and saber-toothed cats were only recently, in geologic terms, eliminated from Earth.

Bears, many of whom are actually omnivores, are considered by most to be the mightiest land predator on the planet today. The strength of the grizzly, kodiak, and polar bear is legendary. They are surprisingly fast runners, and very few animals are prepared to withstand combat with them.

Mammals survive not only on land but in the ocean as well. Whales, dolphins, seals, walruses, otters, and other marine mammals are very successful in their ways of life. The killer whale may be the largest predator on Earth. They feed on other marine mammals and are especially fond of seals.

The largest animal on Earth is a mammal. The blue whale is estimated to weigh around 150 tons and is almost 27 meters (89 feet) long as an adult. It feeds on tiny krill , a shrimplike animal that it catches with a the help of a baleen , a sievelike structure in its mouth.

Many species of whales had been hunted to extinction by the 1960s. Fortunately, an international ban was placed on the hunting of these magnificent mammals and many species are increasing in number.

It is surprising to read that the ancestors of whales were land dwellers, a group of mammals called creodonts. Fossil evidence indicates that these animals may have become increasingly adapted to a marine environmment until they evolved to a completely water-based existence. They are still mammals, however, as evidenced by the formation of a placenta, live birth, and young that suckle for breast milk. Baby whales, although quite large, are completely dependent on their mothers for a long time.

One may find a mammal just about anywhere on Earth. They have evolved to fill almost every possible niche and continue to flourish despite harsh environmental and climatic changes. The current loss of mammals, however, continues at an alarming rate and, over the last 20,000 years, the loss of mammal species, like the mammoth, is staggering. It is hoped this trend will not only halt but, in cases of endangered mammals, be reversed.

Ellen Brook Hall

Bibliography

Hickman, C., L. Roberts, and F. Hickman. Integrated Principles of Zoology. St. Louis, MO: Times Mirror/Mosby College Publishing, 1984.

Lillegraven, J. A., Z. Kielan-Jaworowska, and W. Clemens. Mesozoic Mammals: The First Two-Thirds of Mammal History. Berkeley: University of California Press, 1979.

Macdonald, David. The Encyclopedia of Mammals. New York: Facts on File, 1984.

During the Mesozoic era, marsupials were very common in North America, more common, it is thought, than placental mammals. Marsupials persisted in this part of the world until the mid-to late-Tertiary period.

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

A mammal is a warm-blooded vertebrate (an animal with a backbone) animal with some hair that feeds milk to its young. Mammals are the most diverse as well as the most successful vertebrate, and can be found living in nearly every habitat on Earth. Mammals are also the most advanced or intelligent animal and have become the dominant form of life on Earth. Humans are mammals.

The animals that make up the amazingly diverse class known as Mammalia range from the largest animal that ever lived, the blue whale that can reach 100 feet (30.48 meters) and 150 tons (136.05 metric tons) to the hog-nosed bat of Thailand that is the size of a bumblebee. Mammalia includes human beings, whose intelligence shaped the world as it is today, and the grotesque and slow-moving sloth that hangs upside-down from a tree most of its life. Rats are mammals, and so are dolphins, monkeys, and giraffes.

COMMON TRAITS OF MAMMALS

Despite the extreme differences among species of mammals, all have several traits in common.

Mammary Glands. First, mammals get their name from the characteristic "mammary" glands that females use to feed their young. After a pregnant female gives birth, her mammary glands secrete milk that the young drink by sucking. This liquid substance provides all the nutrients that a young mammal requires to grow and develop. Some mammal species suckle their young for only a few days, while others, like the elephant and humans, may nurse them for more than a year. A mammal also usually gives birth to live young. Except for only three species out of approximately 4,300, the young of every mammal develop inside the body of the mother who delivers or gives birth to live young instead of laying eggs. This makes mammals placental animals (named after the organ called a placenta which nourishes the developing embryo while it is inside its mother's uterus). Another characteristic related to their offspring is that mammals usually care for their young, even after the young cease nursing on mother's milk. Mammal parents both protect the young from enemies and teach them necessary survival skills.

Hair and Teeth. Interestingly, body hair is a mammalian trait, and all mammals have at least some hair or fur on their bodies at some stage in their development. Even smooth-skinned whales and dolphins have hair at birth. Mammal hair and fur is made of long thin strands of protein called keratin. Many mammals have long, stiff hairs around their head or mouth that act as feelers and allow them to get around in the dark. The keratin in some mammals has adapted into a protective device, such as the quills of a porcupine. Hair insulates mammals in much the same way that feathers keep birds warm. This hair insulates and helps keep body heat from escaping. The fact that mammals can generate their own heat (as long as they have enough food to eat) means that they are endothermic or warm-blooded. This is a distinct advantage because it means that mammals can maintain their own internal body heat despite living in a cold climate. Unlike a cold-blooded animal whose body temperature rises and falls with that of its environment (and as a result, becomes lethargic in cold weather), mammals are always ready to spring into action. This is a necessary trait if one is either the hunter or the hunted. Most mammals are also able to cool their bodies in hot weather because they have sweat glands on their skin that produce moisture. This moisture then evaporates and cools the mammals' bodies.

Mammal teeth are also specially adapted to their feeding habits. Mammal carnivores (meat-eaters) have distinctive canine teeth that allow them to catch, hold, kill, and eat their prey. Herbivores (plant-eaters), however, have flat, grinding teeth for breaking down the tough cell walls of plants. Beavers and squirrels have teeth that continue to grow, and an elephant's tusks are really its incisor teeth (what humans call their front teeth).

Senses. Mammals also have highly developed senses, although few species have acute capabilities in all five senses (sight, smell, taste, touch, hearing). Most have an excellent sense of smell since this sense is not only necessary in locating food that may be out of sight, but it is crucially important in alerting an animal to danger. Mammals also usually have an excellent sense of hearing and usually, the larger a mammal's ears, the more it relies on that sense for its survival. For example, a rabbit's primary defense is its quickness and, therefore, its large, long ears serve as an excellent early-warning system.

Skeletal System. As vertebrate animals, all mammals also share a basic skeletal structure. They all have a bony skull that houses a brain and key sense organs (such as eyes, ears, and nose). All have a backbone or vertebral column consisting of individual vertebrae. Except for sloths and manatees, all mammals have exactly seven vertebrae in their necks. Thus a giraffe has seven very large ones, and a mouse also has seven very small ones. Most mammals have four limbs, each of which usually ends with five digits (fingers and toes in humans). Mammals also have a distinctive, four-chambered heart (as birds do). This type of heart keeps oxygen-rich blood separate from the oxygen-deficient blood by having each flow in and out of the heart in its own system. Because of this system, the mammal heart can quickly deliver large amounts of high-energy oxygen on demand.

Behavior. Unlike invertebrates (animals without a backbone), mammals exhibit what is usually called complex behavior. For example, mammals act on instinct, a pattern of behavior that is inborn rather than learned. When a newborn human immediately seeks its mother's breast to suck or when a bear hibernates for the winter, these mammals are following instincts (or natural drive to do something). Mammals have larger brains than other animals and can be said to learn in some ways. If a mammal changes its behavior because of repeated experiences, then mammals can learn. When a mammal avoids a situation that was dangerous in the past and seeks another alternative that was previously beneficial, the mammal also exhibits a type of learning. Such behavior leads to the statement that mammals, and not just humans, are the smartest animals on Earth.

The Brain. Mammals have a large brain with a well-developed cerebral cortex. The cerebral cortex is the part of the brain involved in memory, sensory perception, and learning. Mammals also often live in social groups of different sizes. The smallest social group includes a male, a female, and young. Others live in larger groups called bands, and others in still-larger groups called colonies. The largest group of mammals would be composed of thousands and is called a herd. In bands of mammals, like monkeys, there is always a ranking of individuals with the most dominant member acting as a sort of boss or leader.

Communication. Mammals also establish bonds between one another. Mammals interact with one another by communicating in many different ways. Many use smells to tell friend from stranger or to mark their territory. Visual signals are also used, as when a rabbit flashes a patch of white hair under its tail (danger), or when a dog bares its teeth and lowers its tail. Sound is the most obvious way to communicate, and cries or whistles of danger are different noises than those of mating calls. Altogether, the behavior of mammals can be very complex.

Development of Young. Among the 4,300 species of mammals, all can be placed in one of three groups based on how their young develop: monotremes, marsupials, and placentals. There are only two species of monotremes, which are by far the strangest type of mammals, since they lay eggs. The duck-billed platypus and the spiny anteater lay leathery eggs instead of giving birth to live young. However, they have mammary glands and nurse their young. A marsupial is a mammal whose young complete their development inside the mother's pouch. Kangaroos and koalas, as well as wallabies and opossums, carry their young inside their bodies for a short period, after which they give birth to a tiny, barely developed marsupial that crawls into its mother's pouch where it nurses and grows. All other mammals are placental, meaning that they are nourished inside the female's body until birth. Some mammals, like the horse, are able to walk within minutes of birth, while others, such as a human infant, are helpless and require years of care.

The diversity of mammals can be amazing. For example, bats are the only mammals that can fly, and rodents are the largest group of mammals. Whales, porpoises, and dolphins are aquatic mammals (their fins are limbs) that breathe air, and elephants are the largest land mammals. This being the case, mammals are an interesting group of animals to study and learn from.

[See alsoMammalogy ]

Mammals

The 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 respiration, a lower jaw consisting of a single bone, a left systemic aortic arch leaving the left ventricle of the heart, 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 1 ounce (28.4 g), to the blue whale, which weighs more than 200,000 pounds (90,800 kg). Mammals are found in cold 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. While mammals are not as numerous and diverse as, for example, birds or insects, they have a tremendous impact on the environment, particularly due to the use of Earth’s natural resources by one mammal species: humans.

Mammal species have developed varying adaptations in response to the different environments in which they live. Mammals in cold climates have insulating layers—a thick coat of fur, or a thick layer of fat (blubber)—that help retain body heat and keep the animal’s body temperature constant. Some mammals that live in deserts survive by special adaptations in their kidneys and sweat glands, as well as by their ability to avoid heat by behavioral means. 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.

The care of the young (parental care) is notable among mammals. Born at an average of 10% 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 them to maintain a strong survival rate in the animal kingdom.

The subclass Placentalia contains the majority of living mammals. Placental embryos develop in the mother’s uterus, is nourished by blood from the placenta, and is retained until it reaches an advanced state of development. Marsupialia are found in Australia and in North and South America. Their young develop inside the uterus, usually with a placenta connected to a yolk sac. Young marsupials are born in a very undeveloped state and are sheltered in a pouch (the marsupium) which contains the nipples of the milk glands. Kangaroos, wallabies, and most Australian mammals are marsupials, as is the opossum of the New World. The Monotremata of Australia include the duck-billed platypus and two species of spiny anteaters. Monotremes lay eggs, but have hair and secrete milk like other mammals.