828 Chapter 32

1 FOCUSObjectives32.2.1 Explain how the three

groups of living mammalsdiffer from one another.

32.2.2 Name the major orders ofplacental mammals.

32.2.3 Describe how convergentevolution caused mammalson different continents to besimilar in form and function.

Vocabulary PreviewSay each Vocabulary term aloud andhave students divide the word intosyllables as best they can. Remindstudents that each syllable usuallyhas only one vowel sound.(mon•o•treme, mar•su•pi•al, andpla•cen•ta)

Reading StrategyHave students make a table with theheadings Monotreme, Marsupial, andPlacental. Before they read, tell stu-dents to write what they know aboutthese mammal groups. After theyread, students should add additionalinformation and correct any miscon-ceptions they had.

2 INSTRUCT

Monotremes andMarsupialsBuild Science SkillsComparing and ContrastingHave students make a Venn diagramto compare and contrast the charac-teristics of monotremes and reptiles.In the overlap area, students shouldlist features that monotremes andreptiles share (cloaca, females lay soft-shelled eggs that incubate outsidebody). In the separate areas of the cir-cles, students should writecharacteristics specific to reptiles(many do not care for young, ectother-mic, have scales) and those specific tomonotremes (nourish young with milkfrom body, endothermic, have hair).

Section 32–2

The class Mammalia contains about 4500 species, andthe

diversity of these species is astonishing. From a tiny mouse

nibbling its way along a corncob to an African elephant uproot-

ing a gigantic tree with its tusks and trunk, mammals have the

greatest range of size of any group of vertebrates.

As you have read, tooth structure is one characteristic that

scientists use to classify mammals. Mammals are also classified

by the number and kinds of bones in the head. But the most

important way to categorize living mammals is by the way they

reproduce and develop.

The three groups of living mammals are the monotremes

(MAHN-oh-treemz), the marsupials (mahr-SOO-pee-ulz), and

the placentals. These three groups differ greatly in their means

of reproduction and development.

Monotremes and MarsupialsMonotremes lay eggs. Marsupials bear live young, but

at a very early stage of development. All monotremes are

grouped in a single order, while marsupials are split into several

different orders.

Monotremes Members of the or egg-laying

mammals, share two notable characteristics with reptiles. In

monotremes, the digestive, reproductive, and urinary systems

all open into a cloaca that is similar to the cloaca of reptiles. In

fact, the name monotreme means “single opening.” Reproduction

in monotremes also resembles reproduction in reptiles more

than other mammals. As in reptiles, a female monotreme lays

soft-shelled eggs that are incubated outside her body. The eggs

hatch into young animals in about ten days. Unlike young

reptiles, however, young monotremes are nourished by

their mother’s milk, which they lick from pores on the

surface of her abdomen.

Only three species of monotremes exist today: the

duckbill platypus, shown in Figure 32–9, and two

species of spiny anteaters, or echidnas. These

animals are found in Australia and New Guinea.

monotremes,

32–2 Diversity of Mammals

Key Concepts• How do the three groups of

living mammals differ fromone another?

• How did convergent evolutioncause mammals on differentcontinents to be similar inform and function?

Vocabularymonotrememarsupialplacenta

Reading Strategy: Summarizing As you read,make a list of the major groupsof mammals. Write severalsentences describing thecharacteristics of each group.Then, give an example for each.

� Figure 32–9 Like all monotremes, theplatypus lays eggs that hatch outside thebody but nourishes its young with milkproduced in mammary glands. The unusualsnout of this duckbill platypus can senseelectromagnetic signals put out by the musclesof other animals. The platypus uses its sensitivesnout to locate prey, such as worms andmollusks, that burrow in the sediments.

SECTION RESOURCES

Print:

• Teaching Resources, Lesson Plan 32–2,Adapted Section Summary 32–2, AdaptedWorksheets 32–2, Section Summary 32–2,Worksheets 32–2, Section Review 32–2

• Reading and Study Workbook A, Section 32–2• Adapted Reading and Study Workbook B,

Section 32–2

• Issues and Decision Making, Issues andDecisions, 32

Technology:

• iText, Section 32–2• Transparencies Plus, Section 32–2Ti

m

eSaver

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Make ConnectionsEnvironmental Science Explain tostudents that populations of marsu-pials in Australia and New Guinea aredeclining. This decline is caused inpart by the introduction of rats,sheep, and rabbits to the continent.Ask: These placental mammals thathave been introduced are notpredators. How could they cause adecline in the population of nativemarsupials? (These mammals fillmany of the same niches and competefor the same resources as the nativemarsupials, so there are fewerresources available for the native mar-supials.) Why do you think there arenot many marsupials in other partsof the world? (The placental mam-mals outcompeted the marsupials forresources in the environment. The mar-supials might also have been easierprey for predators.)

Placental MammalsBuild Science SkillsUsing Models Challenge studentgroups to construct a model of a pla-centa with materials such asmodeling clay, pipe cleaners, yarn,netting, tape, or glue. Display photo-graphs or diagrams of a developingplacental embryo as a guide. Havegroups describe how the placentaworks to protect the developingembryo and transfer nutrients to andwastes away from it. Ask: How doesthe placenta allow an embryo todevelop longer than embryos thatdevelop inside eggs? (The egg hasfinite sources of energy for growth anda finite space for storing the wastesproduced. The placenta provides anunlimited supply of energy—from themother’s food intake—and the abilityto continually remove wastes via themother’s body.)

Mammals 829

Figure 32–10Most marsupi-

als, including thiswallaby, are origi-

nally from Australia andNew Guinea. Marsupials bearlive young that usually completetheir development in a pouch. Thepink, newborn wallaby (inset) is stillan embryo but will soon grow into a“joey” that resembles a small adult.

Marsupials Kangaroos, koalas, and wombats are examples of

—mammals bearing live young that usually com-

plete their development in an external pouch. When marsupials

reproduce, the fertilized egg develops into an embryo inside the

mother’s reproductive tract. The embryo is born at a very early

stage of development. It crawls across its mother’s fur and

attaches to a nipple. In most species of marsupials, the nipples

are located in a pouch called the marsupium (mahr-SOO-pee-um)

on the outside of the mother’s body. Marsupials are named after

this structure. Once inside the marsupium, the embryo, looking

much like the one in Figure 32–10, spends several months

attached to the nipple. It will continue to drink milk in its

mother’s pouch until it grows large enough to survive on its own.

How does a marsupial differ from a monotreme?

Placental MammalsPlacental mammals are the mammals with which you are

most familiar. Mice, cats, dogs, whales, elephants, humans,

and the sea lions in Figure 32–11 all fall within this category.

This group gets its name from an internal structure called the

which is formed when the embryo’s tissues join with

tissues from within the mother’s body.

In placental mammals, nutrients, oxygen, carbondioxide, and wastes are exchanged efficiently betweenembryo and mother through the placenta. The placenta

allows the embryo to develop for a much longer time inside the

mother—from a few weeks in mice and rats to as long as two

years in elephants. After birth, most placental mammals care

for their young and provide them with nourishment by nursing.

Figure 32–12, on the following pages, describes the main orders

of placental mammals.

placenta,

marsupials

� Figure 32–11 The California sea lion is an example of a placentalmammal. In placental mammals,nutrients, oxygen, carbon dioxide,and wastes are exchanged betweenembryo and mother through theplacenta.

Answer to . . . Monotremes lay eggs;

marsupials bear live young.

Inclusion/Special NeedsFind out what students already know aboutmonotremes and marsupials. Many will haveheard about the duck-bill platypus. Show themvideos of the platypus and echidnas, as well asmarsupials. As they watch the videos, encouragethem to note how the animals care for theiryoung and features that classify them as mam-mals. Discuss reasons why these mammals arefound in Australia and New Guinea.

English Language LearnersStudents can make a glossary for the mam-malian orders. In their glossaries, they canillustrate common members of each order andwrite descriptions of the mammals in Englishand their native languages. Under the name ofthe order, students can also write in English andtheir native languages the names of animalsbelonging to the order.

UNIVERSAL ACCESS

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FIGURE 32–12 ORDERS OF PLACENTAL MAMMALS

RODENTS �Rodents have a single pair

of long, curved incisor teeth

in both their upper and lower

jaws, which they use for

gnawing wood and other tough

plant material. Examples: mice, rats

(shown here), voles, squirrels, beavers, porcupines, gophers,

chipmunks, gerbils, prairie dogs, chinchillas.

� SIRENIANSSirenians are herbivores that live in rivers, bays, and warm

coastal waters scattered throughout most of the world.

These large, slow-moving mammals lead fully aquatic lives.

Examples: manatees, dugongs (shown here).

� INSECTIVORESThese insect eaters

have long, narrow

snouts and sharp

claws that are well

suited for digging.

Examples: shrews,

hedgehogs (shown

here), moles.

� CHIROPTERANSWinged mammals—or bats—

are the only mammals capable

of true flight. Bats account for

about one-fifth of all mam-

malian species. They eat

mostly insects or fruit and

nectar, although three species

feed on the blood of other

vertebrates.

� PERISSODACTYLSThis order contains hoofed animals with an odd number of

toes on each foot. Examples: horses, tapirs, rhinoceroses,

and zebras (shown here).

The 12 orders of mammals shown on these pages contain

the vast majority of living placental species. ClassifyingHow are perissodactyls similar to artiodactyls? How are thetwo orders different?

� CETACEANSLike sirenians, cetaceans—the order that includes whales

and dolphins—are adapted to underwater life yet must

come to the surface to breathe. Most cetaceans live and

breed in the ocean. Examples: humpback whales (shown

here), narwhals, sperm whales, beluga whales, river

dolphins.

830 Chapter 32

Use VisualsFigure 32–12 Have students exam-ine the photographs and read thecaption for each mammalian order.Ask: What characteristics are usedto classify placental mammals?(Feeding habits, teeth, limbs) Whichorders of mammals have hooves?(Artiodactyls and perissodactyls)Which order has a single pair oflong, curved incisor teeth in theupper and lower jaws? (Rodents)What mammal belongs to theorder Chiroptera? (Bats) To whichorder do you belong? (Primates)

Build Science SkillsClassifying Give students 10 pic-tures of different mammals. Ifpossible, include mammals that stu-dents are not familiar with. Challengestudents to use the information inFigure 32–12 to identify the order towhich each mammal belongs.Students should list the characteris-tics of each mammal on which theybased their classification.

Use Community ResourcesTake the class to the zoo. Beforegoing, have students use the infor-mation in Figure 32–12 to create afield guide for placental mammals.Students should use their field guideat the zoo to identify the mammalianorder to which each mammalbelongs.

To help my students realize how diverse mam-mals are, I break them into small groups andgive each group a picture of a mammal that rep-resents a particular mammalian order. First, Iinstruct the groups to list the characteristics oftheir animal that identify it as a mammal. Then, Ihave the groups list the characteristics that maketheir animal different from other mammals. Eachgroup creates a poster of their mammalian orderthat describes the characteristics of mammals

belonging to that order. Groups present theirposters to the class. The visualization of themany mammalian orders around the room reallyimpresses upon the students the diversity ofmammals.

—Heidi BusaBiology TeacherMarcellus High SchoolMarcellus, NY

TEACHER TO TEACHER

32–2 (continued)

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Address MisconceptionsSome students might think thatwhales are fishes, not mammals.Review the characteristics of fishes.(Scales, gills, ectotherms, lay eggs) Ask:How do whales breathe? (Theybreathe air with lungs by coming to thesurface of the water.) Do whales layeggs or give birth to live young?(Give birth to live young that theynourish with their own milk) If stu-dents do not know the answers tothese questions, show them picturesof whales illustrating these behaviors,or show them a video about whales.

Make ConnectionsEnvironmental Science Show stu-dents pictures of various endangeredmammals, such as kangaroos, goril-las, cheetahs, rhinoceroses, andmanatees. Discuss why the popula-tions of these mammals aredeclining. Some reasons include lossof habitat, poaching, competition forresources with nonnative mammals,and accidental injury caused byincreased boat traffic. Ask: What isthe ultimate cause for the declinein these mammal populations? (Theactivities of people)

Mammals 831

� PRIMATESMembers of this order are

closely related to the

ancient insectivores but

have a highly developed

cerebrum and complex

behaviors. Examples:lemurs, tarsiers, apes,

gibbons, macaques

(shown here), humans.

� LAGOMORPHSLike rodents, members of this order are

entirely herbivorous. They differ from

rodents by having two pairs of incisors

in the upper jaw. Most lagomorphs

have hind legs that are adapted for

leaping. Examples: Snowshoe hares

(shown here), rabbits.

� ARTIODACTYLSThese hoofed mammals

have an even number of

toes on each foot. Like

perissodactyls, this order

contains mostly large,

grazing animals. Examples:cattle, sheep, goats, pigs,

ibex (shown here), giraffes,

hippopotami, camels,

antelope, deer, gazelles.

� CARNIVORESMany mammals in this order, such as tigers and hyenas, stalk

or chase their prey by running or pouncing, then kill the prey

with sharp teeth and claws. Some animals in this group eat

plants as well as meat. Examples: dogs, foxes, bears,

raccoons, walruses (shown here).

� XENARTHRANSMost of the mammals in this order have

simple teeth without enamel, and a few have

no teeth at all. Examples: sloths, anteaters,

armadillos (shown here).

� PROBOSCIDEANSThese are the mammals with trunks. Some time ago, this order went

through an extensive adaptive radiation that produced many species,

including mastodons and mammoths, which are now extinct. Only two

species, the Asian elephant and this African elephant, survive today.

Answer to . . . Figure 32–12 Both perissodactylsand artiodactyls have feet with hoofs.Perissodactyls have an odd number oftoes; artiodactyls have an even number.

Aristotle classifies animalsDuring the fourth century B.C., the Greek philoso-pher Aristotle was the first to classify animalsbased on their own characteristics rather thanwhether they were helpful or harmful to people.He published very detailed observations of theanimals (and plants) that he knew. These observa-tions led him to divide animals into two groups:those with blood and those without blood.

In addition, he recognized that some animalshave lungs, breathe air, are warm-blooded, andnourish their young with milk. Although he rec-ognized that whales, dolphins, and porpoises hadmammalian characteristics, he placed them in agroup separate from mammals and fishes.

HISTORY OF SCIENCE

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Biogeography of MammalsThe history of Earth’s geography has helped shape today’s

mammals. During the Paleozoic Era, the continents were one

large landmass, and mammals could migrate freely across it. But

as the continents drifted farther and farther apart during the

Mesozoic and early Cenozoic Eras, ancestors of mammal groups

were isolated from one another. Each landmass took with it a

unique array of mammal groups.

Similar ecological opportunities on the differentcontinents have produced some striking examples ofconvergent evolution in mammals. Thousands of kilometers

apart, mammals such as those in Figure 32–13 evolved similar

adaptations in form and function. When some of the landmasses

merged in the late Cenozoic Era, mammals dispersed and

intermingled in new habitats. Living mammals reflect the

diversity that resulted from these events.

� Figure 32–13 Similarecological opportunities ondifferent continents have resultedin convergent evolution amongthese and other mammals.Mammals that feed on ants andtermites evolved not once but fivetimes in different regions. Powerfulfront claws; a long, hairless snout;and a tongue covered with stickysaliva are common adaptations inthese insect-eating animals.

AUSTRALIA

EUROPE

ANTARCTICA

A S I A

INDIANOCEAN

PACIFICOCEAN

PACIFICOCEAN

NORTHAMERICA

A F R I C A

SOUTHAMERICA

ATLANTICOCEAN

Nine-BandedArmadillo

Giant Anteater

Aardvark

ChinesePangolin

CommonEchidna

1. Key Concept Name thethree groups of living mammalsand describe the ways eachdevelops.

2. Key Concept With regardto mammals, what was the resultof continental drift?

3. What is the function of theplacenta?

4. List the major orders of placentalmammals.

5. What characteristic distinguisheslagomorphs from rodents?

6. Critical Thinking InferringHow are powerful front claws andsticky tongues useful adaptationsin mammals that feed on ants?

Comparing andContrastingCreate a compare-and-contrasttable that describes thecharacteristics of monotremes,marsupials, and placentalmammals. Include characteris-tics that they share as well asways in which they differ.

32–2 Section Assessment

832 Chapter 32

Biogeography ofMammalsUse VisualsFigure 32–13 Remind students thatin convergent evolution, species thatlive in similar environments in differ-ent locations evolve similaradaptations. Ask: What food dothese mammals eat? (Ants and ter-mites) What adaptations to eatingants do these mammals have incommon? (Long, hairless snouts; long,sticky tongues; strong, sturdy claws fordigging) What other adaptations dosome of these mammals have?(Scales or spines for protection againstpredators)

3 ASSESSEvaluate UnderstandingCall on students at random todescribe the differences betweenmonotremes, marsupials, and placen-tal mammals. Challenge students toname the orders of placental mam-mals and give an example of each.

ReteachEncourage students to use Figure32–12 to help them construct a tablethat lists the name of each mam-malian order, its distinguishingcharacteristics, and an example.

32–2 (continued)

Monotremes lay eggs; marsupialsbear tiny live young that completetheir development in the mother’spouch; the young of placentalmammals develop for a long timebefore birth. Similarities include allthe mammalian characteristics: furor hair, feeding young with milk,endothermy, four-chamberedheart, and breathing air.

If your class subscribes to the iText,use it to review the Key Concepts inSection 32–2.

32–2 Section Assessment1. Monotremes: lay eggs; marsupials: immature

young finish developing in marsupium; pla-cental mammals: embryo, supported by theplacenta, develops completely inside mother

2. Convergent evolution led to mammals withsimilar adaptations in form and function.

3. Nutrients and oxygen pass from the motherto the embryo through the placenta. Carbondioxide and other wastes pass from theembryo to the mother through the placenta.

4. Insectivores, sirenians, cetaceans, chiropter-ans, rodents, perissodactyls, artiodactyls,carnivores, lagomorphs, xenarthrans, pri-mates, proboscideans

5. Lagomorphs have two pairs of incisors;rodents have one pair.

6. Powerful front claws dig into ant colonies.Ants adhere to sticky tongues.

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