26 A Closer Look at Amniotes

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MAIN IDEAAmniotes can be ectothermic or endothermic.Like all organisms, amniotes are more active when they are warm. Enzymesthat speed up the chemical reactions inside cells are more active at highertemperatures. A warm amniote digests food faster and can send more nutrientsto its tissues. It can also move faster because its muscles contract morequickly and more often. All living organisms absorb heat from the environmentand release heat as a byproduct of metabolism. But all animals manageheat in different ways.You may have heard the term “cold-blooded” to describe asnake or a lizard. This term does not accurately describe reptilesand amphibians, because their blood is not actually cold.Instead, scientists use the term ectotherms to refer to organismswhose body temperatures are determined by their surroundingenvironment. These organisms’ body temperature fluctuateswith the temperature of their environment. They have higherbody temperatures in a warm environment than in a cool one.Ectotherms regulate their body temperature through theirbehavior. For example, many reptiles, such as the chameleon inFIGURE 26.4, bask in sunny places to warm their tissues when theyare cold. Similarly, desert lizards move into shady burrows whenoutside temperatures climb too high. Large animals have aharder time shedding heat than small animals. If an ectothermicanimal is massive enough, it will take a long time to cool down.Large ectotherms, such as crocodiles, can stay warm even whenthe environment is relatively cool.On the other hand, you have probably heard humans andother mammals described as “warm-blooded.” But to describethese organisms more accurately, scientists use the term endotherm.Endotherms are organisms that use their own metabolicheat to keep their tissues warm. More specifically, endothermsregulate their metabolic activity in ways that keep their bodytemperature relatively constant all of the time. They may shiverwhen they get too cold, contracting their muscles to generateextra heat. If they get too hot, they may cool down by sweatingor panting. Many endotherms, such as the polar bear inFIGURE 26.4, are covered with insulation in the form of hair, fatcells, or feathers, which helps them control heat loss.You can think of endotherms and ectotherms as having two differentstrategies for managing energy use. There is a trade-off between an animal’sbody temperature and the amount of energy it uses. Warm tissues workquickly and require more ATP, which requires an animal to eat more. Forexample, lions and crocodiles are both large predators, but a crocodile cansurvive on much less meat than a lion can. In short, ectotherms are less activewhen it is cold but can survive on less food than endotherms. Endothermsare active all the time but must eat more than ectotherms eat.FIGURE 26.4 As an ectotherm,a chameleon increases its bodytemperature by basking in sunlight.Endotherms such as thispolar bear can maintain a relativelyconstant body temperatureeven in cold environments.Chapter 26: A Closer Look at Amniotes 791
DATA ANALYSISCHOOSING GRAPHSChoosing an appropriate type ofgraph to represent data collected inan experiment is an important partof the scientific process.The table to the right contains datathat show the differences in energyrequirements for endotherms andectotherms. Despite having similarsizes, endotherms and ectothermsuse energy in different ways andtherefore require different amountsof food.TABLE 1. BODY MASS AND FOOD INTAKEOrganism Mass (kg) Food Intake (kg/yr)Nile crocodile 150 750Grey kangaroo 45 1108Komodo dragon 45 250Koala 8 252Monitor lizard 8 93Source: Nagy, K.A. Nutrition Abstracts and Reviews Series B:71.1. Graph Choose and construct one graph that canrepresent both sets of data.2. Analyze Explain why there is a difference in energyrequirements between endotherms and ectotherms.The ability to regulate their own temperature served as an importantfunction in the early stages of endotherm evolution. This adaptation gaveendotherms a distinct advantage over ectotherms as Earth’s climate changedmillions of years ago. Because they could stay warm in colder weather, endothermswere able to exploit resources that the ectotherms could not. Manyscientists believe that the ability to regulate their own body temperatureallowed endotherms to survive the catastrophic events that led to theextinction of dinosaurs.Analyze As you move away from Earth’s equator into colder latitudes, why are therefewer ectotherms and more endotherms?26.1 ASSESSMENTREVIEWINGMAIN IDEAS1. How did the development of anamniotic egg allow vertebrates toreproduce on land?2. How does anatomy and circulationdiffer among amniotes?3. What is the difference between anendotherm and an ectotherm?CRITICAL THINKING4. Infer A 30-gram shrew will die ifit cannot eat for a few hours. A30-gram gecko thrives on a fewcrickets every other day. Why mightshrews need food more often?5. Compare Illustrate the path ofblood through a three-chamberedheart when the animal is breathing.Show how the pathway changeswhen the animal is not breathing.ConnectingONLINE QUIZClassZone.comCONCEPTS6. Survivorship When eggs arelaid by a species of reptile orbird, they generally stay in anest that is closely guarded bya mother. How does thisbehavior affect the chancesfor offspring to survive toadulthood? What type ofsurvivorship strategy does thisrepresent?792 Unit 8: Animals
Page 1 and 2: 26CHAPTERA Closer Lookat AmniotesKE
Page 3 and 4: 26.1 AmniotesKEY CONCEPT Reptiles,
Page 5: ConnectingCONCEPTSCirculatory Syste
Page 9 and 10: FIGURE 26.6 Turtles and tortoisesar
Page 11 and 12: MAIN IDEAThere are four modern grou
Page 13 and 14: 26.3 BirdsKEY CONCEPT Birds have ma
Page 15 and 16: FIGURE 26.13 Bird AnatomyBird anato
Page 17 and 18: QUICK LABOBSERVINGComparing Feather
Page 19 and 20: CHAPTER 26MATERIALS• balance• 1
Page 21 and 22: ConnectingCONCEPTSNatural Selection
Page 23 and 24: FIGURE 26.18 The shovel-likebill of
Page 25 and 26: CHAPTER 26OPTIONS FOR INQUIRYUse th
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Page 29 and 30: Critical Thinking23. Analyze A pers

26 A Closer Look at Amniotes

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