CHAPTER These now-extinct mammoths, like many other mammals, evolved layers of fat and thick fur coats, which served them well during ice ages. Today, mammals include some of the largest and most intelligent animals that have ever lived. GUIDE FOR READING After you read the following sections, you will be able to 33-1 Describe the characteristics of mammals, Discuss the importance of endothermy in the evolution of mammals. 33-2 Important Orders of Living Compare methods of mammalian reproduction. Identify the most important orders of placental mammals. Recognize the importance of mammals in the world. Journal Activity YOU AND YOUR WORLD come in all shapes and sizes and have some unique char acteristics. Which mammal interests you most? in your journal, explain why you found this animal interesting. What problems might arise if you were to make this animal a family pet? 33-1 Guide For Reading What are the characteristics of mammals? What are the three main groups of mammals? How do mammals perform essential life functions? The group of animals called mammals includes many di verse species that vary greatly in appearance. range in size from a tiny mouse nibbling its way along a corn cob sev eral times its size to a huge elephant uprooting a gigantic tree with its tusks and trunk. can be found flying in the air, running along the ground, and swimming in the sea. Al though they differ in size and habits, however, all members of the class Mammalia share certain characteristics. What Is a Mammal? are endolliermic animals, which means they are able to generate substantial body heat internally. Most species are experts at maintaining a constant bo'dy tempera ture. use various combinations of fur, hair, and sub cutaneous fat to conserve body heat. Subcutaneous fat is fat located under the skin (sub- means under; cutaneous refers to the skin). (We may not always appreciate having a layer of sub cutaneous fat, but it is there for a reason!) Many mammals also have sweat glands that help cool the body. Sweat produced by sweat glands evaporates from the skin, lowering body tempera ture whenever necessary. f\round the end of the Cretaceous Period, Earth's climate changed dramatically. The Rocky Mountains and other large mountain ranges arose, blocking the flow of warm, moist air from the oceans over the continent. Inland seas and swamps on the side of the mountain ranges away from the ocean dried up, winters became colder, and summers became hotter and drier. These conditions proved lucky for mammals, whose small ancestors had spent millions of years scrambling around in the shadow of the giant dinosaurs. As mammals evolved, they became accomplished endotherms that could survive cooler and more variable climates. So as dinosaurs vanished, the great mammalian radiation began. In this chapter you will see how successful mammals have been and examine some of the adaptations that make them so fascinating. Figure 33-1 One characteristic that unites all mammals is hair. This brown bear and her cubs can sleep through winters cold insulated by their thick coats and a layer of fat beneath their skin.
Evolution of Figure 33-3 Be they lions or whales, all mammals breathe air. Breathing is easy for land mammals, but sea mammals must return to the surface to breathe. Figure 33-2 All female mammals nurse their young, feeding them milk they produce in mammary glands. These glands are the source of the class Mammalia's name. With the exception of several very primitive species that lay eggs, all mammals are viviparous. This means that young mammals develop within the mother for a time and then are born alive. Female mammals have mammary glands, which produce milk to nourish the young for some time after they are born. Mammary glands, which give mammals their name, are probably the most important characteristic that scien tists use to include an animal in class Mammalia. have several kinds of teeth. Combined with their jaws, the teeth of mammals bite, chew, and grind food effi ciently. The teeth and jaws of various mammalian species take many forms, depending upon the species' feeding habits. Sci entists use the teeth of a mammal to classify it in one of the mammalian orders. You will read about these different orders in the next section. have well-developed breathing muscles, includ ing a diaphragm that separates the chest cavity from the abdo men. The diaphragm, along with other muscles in the chest, pulls air into the lungs by expanding the chest cavity. Mam mals have a four-chambered heart consisting of two atria and two ventricles. Each side of the heart, consisting of an atrium and a ventricle, is part of a completely separate circulatory cir cuit. One circuit moves blood to and from the lungs, and the other circuit moves blood to and from cells in the rest of the body. The two circuits make up an efficient system for the transfer of gases with the environment and for the delivery and removal of gases and other materials to and from body cells. The first mammals were very small and, according to fossil evidence, resembled species of tree shrews alive today. They were probably nocturnal, which means they were active pri marily at night. Because they were endotherms, they did not need to obtain heat from the environment in order to remain active thus making them able to function well after dark. The ectothermic dinosaurs, on the other hand, would probably have been rather sluggish after sunset. (Even today there are very few nocturnal lizards, and those that do exist live only in the tropics.) By the end of the Cretaceous Period, the mammals had split into three groups. The first group, and the most primitive, is the monotremes. Today only three species of monotremes sur vive. The most familiar species are the duckbill platypus and the Australian spiny anteater. The second group, the marsupials, includes opossums, kan garoos, wombats, and koalas. Each of these species has a pouch in which its young live for a time. The third group, the placental mammals, include the mammals you are most familiar with. Mice, cats, whales, ele phants, and humans are just a few examples of placental mammals. Because the fossil record of the earliest mammals is incom plete, it is hard to say precisely where and when each of these three groups appeared. However, we do know that placental mammals experienced a period of adaptive radiation in North America and Europe. Marsupials experienced a period of adap tive radiation in Australia, South America, and Antarctica (which was a good deal warmer then than it is today). Form and Function in have limbs and organ systems that have evolved many shapes to serve many functions in different environ ments. The specialized adaptations are far too numerous to ex plore here, so we shall mention only a few of the more interesting ones. FEEDING Carnivorous mammals, such as cats and dogs, have strong, sharp teeth called incisors and canines that are used for biting and ripping flesh from their prey. Some extinct carnivores, such as saber-toothed cats, had enormous canines. Even the molars of carnivorous mammals are sharp, for they are used to slice meat into small pieces to speed digestion. Car nivores use an up-and-down chopping movement of their jaws to chew their food. The behavioral and physical characteristics of many mam mals allow them to capture prey. For example, some carnivores have sharp claws on their feet with which they grab onto prey. Their bodies are built to produce the quick bursts of speed Figure 33-4 The fossil record shows that the first mammals resembled this tree shrew. Tree shrews are omniuores; they eat both plants and animals. 739
Guide For Reading How do the methods of reproduction in mammals differ? What are the most important orders of placenta] mammals? How do mammals fit into the world? Figure 33-13 The echidna is known as the spiny anteater (top). Its spines protect it from enemies. Its long sticky tongue laps up the ants it eats. These baby opossums are hitching a ride on their mother's hack, where they are safe (bottom). When their mother locates food, the babies climb off to eat. 33-2 Important Orders of Living Scientists use several important characteristics to clas sify mammals. The structure of teeth and the number and kinds of bones in the head are two important features by which mammals are classified. But perhaps the most impor tant characteristic used to classify mammals is the method of reproduction. As you have already learned, mammal species show three different methods of reproduction. Some mammals lay eggs. Some mammals give birth to young that are not well developed at birth and must therefore spend time developing in a special pouch in their mother's body. Some mammals in fact, most retain embryos within the mother's body, where the embryos grow and develop, nourished by the mother. At birth, these young are more well developed than the young of mammals that spend time in their mother's pouch. Monotremes Monotremes, or egg-laying mammals, are very rare. In fact, only three species of monotremes exist today, living in isolated parts of Australia and New Guinea. You have already learned about the duckbill platypus, the most familiar monotreme. The spiny anteater, or echidna, is another monotreme. It has a jaw shaped like a bird's bill, strong clawed feet with which it can dig quickly, and long sharp spines among its body hairs. Marsupials mammals also have slightly higher metabolic rates. Placental mammals are much more abundant today than their marsupial cousins. The most important orders of placental mammals are briefly described here. ORDER INSECTIVORA This order, whose name means insect eaters, includes tree shrews, hedgehogs, shrews, and moles. Shrews, several of which are about the size of a mouse, have extremely high metabolic rates and must eat almost con stantly to stay alive. Biologists believe that the first mammals looked and behaved much like certain modern tree shrews. Re member, however, that this does not mean that mammals evolved from tree shrews as we know them today. It simply means that living tree shrews have many characteristics of primitive mammals. ORDER CHIROPTERA This order contains the many dif ferent species of bats, which in fact account for one quarter of all mammal species. Bats are closely related to insectivores, al though different types of bats eat many different kinds of foods. Some bats eat only insects, whereas others eat only fruits. Still others, such as vampire bats, feed on the blood of other mam mals. Many bats are active only at night. Night-flying bats use echolocation to help them navigate while in flight. By emitting high-pitched sounds that bounce off objects, the bats can cal culate distances and determine locations of tiny insect prey. Some fruit-eating bats, like the giant flying foxes, feed during the day. Colonies of these bats sleep together in trees, hanging upside down and wrapping their wings around their body. Figure 33-14 Moles dig tunnels under ground (left). In the subterranean darkness, these animals eat the insect larvae that live in the soil., Bats are the only true flying mammals (right). They glide along, effortlessly flapping their delicate wings. Like moles, many species of bats also eat insects, which they catch in the air. The bats in this photograph, however, eat frogs. Marsupials are pouched mammals. A number of fascinating marsupials, such as kangaroos and koalas, are found in Australia. Kangaroos and their close relatives, the wallabies, are herbivores that feed primarily on range grasses. Koalas spend most of their time in trees and eat only the leaves of eucalyptus trees. Opossums are the only marsupials found in North America today. Active mostly at night, opossums spend the day sleeping in protected spots. These animals eat a variety of insects, birds, and small mammals. A newborn opossum is about the size of a bee. Just after birth, it uses its tiny hands and feet to grasp onto its mother's hair and crawl to the safety of her pouch. In the past, many other marsupials lived in South America. These included many species similar in form to wolves, bears, camels, and moles (none of which are actually marsupials). Placentals Living placental mammals are placed in sixteen orders, most of which contain animals familiar to you. In addition to having different reproductive habits than marsupials, placental 747
Figure 33-15 Common in certain parts of the United States, armadillos are in the order Edentata (top). The porcupine depends upon its many sharp quills for protection (center). One function of the huge ears of a rabbit is to give off excess body heat to the environment through the many blood vessels in these organs (bottom). ORDER EDENTATA The name of this order means with out teeth, although some of the mammals included here have small teeth. The edentates include such odd animals as sloths, anteaters, and armadillos. Sloths are South American animals that live most of their life hanging upside down in trees. Many sloths move so slowly that at times they scarcely seem alive. Anteaters have long tapered snouts and powerful front legs with sharp claws. They feed by ripping open ant nests and col lecting the scurrying insects with a long and sticky tongue. ORDER RODENTIA This order includes many amusing mammals, as well as many destructive ones. Mice, rats, squir rels, beavers, porcupines, and gophers are all rodents. Rodents have two long front teeth, which they use for chewing wood and other tough plant material. These two front teeth continue to grow during a rodent's life. The constant gnawing on tough plant material wears the teeth down. Most rodents are small and have a short gestation period. Rats and mice are very adaptable animals that eat a wide variety of foods. Long ago, both these rodents moved in with humans and have traveled with us all over the world. ORDER LAGOMORPHA The familiar rabbits and hares comprise this order. In many ways, lagomorphs resemble ro dents. They have sharp front teeth and eat plant material. Their gestation period is short, and the number of young they pro duce is high. Many of these animals compete with humans for food. Several species, such as jack rabbits and cottontails, are widely distributed across the United States. ORDER CARNIVORA Carnivores are meat eaters. Many familiar animals including cats, dogs, wolves, bears, weasels, hyenas, and seals are in this order. Most are terrestrial, stalk ing and chasing their prey by running and pouncing, and then killing them with sharp teeth and claws. Carnivores such as seals and walruses had ancestors that at one time lived on land, but these mammals have since returned to the ocean, where they feed on fish, mollusks, and sea birds. Although quite agile in water, aquatic carnivores return to land to breed and bear their young. On land these animals move with diffi culty. Their appendages, so useful in the sea, are not very ef fective at moving their body from place to place on land. ORDER CETACEA This order contains truly aquatic mammals: whales, dolphins, and porpoises. Although they still breathe air, these mammals have lungs and a circulatory sys tem designed to permit long, deep dives. Their thick layer of subcutaneous fat, called blubber, keeps them warm in even the coldest water. Cetaceans have lost both their external ears and their hind legs. These animals mate and bear their young in the water. On land, they are completely helpless. All cetaceans are carnivores. A few, such as the great blue whale and the hump back whale, are filter feeders and live by eating plankton. ORDER SIRENIA These strange aquatic mammals are re lated to elephants. They are peaceful, slow-moving herbivores that live in rivers and streams in parts of Africa, South America, and Florida. Some species are also found in tropical oceans and the Caribbean. The manatee, or sea cow, lives in quiet waters in southern Florida, where it is often injured by careless boaters. The propellers of boat engines cut the manatee's back as it swims along just below the surface of the water. ORDER ARTIODACTYLA This order contains the large grazing animals: cattle, sheep, goats, hippopotami, giraffes, and pigs. For mammals in this order, the original five toes on each foot have been reduced to two. Thus artiodactyls are called even-toed ungulates. The word ungulate means hoofed mammal. ORDER PERISSODACTYLA Horses, zebras, tapirs, and rhinoceroses the odd-toed ungulates-make up this order Many grazing animals with habits similar to those of even-toed mammals are included in this order. Some odd-toed ungulates have hooves formed from the center toe of each foot. ORDER PROBOSCIDEA These are the mammals with trunks, the great elephants. Some time ago, this order had a reasonably large adaptive radiation that produced about thir y species. Included in those species were the mammoths and mastodons, which today are extinct. Only two species, the In dian elephant and the African elephant, presently survive. Both species are in danger of becoming extinct. ORDER PRIMATES This order, which includes our own species, is closely related to the ancient insectivores. Of all the animals, primates have the most highly developed cerebrum and the most complicated behaviors. The most primitive living primates, the lemurs, are small tree dwellers. The primates most people call monkeys or apes represent two main branches within this order. Figure 33-16 Carnivores such as the walrus (left) and the hyena (right) are placed in the same order. The walrus moves along the ocean bottom, using its tusks to find the clams and other shellfish it eats. The hyena is an efficient hunter and often kills small antelopes. Figure 33-17 Manatees live in tropical water, often floating. Their diet consists of water plants. 749
regions to the warmest equatorial regions. And in all these re gions, other mammal species live alongside humans. In many parts of the world, herbivorous mammals are major consumers of plant material. For example, huge herds of grazing zebras and wildebeests eat their way across the sa vannas of Africa. Herds of reindeer and musk oxen move across the tundra, eating small plants and lichens. These peaceful grazers are, in turn, food for carnivorous mammals. Lions, leopards, cheetahs, wild dogs, wolves, and other carnivores hunt and kill plant eaters for food. In the air, flying bats and gliding squirrels move gracefully in search of food. Bats eat enormous numbers of mosquitoes Figure 33-18 The mountain goat (left) has two toes and is included with other two-toed mammals in the order Artiodactyla. The tapir (right) has three toes and is grouped with other animals that have an odd number of toes. Figure 33-19 Among the most intelligent animals, the black lemur (left) and the chimpanzee (right) are both primates. Today, chimpanzees and many species of lemurs are endangered. Very early in their evolutionary history, primates (as a group) were split apart by the moving continents. One branch, the New World monkeys, includes the squirrel monkey and the spider monkey. These monkeys live almost entirely in trees. They have long arms for swinging from branch to branch and long prehensile tails, which they use for grasping while climbing. The other branch, the Old World monkeys, include chim panzees, gorillas, and the ancestors of humans. Many Old World monkeys still spend much of their time in trees, but they all lack prehensile tails. How Fit into the World evolved from early reptile ancestors during the Mesozoic Era about two hundred million years ago. During this era, and later during the Cenozoic Era, mammals underwent massive adaptive radiations. Today mammals are distributed throughout the world. Humans, one of the important mamma lian species, inhabit areas that range from the very cold polar also inhabit the oceans. Whales, the largest ani!and other mals insects. to have Gliding lived on squirrels Earth, are feed probably on nuts the most and familiar seeds. ex ample. Despite their huge size, many species are today in danger of becoming extinct. Domesticated mammals such as dogs, cows, sheep, and goats have a significant influence on human culture. Many of these animals provide food in the form of meat and dairy prod ucts such as milk, butter, and cheese. Others help humans find food. Dogs are used to hunt, and monkeys are used to harvest coconuts and other fruits. Some mammals have a negative impact on human life. Car nivores prey on domesticated animals. Rodents such as rats and mice damage crops and eat stored food. Some mammals carry diseases that can affect humans. For example, rats harbor fleas that can spread the plague. Dogs, squirrels, and other wild animals can transmit the virus that causes rabies. can have a profound effect on the environment. Elephants destroy huge numbers of trees as they feed. Over grazing by cattle and rabbits can turn an area of prime farm land into a virtual dust bowl. Beavers flood areas and can create ponds when they build their dams. Humans in particular are capable of altering the environment in many ways both good and bad. Figure 33-20 People used natural fibers and fur to weave cloth long before synthetic fibers were developed. In this photograph, sheep are being shorn of their fine wool. Figure 33-21 Animals often make profound changes to the natural envirdnment. This beautiful dam was constructed by beavers, not by engineers. The pond that formed behind the dam provides safety for the beavers and a home for many other kinds of animals. 1 SECTION REVIEW 1. What is a placental mammal? How does a placental mammal differ from a pouched mammal? 2. How do monotremes provide evidence for the evolution of mammals from ancient reptiles? 3. Name five different orders of placental mammals and give an example of each. 4. In what four ways do mammals influence human life? 5. Critical Thinking Making Predictions How could you predict the diet of a mammal by looking at its teeth? 751