Section 32–2
32–2 Diversity of Mammals
1 FOCUS
Objectives
32.2.1 Explain how the three
groups of living mammals
differ from one another.
32.2.2 Name the major orders of
placental mammals.
32.2.3 Describe how convergent
evolution caused mammals
on different continents to be
similar in form and function.
Vocabulary Preview
Say each Vocabulary term aloud and
have students divide the word into
syllables as best they can. Remind
students that each syllable usually
has only one vowel sound.
(mon•o•treme, mar•su•pi•al, and
pla•cen•ta)
T
Key Concepts
• How do the three groups of
living mammals differ from
one another?
• How did convergent evolution
cause mammals on different
continents to be similar in
form and function?
Vocabulary
monotreme
marsupial
placenta
Reading Strategy:
Summarizing As you read,
make a list of the major groups
of mammals. Write several
sentences describing the
characteristics of each group.
Then, give an example for each.
he class Mammalia contains about 4500 species, and the
diversity of these species is astonishing. From a tiny mouse
nibbling its way along a corncob to an African elephant uprooting 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 Marsupials
Monotremes 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 monotremes, 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.
Reading Strategy
Have students make a table with the
headings Monotreme, Marsupial, and
Placental. Before they read, tell students to write what they know about
these mammal groups. After they
read, students should add additional
information and correct any misconceptions they had.
2 INSTRUCT
Monotremes and
Marsupials
Figure 32–9
Like all monotremes, the
platypus lays eggs that hatch outside the
body but nourishes its young with milk
produced in mammary glands. The unusual
snout of this duckbill platypus can sense
electromagnetic signals put out by the muscles
of other animals. The platypus uses its sensitive
snout to locate prey, such as worms and
mollusks, that burrow in the sediments.
Build Science Skills
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Chapter 32
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Comparing and Contrasting
Have students make a Venn diagram
to compare and contrast the characteristics of monotremes and reptiles.
In the overlap area, students should
list features that monotremes and
reptiles share (cloaca, females lay softshelled eggs that incubate outside
body). In the separate areas of the circles, students should write
characteristics specific to reptiles
(many do not care for young, ectothermic, have scales) and those specific to
monotremes (nourish young with milk
from body, endothermic, have hair).
Make Connections
Marsupials Kangaroos, koalas, and wombats are examples of
marsupials —mammals bearing live young that usually complete 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 Mammals
Placental 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
placenta, which is formed when the embryo’s tissues join with
tissues from within the mother’s body.
In placental mammals, nutrients, oxygen, carbon
dioxide, and wastes are exchanged efficiently between
embryo 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.
Figure 32–10
Most marsupials, including this
wallaby, are originally from Australia and
New Guinea.
Marsupials bear
live young that usually complete
their development in a pouch. The
pink, newborn wallaby (inset) is still
an embryo but will soon grow into a
“joey” that resembles a small adult.
Figure 32–11
The California
sea lion is an example of a placental
mammal.
In placental mammals,
nutrients, oxygen, carbon dioxide,
and wastes are exchanged between
embryo and mother through the
placenta.
Inclusion/Special Needs
Find out what students already know about
monotremes and marsupials. Many will have
heard about the duck-bill platypus. Show them
videos of the platypus and echidnas, as well as
marsupials. As they watch the videos, encourage
them to note how the animals care for their
young and features that classify them as mammals. Discuss reasons why these mammals are
found in Australia and New Guinea.
English Language Learners
Students can make a glossary for the mammalian orders. In their glossaries, they can
illustrate common members of each order and
write descriptions of the mammals in English
and their native languages. Under the name of
the order, students can also write in English and
their native languages the names of animals
belonging to the order.
Environmental Science Explain to
students that populations of marsupials in Australia and New Guinea are
declining. This decline is caused in
part by the introduction of rats,
sheep, and rabbits to the continent.
Ask: These placental mammals that
have been introduced are not
predators. How could they cause a
decline in the population of native
marsupials? (These mammals fill
many of the same niches and compete
for the same resources as the native
marsupials, so there are fewer
resources available for the native marsupials.) Why do you think there are
not many marsupials in other parts
of the world? (The placental mammals outcompeted the marsupials for
resources in the environment. The marsupials might also have been easier
prey for predators.)
Placental Mammals
Build Science Skills
Using Models Challenge student
groups to construct a model of a placenta with materials such as
modeling clay, pipe cleaners, yarn,
netting, tape, or glue. Display photographs or diagrams of a developing
placental embryo as a guide. Have
groups describe how the placenta
works to protect the developing
embryo and transfer nutrients to and
wastes away from it. Ask: How does
the placenta allow an embryo to
develop longer than embryos that
develop inside eggs? (The egg has
finite sources of energy for growth and
a finite space for storing the wastes
produced. The placenta provides an
unlimited supply of energy—from the
mother’s food intake—and the ability
to continually remove wastes via the
mother’s body.)
Answer to . . .
Monotremes lay eggs;
marsupials bear live young.
Mammals
829
32–2 (continued)
FIGURE 32–12 ORDERS OF PLACENTAL MAMMALS
Use Visuals
The 12 orders of mammals shown on these pages contain
the vast majority of living placental species. Classifying
Figure 32–12 Have students examine the photographs and read the
caption for each mammalian order.
Ask: What characteristics are used
to classify placental mammals?
(Feeding habits, teeth, limbs) Which
orders of mammals have hooves?
(Artiodactyls and perissodactyls)
Which order has a single pair of
long, curved incisor teeth in the
upper and lower jaws? (Rodents)
What mammal belongs to the
order Chiroptera? (Bats) To which
order do you belong? (Primates)
Build Science Skills
How are perissodactyls similar to artiodactyls? How are the
two orders different?
INSECTIVORES
These insect eaters
have long, narrow
snouts and sharp
claws that are well
suited for digging.
Examples: shrews,
hedgehogs (shown
here), moles.
SIRENIANS
Sirenians 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).
Classifying Give students ten pictures of different mammals. If
possible, include mammals that students are not familiar with. Challenge
students to use the information in
Figure 32–12 to identify the order to
which each mammal belongs.
Students should list the characteristics of each mammal on which they
based their classification.
Use Community Resources
Take the class to the zoo. Before
going, have students use the information in Figure 32–12 to create a
field guide for placental mammals.
Students should use their field guide
at the zoo to identify the mammalian
order to which each mammal
belongs.
CETACEANS
Like 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.
CHIROPTERANS
Winged mammals—or bats—
are the only mammals capable
of true flight. Bats account for
about one fifth of all mammalian species. They eat
mostly insects or fruit and
nectar, although three species
feed on the blood of other
vertebrates.
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.
PERISSODACTYLS
This order contains hoofed animals with an odd number of
toes on each foot. Examples: horses, tapirs, rhinoceroses,
and zebras (shown here).
TEACHER TO TEACHER
To help my students realize how diverse mammals are, I break them into small groups and
give each group a picture of a mammal that represents a particular mammalian order. First, I
instruct the groups to list the characteristics of
their animal that identify it as a mammal. Then, I
have the groups list the characteristics that make
their animal different from other mammals. Each
group creates a poster of their mammalian order
that describes the characteristics of mammals
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Chapter 32
belonging to that order. Groups present their
posters to the class. The visualization of the
many mammalian orders around the room really
impresses upon the students the diversity of
mammals.
—Heidi Busa
Biology Teacher
Marcellus High School
Marcellus, NY
Address Misconceptions
CARNIVORES
Many 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).
ARTIODACTYLS
These 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.
XENARTHRANS
Most 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).
PRIMATES
Members 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.
PROBOSCIDEANS
These 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.
Some students might think that
whales are fishes, not mammals.
Review the characteristics of fishes.
(Scales, gills, ectotherms, lay eggs) Ask:
How do whales breathe? (They
breathe air with lungs by coming to the
surface of the water.) Do whales lay
eggs or give birth to live young?
(Give birth to live young that they
nourish with their own milk) If students do not know the answers to
these questions, show them pictures
of whales illustrating these behaviors,
or show them a video about whales.
Make Connections
Environmental Science Show students pictures of various endangered
mammals, such as kangaroos, gorillas, cheetahs, rhinoceroses, and
manatees. Discuss why the populations of these mammals are
declining. Some reasons include loss
of habitat, poaching, competition for
resources with nonnative mammals,
and accidental injury caused by
increased boat traffic. Ask: What is
the ultimate cause for the decline
in these mammal populations? (The
activities of people)
LAGOMORPHS
Like 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.
HISTORY OF SCIENCE
Aristotle classifies animals
During the fourth century B.C., the Greek philosopher Aristotle was the first to classify animals
based on their own characteristics rather than
whether they were helpful or harmful to people.
He published very detailed observations of the
animals (and plants) that he knew. These observations led him to divide animals into two groups:
those with blood and those without blood.
In addition, he recognized that some animals
have lungs, breathe air, are warm-blooded, and
nourish their young with milk. Although he recognized that whales, dolphins, and porpoises had
mammalian characteristics, he placed them in a
group separate from mammals and fishes.
Answer to . . .
Figure 32 –12 Both perissodactyls
and artiodactyls have feet with hoofs.
Perissodactyls have an odd number of
toes; artiodactyls have an even number.
Mammals
831
32–2 (continued)
Biogeography of
Mammals
Use Visuals
Figure 32 –13 Remind students that
in convergent evolution, species that
live in similar environments in different locations evolve similar
adaptations. Ask: What food do
these mammals eat? (Ants and termites) What adaptations to eating
ants do these mammals have in
common? (Long, hairless snouts; long,
sticky tongues; strong, sturdy claws for
digging) What other adaptations do
some of these mammals have?
(Scales or spines for protection against
predators)
3 ASSESS
Evaluate Understanding
Call on students at random to
describe the differences between
monotremes, marsupials, and placental mammals. Challenge students to
name the orders of placental mammals and give an example of each.
NORTH
AMERICA
Nine-Banded
Armadillo
ASIA
PACIFIC
OCEAN
AFRICA
PACIFIC
OCEAN
SOUTH
AMERICA
INDIAN
OCEAN
AUSTRALIA
Common
Echidna
ANTARCTICA
Giant Anteater
Aardvark
Figure 32–13
Similar
ecological opportunities on
different continents have resulted
in convergent evolution among
these and other mammals.
Mammals that feed on ants and
termites evolved not once but five
times in different regions. Powerful
front claws; a long, hairless snout;
and a tongue covered with sticky
saliva are common adaptations in
these insect-eating animals.
Reteach
Encourage students to use Figure
32–12 to help them construct a table
that lists the name of each mammalian order, its distinguishing
characteristics, and an example.
Biogeography of Mammals
The 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 different
continents have produced some striking examples of
convergent 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.
32–2 Section Assessment
1.
Monotremes lay eggs; marsupials
bear tiny live young that complete
their development in the mother’s
pouch; the young of placental
mammals develop for a long time
before birth. Similarities include all
the mammalian characteristics: fur
or hair, feeding young with milk,
endothermy, four-chambered
heart, and breathing air.
Chinese
Pangolin
EUROPE
ATLANTIC
OCEAN
Key Concept Name the
three groups of living mammals
and describe the ways each
develops.
2.
Key Concept With regard
to mammals, what was the result
of continental drift?
3. What is the function of the
placenta?
4. List the major orders of placental
mammals.
5. What characteristic distinguishes
lagomorphs from rodents?
6. Critical Thinking Inferring
How are powerful front claws and
sticky tongues useful adaptations
in mammals that feed on ants?
Comparing and
Contrasting
Create a compare-and-contrast
table that describes the
characteristics of monotremes,
marsupials, and placental
mammals. Include characteristics that they share as well as
ways in which they differ.
32 –2 Section Assessment
If your class subscribes to the iText,
use it to review the Key Concepts in
Section 32 –2.
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Chapter 32
1. Monotremes: lay eggs; marsupials: immature
young finish developing in marsupium; placental mammals: embryo, supported by the
placenta, develops completely inside mother
2. Convergent evolution led to mammals with
similar adaptations in form and function.
3. Nutrients and oxygen pass from the mother
to the embryo through the placenta. Carbon
dioxide and other wastes pass from the
embryo to the mother through the placenta.
4. Insectivores, sirenians, cetaceans, chiropterans, rodents, perissodactyls, artiodactyls,
carnivores, lagomorphs, xenarthrans, primates, 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.