Section 33–2
33–2 Controlling Body Temperature
1 FOCUS
O
n a spring morning, after a cold night, a tortoise lies on a
rock basking in the sun. Nearby, a snake slides out of its
burrow beneath a rotting stump. In a tree overhead, a young
robin puffs up its downy feathers. As you walk out of the water
after an early swim, your skin gets goose bumps and you shiver.
All these activities are examples of the different ways that
vertebrates control their body temperature.
Objectives
33.2.1 Explain how the control of
body temperature is an
important aspect of
vertebrate life.
33.2.2 Contrast ectotherms and
endotherms.
Key Concepts
• How is the control of body
temperature an important
aspect of vertebrate life?
• What is the difference
between ectotherms and
endotherms?
Body Temperature and Homeostasis
Vocabulary
ectotherm
endotherm
Vocabulary Preview
Reading Strategy:
Finding Main Ideas Before
Review the terms endotherm and
ectotherm with students. On the
board, create a list of what students
already know about these terms.
you read, skim the section to
identify the key-idea sentences
about body temperature
control. Then, carefully read the
section, making a list of supporting details for each main idea.
Reading Strategy
As students read the section, encourage them to take notes in a
compare/contrast table to help them
differentiate between endothermy
and ectothermy.
2 INSTRUCT
Body Temperature
and Homeostasis
Use Visuals
Figure 33–5 Invite a student volunteer to identify a penguin as an
endotherm or ectotherm.
(Endotherm) Ask: How is a penguin
able to maintain homeostasis in a
cold climate? (Generates own body
heat; feathers act as insulation) Why
is it important for a penguin to be
able to maintain a constant internal temperature? (Essential life
functions are carried out most efficiently when an animal’s internal
temperature stays within a certain
range.)
Figure 33–5 Birds and other
endotherms are able to generate their
own body heat.
The internal
control of body temperature allows
these emperor penguins to live in
cold Antarctic climates, where their
feathers act as insulation.
SECTION RESOURCES
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• Teaching Resources, Section Review 33–2
• Reading and Study Workbook A, Section 33–2
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Section 33–2
• Lesson Plans, Section 33–2
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Chapter 33
r
854
Recall from Chapter 2 that many of the chemical reactions that
are important in metabolism are influenced by temperature. For
this reason, essential life functions can be carried out most
efficiently when an animal’s internal body temperature is
within a particular “operating range.” For muscles to operate
quickly and efficiently, for example, their temperature can
neither be too low nor too high. If muscles are too cold, they may
contract slowly, making it difficult for the animal to respond
quickly to events around it. If an animal gets too hot, on the
other hand, its muscles may tire easily and other body systems
may not function properly.
Because most chordates are vertebrates, and mechanisms
for controlling body temperature are well developed among
vertebrates, this section will focus exclusively on that group.
The control of body temperature is important for
maintaining homeostasis in vertebrates, particularly in
habitats where temperature varies widely with time of
day and with season. Vertebrates, such as the penguins
in Figure 33–5, have a variety of ways to control their
body temperature. All of these ways incorporate
three important features: a source of heat for the
body, a way to conserve that heat, and a
method of eliminating excess heat when
necessary. In terms of how they generate
and control their body heat, vertebrates
can be classified into two basic groups:
ectotherms and endotherms.
Temperature Control in Chordates
50
40
Comparing Ectotherms
and Endotherms
Cat
Rabbit
30
Alligator
Snake
Endotherms, such as humans, depend on their metabolism to maintain high body temperatures. Ectotherms,
on the other hand, depend primarily on heat from
the environment to regulate their body temperatures.
The accompanying graph shows the internal body
temperatures maintained by several ectotherms and
endotherms at different environmental temperatures.
1. Using Tables and Graphs Which chordate has
the highest body temperature when the environmental temperature is between 0˚ and 10˚C?
Which chordate has the lowest body temperature
under those same conditions?
2. Inferring Which animals shown in the graph are
ectotherms? Which are endotherms? Explain your
answers.
Pigeon
20
Lizard
10
0
0
10
20
30
40
50
3. Predicting Describe the patterns of activity you
would expect for the animals shown in this graph
if they lived in your local environment. Would
you expect all of the animals to be equally active
year-round? If not, why not?
Ectothermy On cool, sunny mornings, lizards often bask
in the sun. This doesn’t mean that they are lazy! A lizard is
an ectotherm, which means that its body temperature is
mainly determined by the temperature of its environment.
Most reptiles, fishes, and amphibians are
ectotherms—animals whose body temperatures are
controlled primarily by picking up heat from, or losing
heat to, their environment. Ectotherms often warm up by
basking in the sun, and may cool down by seeking shelter in
underground burrows.
Ectotherms have relatively low rates of metabolism when
they are resting. Thus, their bodies do not generate much heat.
When active, an ectotherm’s muscles generate heat, just as your
muscles do. However, because its body lacks effective insulation,
the heat is lost to the environment fairly easily.
Ectothermy and endothermy are
both derived from the Greek word
therme, meaning “heat.” The
prefix endo- is a Greek word
meaning “within.” Therefore, the
word endotherm literally means
“heat from within.” What do you
think the prefix ecto- means?
Ecto- means “outer,” “outside,” or
“external.”
N S TA
Download a worksheet
on homeostasis for students to
complete, and find additional teacher
support from NSTA SciLinks.
Endothermy An endotherm is an animal whose body
temperature is controlled from within.
Birds and mammals are endotherms, which means they can generate and
retain heat inside their bodies. Endotherms have relatively
high metabolic rates that generate a significant amount of heat,
even when they are resting. Birds conserve body heat primarily
through insulating feathers, such as down. Mammals have body
fat and hair for insulation. Mammals can get rid of excess heat
by panting, as dogs do, or by sweating, as humans do.
Have student volunteers describe the
line on the graph for each of the animals. Elicit descriptions in which they
explain the animal’s body temperature as a function of environmental
temperature. For example, the body
temperature of the alligator is about
3°C higher than the environmental
temperature.
1. Highest: pigeon; lowest: lizard
2. The lizard, snake, and alligator are
ectotherms. Their body temperature
fluctuates based on the environmental temperature. The rabbit, cat, and
pigeon are endotherms. Their body
temperature remains relatively the
same as the environmental temperature changes.
3. In areas with cold winters, the
ectotherms would not be active during the winter. They would be active
during warm months. The endotherms might be active all year,
regardless of temperature changes.
N S TA
For: Links on
homeostasis
Visit: www.SciLinks.org
Web Code: cbn-9332
Give an example of an ectotherm and an endotherm.
Inclusion/Special Needs
Remind students that they are endotherms.
Challenge them to identify behaviors that they
use to conserve body heat and to eliminate
excess body heat. (Examples include wearing
coats and mittens to add insulation and protect
against the loss of body heat and swimming to
cool off.)
Advanced Learners
Challenge students to compare human characteristics with those of other endothermic
vertebrates adapted to live in cold environments. Ask students to infer the type of
environment to which humans are naturally
adapted. (Warm environments) Invite students to
develop a computer presentation that describes
how humans have adapted to live in cold environments.
Answer to . . .
Ectotherms: fishes,
amphibians, and reptiles; endotherms:
birds and mammals
Comparing Chordates
855
Comparing Ectotherms and Endotherms
33–2 (continued)
Comparing
Ectotherms and
Endotherms
Build Science Skills
Making Judgments Conduct a
mock debate on the pros and cons of
ectothermy. You can either stage an
entire class debate or have small
groups debate each other. You might
also want students to switch sides so
that they have the opportunity to
argue for both types of temperature
control.
Evolution of
Temperature
Control
Build Science Skills
Figure 33–6
Unlike birds
and mammals, which can regulate
their body temperature from
within, lizards and other
ectotherms rely on their surroundings to gain or lose body heat. The
venomous gila monster, for example, makes its home in arid regions
of the southwestern United States
and Mexico, most often in desert
and grassland biomes. To cool
down, it burrows below the ground.
Formulating Hypotheses
Challenge students to use their own
ideas about evolution and the control
of body temperature to devise a
hypothesis to explain when endothermy first evolved. Encourage
students to refer to additional
resources to increase their knowledge.
Students should note the evidence
on which their hypothesis is based.
In an absolute sense, neither endothermy nor ectothermy is
superior. Each strategy has advantages and disadvantages in
different environments. For example, endotherms move around
easily during cool nights or in cold weather because they
generate and conserve their own body heat. That’s how musk
ox live in the tundra and killer whales swim through polar
seas. But the high metabolic rate that generates that heat
requires a lot of fuel. The amount of food needed to keep a
single cow alive would be enough to feed ten cow-sized lizards!
Ectothermic animals, like the gila monster shown in
Figure 33–6, need much less food than similarly sized
endotherms. In environments where temperatures stay warm
and fairly constant most of the time, ectothermy is a more
energy-efficient strategy. But large ectotherms run into trouble
in habitats where temperatures get cold at night or stay cold for
long periods, such as boreal forest biomes. It takes a long time
for a large animal to warm up in the sun after a cold night. Most
large lizards and amphibians live in warm areas such as
tropical rain forest biomes.
Evolution of Temperature Control
There is little doubt that the first land vertebrates were
ectotherms. But there is some doubt as to when endothermy
evolved. Although modern reptiles are ectotherms, some
biologists hypothesize that at least some of the dinosaurs were
endotherms. Others hypothesize that endothermy evolved a
long time after the appearance of the dinosaurs, so that all the
dinosaurs were ectotherms. Evidence suggests that endothermy
has evolved more than one time. It developed once along the evolutionary line of reptiles that led to birds and once along the
evolutionary line of reptiles that led to mammals.
33–2 Section Assessment
3 ASSESS
Evaluate Understanding
Ask students to name the three features of body temperature control.
Then, have students give examples of
each feature for both endotherms
and ectotherms.
Reteach
1.
Key Concept What
important function does the
control of body temperature
serve in chordates?
2.
Key Concept Compare
and contrast ectotherms and
endotherms.
3. What three features are needed
to control an animal’s body
temperature?
4. How does endothermy affect an
animal’s need for food?
5. Critical Thinking Inferring
Why is it unlikely that you would
find a giant lizard living in the
wild in North Dakota?
Comparing and
Contrasting
Construct a table that
compares ectothermy and
endothermy. Factors you
should compare include:
how body temperature is
controlled; relative rates of
metabolism; relative amounts
of food eaten; advantages;
disadvantages; and examples
of animals with each method
of temperature regulation.
Students can create a Venn diagram
to compare and contrast endothermy
and ectothermy.
33–2 Section Assessment
Tables should include the six factors specified, as well as animal
examples.
If your class subscribes to the iText,
use it to review the Key Concepts in
Section 33–2.
856
Chapter 33
1. Helps maintain homeostasis
2. Ectotherms obtain heat from outside the
body and have a low metabolic rate. Endotherms can generate and retain heat inside
the body and have a high metabolic rate.
3. A source of heat for the body, a way to
conserve heat, and a method of eliminating
excess heat
4. Endotherms need more food in order to
release enough energy to generate body
heat.
5. Winters in North Dakota are too cold for a
giant lizard to maintain enough warmth for
activity.