Sunshine StateSTANDARDSSC.F.2.3.3: The studentknows that generallyorganisms in a popula-tion live long enoughto reproduce becausethey have survivalcharacteristics.

THINK ABOUT

How diverse are mammals?

Mammals have adapted to sur-vive in many environments and come in many shapes and sizes.Whales live in the ocean, and goats may live near mountain peaks. Some monkeys live in tropical forests, and polar bears survive in frozen areas. An elephant might not fit in your classroom, but the tinyshrew shown here could fit on your finger. As you read this chapter,think about the characteristics of mammals that help them to survivein such a variety of ways.

Mammals are a diverse group.The group of vertebrates called mammals includes many familiar animals. Mice are mammals, and so are cows, elephants, and chim-panzees. You are a mammal, too. Bats are mammals that can fly. Somemammals, including whales, live in water.

Some mammal species, such as raccoons and skunks, have adaptedto live in many sorts of environments, including cities. Others, such ascheetahs and polar bears, have adaptations for meeting their needs injust a few environments.

Mammals are a diverse animal group. Although there are less than5000 species of mammals on Earth, mammal species come in manyshapes and sizes, and have many different ways of moving, finding

472 Unit 3: Diversity of Living Things

VOCABULARYmammal p. 473placenta p. 476gestation p. 476

BEFORE, you learned

• Endotherms can stay active incold environments

• Many bird adaptations arerelated to flight

• Birds lay hard-shelled eggs and usually take care of their young

NOW, you will learn

• About mammals as endotherms• About the diversity of adapta-

tions in mammals• That mammals produce milk,

which is food for their young

KEY CONCEPT

Mammals live in manyenvironments.

NOTETAKING STRATEGYUsing a strategy of yourchoice, take notes on theidea that mammals are adiverse group. Be sure toinclude mammal characteristics.

food, and eating. These are some of the characteristics that distinguishfrom other animals:

• All mammals have hair during some part of their lives.

• Most mammal species have teeth specialized for consuming particular kinds of food.

• All mammal species produce milk, with which they feed their young.

Mammals are endotherms.You have learned that endothermic animals are able to stay active incold environments. This is because endotherms maintain a constantbody temperature. Mammals are endotherms. This means that theyuse some of the food they consume to generate body heat. Mammalsalso have adaptations for controlling body temperature.

HairMany species of mammals have bodies covered with hair. Like birds’ feathers, hair isan adaptation that allows mammals to havesome control over the warmth or coldness of their bodies. Mammals that live in coldregions, like polar bears, have hair that keepsthem warm. Desert mammals, such ascamels, have hair that protects them fromextreme heat.

Most mammals have at least two types ofhair. Soft, fluffy underhairs keep heat close totheir bodies, like the down feathers of birds.Water-resistant guard hairs cover the under-hairs and give the animal’s fur its color.

Some species of mammals also have specialized hairs. A specialized structure isone that performs a particular function. Forexample, whiskers are sensory hairs that arepart of an animal’s sense of touch. Porcupines’quills are hairs that function in self defense.

Check Your Reading What are three functions of hair?

mammalsRESOURCE CENTERCLASSZONE.COM

Learn more about mammals.

Colored guard hairs give this tiger its stripes.Its whiskers are specialized sensory hairs. Italso has underhairs, which you cannot see.

This porcupine’s quills are specialized guard hairs.

whiskers

quills

Chapter 13: Vertebrate Animals 473

474 Unit 3: Diversity of Living Things

Body FatSome mammal species that live in water, such as dolphins, have verylittle hair. These mammals have a layer of fat, called blubber, that playsan important role in maintaining body temperature. The blubber islocated between the animal’s skin and muscles, and provides itsorgans with insulation from heat and cold.

Body fat can also be a storage place for energy. When a mammalconsumes more food than it needs, the extra energy may be stored inits fat cells. Later, if the animal needs energy but cannot find food, itcan use the energy stored in the fat.

For example, animals that hibernate, such as woodchucks, may eat a lot more than they need to survive at times when plenty of foodis available. This makes them fat. Then, while they are hibernating,they do not have to eat, because their body fat provides them with theenergy they need.

Check Your Reading What are two ways that body fat functions in mammals?

How well does fat keep a mammal warm?A layer of body fat between the muscles and the skin allows some mammals tosurvive in very cold places. In this investigation, you will experience how well thatadaptation works by making a blubber glove model.

PROCEDURE

Half fill one large, zip-lip plastic bag with vegetable shortening. Turn another bag inside out and place it inside the bag with the shortening.Zip the edges of the two bags together so that the shortening is sealedbetween them.

Place one hand inside this “blubber glove.” Place the other hand inside the third bag. Then submerge both hands in a tub of ice water. Which hand stays warmer? For how long? Record your observations.

WHAT DO YOU THINK?How well did the blubber glove insulate your hand?

CHALLENGE Would you expect an animal that lives in a very hot environment to have a thick layer of body fat?

2

1

How Body Fat InsulatesHow Body Fat InsulatesSKILL FOCUSMaking models

MATERIALS• 3 zip-lip plastic

bags, 1/2-gallonsize

• can of vegetableshortening

• bowl of icewater

TIME15 minutes

Chapter 13: Vertebrate Animals 475

Mammals have adapted to manyenvironments.

Scientists think mammals appeared on Earth about 200 million yearsago. Fossil evidence suggests that the first mammals were small landvertebrates with four limbs, a tail, and specialized teeth. They probablyhad fur and were most active at night.

Over millions of years, those early mammals adapted to live inmany different environments and became the diverse group of speciesthey are today. Moles, for example, live almost entirely underground.They have strong limbs for digging and organs specialized for sensinginvertebrate prey in the dark. Spider monkeys, on the other hand, livemostly in trees. A spider monkey can use all four of its long limbs andits flexible, grasping tail to move through a forest without touchingthe ground.

The first mammals lived on land, but over time some species adapted to live in watery environments. Some of them, such as ottersand walruses, live mostly in water but can also be found on land.Others, such as dolphins and whales, have bodies so completely adaptedfor life in the water that they no longer have a way of moving on land.If you look carefully at the diagram above, however, you will see that adolphin’s body plan differs in some important ways from that of a fish.

A dolphin’s blowhole, flippers, tail flukes, and blubber are featuresthat allow it to meet its needs in water.

blubber

flukes

lung

flipper

blowhole

vertebrae

heart

Mammal Adaptations for a Water Environment

The length of gestation varies greatly inplacental mammals.

Length of Gestation

Mam

mal

s

Time (weeks)0 15 30 45 60 75 90 105

Mouse 3 weeks

Dog 9 weeks

Human 39 weeks

Horse 40–44 weeks

Elephant 84–88 weeks

476 Unit 3: Diversity of Living Things

Mammals have reproductive adaptations. Mammals reproduce sexually. Before a mammal can produce off-spring, it finds a mate. Some mammals, such as lions, live in groupsthat include both males and females. However, most mammals livealone most of the time. They find a mate when they are ready toreproduce. Some mammal species breed only at certain times of theyear. Others can reproduce throughout the year.

Development Before BirthFertilization occurs internally in mammals. In almost every species ofmammal, the offspring develop inside the female’s body. Many mam-mals have a special organ called a that transports nutrients,water, and oxygen from the mother’s blood to the developing embryo.The embryo’s waste materials leave through the placenta and aretransported out of the mother’s body along with her waste.

The time when a mammal is developing inside its mother is calledAs you can see in the diagram, the length of the gestation

period is different for different species. Gestation ends when theyoung animal’s body has grown and developed enough for it to survive outside the mother. Then it is born.

Check Your Reading Where do the offspring of most mammals develop before they are born?

gestation.

placentaVOCABULARYRemember to make a word magnet for the terms placenta andgestation.

KEY CONCEPTS1. Why is a bat classified as a

mammal instead of a bird?Why are whales classified asmammals and not fish?

2. Name two adaptations thatallow mammals to have controlover body temperature.

3. How does the way that mam-mals feed their young differfrom the ways other animalsfeed their young?

CRITICAL THINKING4. Apply One day on your way

home from school, you see ananimal that you have neverseen before. What clues wouldyou look for to tell you whattype of animal it is?

5. Synthesize Make a Venn diagram that shows how thevertebrate animal groups youlearned about in this chapterare similar and different.

CHALLENGE6. Evaluate This section

begins with the statement“Mammals are a diversegroup.” Explain what thismeans and why it is a truestatement. How does thediversity of mammals com-pare with the diversity ofother living things, such asbacteria, plants, arthropods,or fish?

Chapter 13: Vertebrate Animals 477

Not all mammals fully develop inside their mothers. The duck-billed platypus and two species of spiny anteaters have young thathatch from eggs. Mammals in a group called the marsupials (mahr-SOO-pee-uhlz), which includes kangaroos, develop inside the motherat first but are born when they are still extremely small. Right afterbirth, a young marsupial climbs across its mother to a special pouchon the outside of her body. It completes its development there. Onlyone marsupial, the opossum, lives in North America.

Raising YoungMilk, a high-energy liquid food full of proteins, fats,sugars, and other nutrients, is the first food youngmammals consume. Each species’ milk has a differentcombination of these ingredients.

Mammals’ bodies have special glands for producingmilk, called mammary glands. In almost all mammalspecies, only females’ mammary glands function. Thismeans that only female mammals can feed the youngwith milk. In most mammal species, females gestateand care for the offspring alone. However, in somemammal species, the male helps raise the young.

Different species of mammals are born at varyingstages of development. Most mice are helpless, blind, and naked atbirth, while giraffes can walk soon after they are born. The length oftime a young mammal needs care from an adult varies. Some sealsnurse their young for less than a month before leaving them to surviveon their own. Some whales live alongside their young for a muchlonger time. However, humans may be the mammal species that takescare of their offspring for the longest time of all.

This dog is feeding herpuppies with milk.