It is late January in the Appalachian Mountains. In a rocky

32 1 Introduction to the Mammals Section 32 1 It is late January in the Appalachian Mountains. In a rocky den beneath the snowdrifts, a black bear has just given birth. Two tiny cubs are nursing on their mother s rich milk. It is bitterly cold outside, but the mother s dense fur and thick layer of fat keep her and her cubs comfortably warm. When spring arrives, the hungry bears will emerge from the den. For the next two years, the cubs will follow their mother as she teaches them to search for food and defend themselves. Bears are mammals, members of the class Mammalia. All mammals are characterized by two notable features: hair and mammary glands. In female mammals, mammary glands the feature for which mammals are named produce milk to nourish the young. In addition to having hair and the ability to nourish their young with milk, all mammals breathe air, have four-chambered hearts, and are endotherms that generate their heat internally. Evolution of Mammals Neither mammary glands nor hair are preserved in the fossil record. But mammals have several other characteristics that help scientists to identify mammalian fossils. These characteristics include a lower jaw consisting of a large, teeth-bearing bone connected directly to the skull by a joint; complex teeth that are replaced just once in a lifetime; and distinctive features of the limbs and the backbone. Mammals are descended from ancient reptiles. According to the fossil record, the ancestors of modern mammals diverged from ancient reptiles during the Carboniferous Period. For millions of years, various mammal-like reptiles lived alongside other reptile groups. The first true mammals appeared during the late Triassic Period, about 220 million years ago. These mammals were very small and probably resembled modern tree shrews, like the one in Figure 32 1. While dinosaurs ruled the Cretaceous Period, from about 145 to 65 million years ago, mammals were generally small and remained out of sight. These mammals were probably nocturnal, or active at night. After the disappearance of the dinosaurs at the end of the Cretaceous Period, mammals underwent a burst of adaptive radiation. They increased in size and occupied many new niches. In fact, the Cenozoic Era, which followed the Cretaceous Period, is usually called the Age of Mammals. Three major groups of mammals had evolved by the beginning of the Cenozoic Era. Surviving members of these groups include today s monotremes, marsupials, and placental mammals. SECTION RESOURCES Key Concepts What are the characteristics of mammals? When did mammals evolve? How do mammals maintain homeostasis? Vocabulary mammary gland subcutaneous fat rumen diaphragm cerebral cortex Reading Strategy: Asking Questions Before you read, rewrite the headings in the section as how, why, or what questions about mammals. As you read, write brief answers to these heading questions. Figure 32 1 The first mammals appeared on Earth about 220 million years ago. They may have resembled this tree shrew from Madagascar, shown here clutching a beetle. Like this tree shrew, early mammals probably ate insects. 1 FOCUS Objectives 32.1.1 List the characteristics of mammals. 32.1.2 Tell when mammals evolved. 32.1.3 Describe how mammals perform essential life functions. Vocabulary Preview Explain that the prefix sub- means under, and cutaneous means having to do with the skin. Then, ask: Where do you think subcutaneous fat is located? (Under the skin) Reading Strategy Have students also write how, why, or what questions for the green subheadings in the section. Remind students to leave room under each question to write its answer as they read the section. 2 INSTRUCT Evolution of Mammals Posing Questions Challenge students to consider what types of questions a paleontologist might ask to determine whether or not a new fossil find is the remains of an early mammal. You might have groups of students work together to develop a list of questions. (Possible question topics include presence of mammalian tooth structure, limb attachment to backbone, jaw structure, or number of limbs.) Print: Teaching Resources, Lesson Plan 32 1, Adapted Section Summary 32 1, Adapted Worksheets 32 1, Section Summary 32 1, Worksheets 32 1, Section Review 32 1 Reading and Study Workbook A, Section 32 1 Adapted Reading and Study Workbook B, Section 32 1 Investigations in Forensics, Investigation 9 Lab Worksheets, Chapter 32 Real-World Lab Technology: itext, Section 32 1 Transparencies Plus, Section 32 1 BioDetectives DVD, Wrongly Accused: Science and Justice Mammals 821

32 1 (continued) Form and Function in Mammals Inferring Borrow various animal hides from a zoo or natural history museum. Challenge students to make inferences about the animals habitats based on the types of hairs in their coats. (In general, animals in cold climates have heavy coats with two layers of hairs. Animals in warm environments have thinner coats or no hair at all. Exceptions in cold climates include dolphins, whales, and walruses, which are protected by layers of blubber.) Make Connections Environmental Science Relate the diversity of mammalian modes of feeding to the adaptive radiation of mammals in the Cenozoic Era. Ask: Why do you think the first mammals were insectivores? (Possible answers: Insects provided a lot of energy to an endothermic mammal; mammals small size restricted their predation to insects and perhaps dinosaur eggs.) What change in conditions opened up many new energy sources to the early mammals? (The extinction of dinosaurs left many niche resources open; mammals were equipped to live successfully in the changing climate, and lack of competition and predation allowed them to develop many new, diverse niches.) Point out that the changing climate also affected plant life, and many mammals coevolved with plants. One example is grasses evolving along with mammalian herbivores in grassland ecosystems. Figure 32 2 As endotherms, mammals are capable of adjusting their heat internally. When they get too warm, some mammals, such as this gray wolf cub, pant to rid their bodies of excess heat. Figure 32 3 The teeth of certain whales, such as this humpback, have been replaced by huge, stiffened plates called baleen. The fringed baleen strains out small animals and plankton from the mouthfuls of water that the whale takes in. Inferring What kind of animals do humpback whales eat? Form and Function in Mammals The mammalian has adapted in varied ways to a great many habitats. As a member of this class of chordates, you may be familiar with some of these adaptations. Body Temperature Control Like birds, mammals are endotherms; their bodies can generate heat internally. Mammals and birds especially small ones have a much higher metabolic rate than most other chordates. The high rate of metabolism helps mammals generate heat. Mammals also have external hair that helps them keep warm. Hair is part of the integumentary system, which is the outer covering of the the skin and all structures associated with the skin. Subcutaneous (sub-kyoo- TAY-nee-us) fat, which is a layer of fat located beneath the skin, also helps conserve heat. Many mammals have sweat glands that help cool the. Sweating is regulated by an internal negative feedback mechanism, which you learned about in Chapter 26. When its internal temperature becomes too high, the mammal begins to sweat. The evaporation of the sweat then cools the. The mammal then stops sweating. Mammals that lack sweat glands, like the wolf in Figure 32 2, often pant to rid themselves of excess heat. The ability of mammals to regulate their heat from within is an example of homeostasis. This ability also allows mammals to move about in the cold, while most other animals would seek shelter. Feeding Because of its high metabolic rate, a mammal must eat nearly 10 times as much food as a reptile of the same size to maintain homeostasis. Some mammals, such as rabbits and giraffes, eat only plants. Others, including cats and weasels, are meat-eaters. Bears and humans are omnivores, consuming all types of food. Certain whales, like the one in Figure 32 3, are filter feeders. Early mammals ate insects. As mammals evolved, the form and function of their jaws and teeth became adapted to eat foods other than insects. The joint between the skull and lower jaw became stronger than that of reptiles. This joint allowed mammals to evolve larger, more powerful jaw muscles and different ways of chewing. SUPPORT FOR ENGLISH LANGUAGE LEARNERS Comprehension: Key Concept Beginning On the board, rewrite the boldface sentence (paragraph 2, page 821) as individual sentences that each express one characteristic of mammals, e.g., All mammals breathe air. Explain each characteristic. Then, have students construct a concept circle with Characteristics of Mammals in the center and the characteristics connected to the center by lines. Intermediate Read aloud the boldface sentence (paragraph 2, page 821). Ask individual students, including some ESL students, to identify characteristics of mammals. Then, pair ESL students with English-proficient students. Each pair should write a summary, in their own words, of the characteristics of mammals. The student pairs should share their summaries with the class. 822 Chapter 32

FIGURE 32 4 JAWS AND TEETH OF MAMMALS The specialized jaws and teeth of mammals are adapted for different diets. Carnivorous mammals use sharp canines and incisors to grip and slice flesh from their prey. Their jaws usually move up and down as they chew. Herbivorous mammals use flat-edged incisors to grasp and tear vegetation, and flattened molars to grind the food. Their jaws generally move from side to side. Jaw joint CARNIVORE Canines Canines are pointed teeth. Carnivores use them for piercing, gripping, and tearing. In herbivores, they are reduced or absent. Wolf Incisors Chisel-like incisors are used for cutting, gnawing, and grooming. Modern mammals have specialized teeth incisors, canines, molars, and premolars which you can see in Figure 32 4. Observe that the structure of carnivores teeth is different from that of herbivores teeth. Mammals teeth enable food to be processed efficiently. The more efficiently an animal can obtain and process its food, the more energy it can obtain. A mammal s digestive tract breaks down and absorbs the type of food that it eats. Because digestive enzymes can quickly break down meat, carnivores have a relatively short intestine. Tough, fibrous plant tissues take much more time to digest, so most herbivores have a much longer intestine. Many herbivores also have specialized digestive organs to break down plant matter. Cows and their relatives have a stomach chamber called the rumen, in which newly swallowed plant food is stored and processed. The rumen contains symbiotic bacteria that digest the cellulose of most plant tissues. After some time, the grazer regurgitates the food from the rumen into its mouth. The partially digested food is chewed and swallowed again. After several cycles, it moves through the rest of the stomach and into the intestines. HERBIVORE Horse Molars and premolars Molars crush and grind food. The ridged shape of the wolf s molars and premolars allows them to interlock during chewing, like the blades of scissors. The broad, flattened molars and premolars of horses are adapted for grinding tough plants. Jaw joint The word incisor comes from the Latin word incidere, which means to cut. In surgery, what is an incision? Use Visuals Figure 32 4 Have students compare and contrast the herbivore and carnivore teeth in the illustration. Make a Venn diagram on the board to note the similarities and differences. Ask: Why do you think it is advantageous for herbivores to have flat molars? (The grinding motion of the teeth and jaws helps break apart tough plant fibers.) Could a dog successfully live on a diet of grass? (No) Why? (Dogs have the teeth of a carnivore; although the teeth are sharp, they could not effectively grind plant material to break it apart. The dog s digestive system is also not structured to digest and absorb all the nutrients from plant tissue.) Point out how carnivores use their jaws differently from herbivores. Carnivores use mostly up and down motion to chew food. Herbivores use side-to-side motion to make the plant matter (usually long fibrous stems) small enough to be swallowed. Observing Give students a celery stick. Have them chew and swallow the celery. While they chew, challenge students to observe in a mirror how their teeth and jaws work to chew the food. Discuss students observations, focusing on the characteristics of their teeth that enabled them to chew the food. Remind students that they are omnivores with teeth adapted for eating both plants and animals. An incision is a cut made into a tissue or an organ. What is the function of a rumen? Digestion is ruminantary Ruminants quickly chew grass just until it s small enough to swallow. It moves to the first chamber of the stomach, the rumen, which stores large amounts of food. It is here that simple carbohydrates, proteins, and cellulose are broken down by cellulose-digesting microorganisms. While resting, the ruminant regurgitates food from the rumen and rechews it. After the food is reswallowed, it moves into the reticulum, which screens out larger food particles, allowing smaller particles to move into the omasum. There, excess water is removed from the food by squeezing and grinding. Then, food moves into the abomasum, where acids and digestive enzymes break down protein, as in a carnivore s stomach. there, the food enters the intestines, where digestion continues and nutrients and water are absorbed; then wastes are eliminated. Answers to... It stores and processes newly swallowed plant food. Figure 32 3 Plankton and small marine animals Mammals 823

32 1 (continued) Use Visuals NSTA Download a worksheet on mammals for students to complete, and find additional teacher support from NSTA SciLinks. Figure 32 5 Have students trace the path of blood through the heart. They can compare the mammalian heart in Figure 32 5 to the reptilian heart in Figure 31 4 on page 801. Remind students that mammals evolved from reptiles. Ask: How does the mammalian heart differ from the reptilian heart? (The mammalian heart has two separate ventricles; most reptiles have a single ventricle.) Why is it advantageous to have two separate ventricles? (The oxygen-poor blood never mixes with the oxygen-rich blood, so the blood going to the has the highest possible level of oxygen.) Observing Some students might enjoy dissecting a mammalian heart. Obtain a cow, pig, or sheep heart from a butcher. Students also need dissecting tools and a dissecting tray. Students should wear disposable plastic gloves while dissecting the heart and wash their hands thoroughly when finished. Have students diagram the structure of the heart and label its parts. You can display the dissected hearts and the diagrams for the class to observe. NSTA For: Links on mammals Visit: www.scilinks.org Web Code: cbn-9321 Figure 32 5 All mammals, including this brown bear, have a four-chambered heart that pumps blood in two separate circuits around the. Interpreting Graphics According to the diagram, which chamber receives blood that is low in oxygen? Respiration All mammals, even those that live in water, use to breathe. These are controlled by two sets of muscles. Mammals inhale when muscles in the chest lift the rib cage up and outward, increasing the volume of the chest cavity. At the same time, a powerful muscle called the diaphragm (DYuh-fram) pulls the bottom of the chest cavity downward, which further increases its volume. As a result, air is pulled into the. When the chest muscles lower the rib cage and the diaphragm relaxes, the volume of the chest cavity decreases. This action pushes air out of the. Circulation The mammalian circulatory system is divided into two completely separate loops with a four-chambered heart, shown in Figure 32 5. The right side of the heart receives oxygen-poor blood from all over the and pumps it to the. After picking up oxygen in the, blood returns to the left side of the heart. This oxygen-rich blood is then pumped through blood vessels to the rest of the. The two separate circuits one to and from the, and the other to and from the rest of the efficiently transport materials throughout the. Excretion Mammals have highly developed kidneys that help control the composition of fluids. Mammalian kidneys extract nitrogenous wastes from the blood in the form of urea. Urea, other wastes, and water combine to form urine. the kidneys, urine flows to a urinary bladder, where it is stored until it is eliminated. The kidneys of mammals help maintain homeostasis by filtering urea from the blood, as well as by excreting excess water or retaining needed water. They also retain salts, sugars, and other compounds the cannot afford to lose. Because they are so efficient at controlling and stabilizing the amount of water in the, the kidneys enable mammals to live in many habitats, such as deserts, in which they could not otherwise survive. To Right atrium Right ventricle To To Left atrium Left ventricle Complete To division Respiratory system makes noise Mammals make vocalizations for various reasons. Some vocalizations warn other species members of danger. Others are used to find mates or to defend territories. To make these vocalizations, mammals use the respiratory system. As air moves into the pharynx and through the trachea to the, it passes through the larynx. The larynx is the location of the vocal cords. The vocal cords are a pair of folds in the cartilaginous walls of the larynx. The space between these folds is the glottis. When air is expelled from the, it vibrates the vocal cords to produce a sound. The cartilage in the larynx can be highly specialized from one species to another to produce distinct sounds. Speech is produced by shaping vocal sounds into patterns using the mouth, tongue, and lips. 824 Chapter 32

Response Mammals have the most highly developed brains of any animals. As you can see in Figure 32 6, the brain consists of three main parts: the cerebrum, the cerebellum, and the medulla oblongata. The cerebrum makes possible such complicated behaviors as thinking and learning. The cerebellum controls muscular coordination. The medulla oblongata regulates involuntary functions, or those that are not under conscious control, such as breathing and heart rate. A mammal s cerebrum contains a well-developed outer layer called the cerebral cortex, which is the center of thinking and other complex behaviors. Some activities, such as reading this textbook, are possible only with the human cerebral cortex. However, mammals other than humans also exhibit complex behaviors, such as storing food for later use. Mammals rely on highly developed senses to detect and respond to stimuli from their external environment. Many mammals have well-developed senses of smell and hearing. You probably know, for example, that dogs can easily identify people by their particular scent. Although mammalian ears all have the same basic parts, they differ in their ability to detect sound. For example, dogs, bats, and dolphins can detect sounds at much higher frequencies than humans can. In fact, bats and dolphins can find objects in their environment using the echo of their own high-frequency sounds. Other mammals, such as elephants, can detect sounds at much lower frequencies. Many mammals have some color-sensing structures in their eyes, yet the ability to distinguish colors may vary among different species. Color vision is most useful to diurnal animals those that are active during daylight. Although mammals such as cats can detect color, they may not see the full range of colors that humans and some other primates can. What is the function of the cerebral cortex? Chemical Controls The nervous system is not the only system that controls processes. Mammals, like other vertebrates, have endocrine glands that are part of an endocrine system. Endocrine glands regulate activities by releasing chemicals called hormones that affect other organs and tissues. Hormones produced by a gland in a mammal s neck, for example, help regulate the amount of calcium in the bones. Hormones are carried by the blood to the organs that they affect. Fighting Disease All organisms live in an environment that contains disease-causing microorganisms, or pathogens. The immune systems of mammals and other vertebrates function to protect animals from disease. When mammals do get sick, their immune systems help them recover. Mammalian immune systems consist of barriers, such as the skin, that prevent pathogens from entering the. In addition, specialized cells and chemicals recognize and destroy pathogens. Olfactory bulb Cerebrum Optic lobe Cerebellum Medulla oblongata Spinal cord Figure 32 6 Mammals have large brains in proportion to their size. Most of the brain is taken up by an enlarged cerebrum, which contains a well-developed cerebral cortex. Inferring How would a large cerebrum be advantageous to a mammal? Inferring Explain that the number of rods and cones in the retina correlates with whether an animal is nocturnal or diurnal. Rods are photoreceptors that are sensitive to light but do not distinguish color. Cones are photoreceptors that do not function in night vision because they require more light to be stimulated. In daylight, cones distinguish colors. Challenge students to infer the relative compositions of rods and cones in the eyes of nocturnal and diurnal mammals. (Nocturnal animals usually have many more rods than cones because they require sharp vision during the night when they are active. Diurnal animals have more cones, which enable them to have very sharp vision in daylight.) Address Misconceptions Emphasize to students that all vertebrates, not just mammals, have a system for fighting disease. Pathogens, such as fungi, bacteria, and viruses, infect fishes, amphibians, reptiles, and birds, as well as mammals. Explain that as in mammals, other vertebrates have barriers to pathogens that include skin and mucus secretions. Vertebrates also have white blood cells that destroy pathogens. Why a larger mammalian brain? Some scientists think the mammalian brain evolved as a result of the needs of a nocturnal animal. Many reptiles and birds are active during the day and depend substantially on eyesight to find food. Visual information, especially the threedimensional impressions that result from binocular vision, needs little analysis and thus relatively less brain matter. A nocturnal animal, however, must also depend significantly on information from scent and sound. As the animal moves, it must compare and integrate perceptions from three senses, a process that requires a relatively more complex brain. Furthermore, an animal that could associate such information with past events compare past to present might have a selective advantage. Answers to... The cerebral cortex is the center of thinking and other complex behaviors. Figure 32 5 The right atrium Figure 32 6 The increased ability for complex thinking increases a mammal s adaptability. Mammals 825

32 1 (continued) Use Visuals Figure 32 7 Have students compare the mammalian limbs in Figure 32 7. Ask: What limb characteristics are common to mammals that run? (Longer, less flexible, no side digits) How do the limbs of digging mammals differ from those of climbing mammals? (Climbing mammals have longer, more slender digits and limbs. Digging mammals have shorter, thicker digits and stocky limbs.) How are the limbs of swimming mammals similar to those of flying mammals? (Both have limbs and digits that are modified to support either the flipper or flaps of skin that form the wings.) Classifying Provide skeletons of different mammals for students to examine. You can use diagrams if skeletons are not available. Challenge students to divide the skeletons into groups based on how the mammals move. Students should identify the characteristics of the skeletons that helped them make their classifications. Share with students the identity of each skeleton. Point out the features of the skeleton that could be used to make the correct classification. Use Community Resources Invite a zookeeper or zoologist to the class to describe the adaptive significance of maternal care in mammals. Ask the speaker to contrast the number of offspring produced by mammals with that of other vertebrates and to explain the current thinking that describes why mammals expend so much energy for maternal care. Figure 32 7 The limbs and digits (fingers and toes) of many mammals are adapted to their particular way of life. Note the variety of lengths and shapes of the limb bones that different mammals use for movement. Homologous bones are the same color in all the drawings. Applying Concepts Which structure shown in this figure would most closely resemble the limbs and digits of a whale? Figure 32 8 Still wobbly, a newborn wildebeest rises to its feet minutes after birth. Its mother will nurse and protect the calf until it is able to live on its own. Monkey Climbers Climbing mammals have long, flexible fingers and toes that can grasp vines and branches. They also have a flexible wrist joint. Horse Runners Running mammals need long limbs that can absorb shock. These animals have lost the side digits on their front and back feet. They stand on the tips of their remaining toes, which are called hooves. Movement Mammals have evolved a variety of adaptations that aid in movement, including a backbone that flexes both vertically and side to side. This flexibility allows mammals to move with a bouncing, leaping stride. Shoulder and pelvic girdles have become more streamlined and flexible, permitting both front and hind limbs to move in a variety of ways. Compare the adaptations of mammalian limbs shown in Figure 32 7. Variations in the limb bones and muscles allow mammals to run, walk, climb, burrow, hop, pounce, swing, fly, leap, and swim. Depending on their lifestyle, mammals may use any number of these methods to move about. Reproduction Mammals reproduce by internal fertilization. The male deposits sperm inside the reproductive tract of the female, where fertilization occurs. As you will learn in the next section, mammals are classified into three groups, based on their modes of development and birth. Regardless of the mode of development, all newborn mammals, such as the newborn wildebeest in Figure 32 8, feed on their mother s milk. Young mammals generally need care when they are born and for a long time afterward. During this period, they are cared for by one or both parents. Maternal care is an important mammalian characteristic, and the bond between mother and young is very close. Males of many species also play a role in caring for the young. Parental care helps ensure that young mammals will survive and reproduce. Mammalian parental behavior is an adaptation that is the result of natural selection and other evolutionary processes. The duration and intensity of parental care varies among different species. Some mammals have a prolonged period when the young and the mother live together. During that period, the juvenile learns from its caregiver the behaviors it needs to survive. Elephants care about their young Elephants have complex social interactions and behaviors associated with raising their young. The gestation period for elephants is about 22 months. Females first start mating around the age of 20 years. They will continue to have calves every two to four years until they reach about 50 years of age. Calves are highly dependent on their mothers for food for their first two years. These calves are cared for not only by their mothers but also by the other females in the herd. Older females help new mothers. Younger females play with the babies; this helps the young females prepare to become mothers. When a baby has finished weaning from its mother, it reaches an age of adolescence. Young males leave the herd, often joining bachelor herds. Young females stay with the herd and help care for other newborns. 826 Chapter 32

Mole Bat Seal Diggers Digging mammals have strong, thick claws, especially on their front feet. Their limbs are short and stocky, with large projections that anchor powerful muscles. Flyers The arms and hands of bats are modified to support flaps of skin that form wings. Some mammal species, such as lions and elephants, live in groups in which the young may be cared for by adults other than the parents. Group living provides young mammals with the opportunity for complex social interaction among adults and juveniles. Interrelationships of Organ Systems In mammals and other animals, organ systems are interdependent in order to maintain a homeostatic environment. All systems depend on the circulatory system to transport materials. The respiratory system, for example, ensures that oxygen enters the, but the blood carries oxygen to cells. Similarly, blood carries waste products to the kidneys, which remove the waste products from the. Nerve impulses from cells in the nervous system carry information to and from organs in every system. The bones of the skeletal system could not grow and maintain themselves without calcium and other materials that enter the through the digestive system. An animal s organ systems work together to meet the needs of the as a whole. Swimmers Swimming mammals concentrate most of their movement between the arm and shoulder girdle. Their limbs are modified into broad, flat paddles, with the bones of their hands or feet extended to make a flipper. Applying Concepts Invite students to think about eating. Ask: What organ systems are involved in eating? (Nervous system: smelling, tasting, stimulating bringing food into mouth and chewing; muscular system: moving jaws for chewing, transporting food into stomach; skeletal system: jaws and teeth for chewing; digestive system: digesting food and absorbing nutrients; circulatory system: carrying nutrients to cells; excretory system: excreting wastes) 3 ASSESS Evaluate Understanding Invite student volunteers to write on the board one characteristic of mammals. When students have exhausted all their ideas, add to the list, if necessary, to make it complete. Then, ask students to match the characteristics to the life functions that mammals perform. Reteach Have student pairs study the mammals pictured in this section and make a list of characteristics that mammals have in common. Students can compare their lists with the description of mammals given at the beginning of the section. 32 1 Section Assessment 1. Key Concept Name the characteristics that are common to all mammals. 2. Key Concept When did mammalian ancestors diverge from the other reptiles? 3. Key Concept List two ways in which mammals maintain homeostasis. 4. What is the function of the endocrine system? 5. Critical Thinking Comparing and Contrasting Compare the functions of the respiratory and circulatory systems. Then, explain how the structure of a mammal s heart helps these two systems work together to deliver oxygen to cells. Structure and Function Compare the structure of a mammal s brain to that of a fish, as shown in Chapter 30, Figure 30 14. What structures are more prominent in each animal s brain? How might these differences relate to the way the animals live? In mammals, the cerebrum is much larger, relative to the rest of the brain, than in fishes. In contrast, fishes have much larger olfactory bulbs. Mammals depend heavily on behaviors such as learning that are regulated by the cerebral cortex. Fishes are highly dependent on the sense of smell to capture prey. 32 1 Section Assessment 1. Hair, mammary glands, breathe air, have four-chambered hearts, endotherms 2. During the Carboniferous Period 3. Sample answers: regulating heat from within, excreting or retaining liquid with the kidneys, eating a variety of foods 4. Regulates activities by releasing hormones 5. The respiratory system brings air (oxygen) into the. The circulatory system carries oxygen to the and oxygen-poor blood to the. The heart pumps blood to and from the and to and from the so that oxygen is delivered to cells and oxygen-poor blood is delivered to the. If your class subscribes to the itext, use it to review the Key Concepts in Section 32 1. Answer to... Figure 32 7 The limbs of a seal, a swimming mammal Mammals 827