2 ◆ C Monarch Migration

MonarchMigrationA lthough the beautiful black and orange wings of

the monarch butterfly are a common sight duringsummer in the United States, as fall and coldertemperatures arrive, the butterflies disappear. Each

fall they begin a seasonal migration. Scientists have had somesuccess in unlocking the mystery of monarch migrationthrough the use of scientific methods. Through this example,you can see how each step of this scientific method contributesto reliable results that can lead to better-informed conclusions.

The monarch population that lives west of the RockyMountains flies to the coast of California. The eastern popula-tion of monarchs flies to the mountains of central Mexico.Sometimes they travel up to 145 km per day. Some easternmonarchs, such as those living in southern Canada, fly morethan 3,200 km to reach their winter home.

NavigationJust as astonishing as the distance trav-

eled by these insect voyagers is their abilityto find their way. Since no butterfly com-pletes the entire round-trip, the butterfliescannot learn the route from others. So,how do butterflies that have never madethe trip before find their way from Canadaand the eastern United States to Mexico?This is the question that some entomolo-gists—scientists who study insects—setout to answer.

Scientific Methods

Figure 1 Eastern monarchbutterflies lay their eggs onmilkweed plants.

Figure 2 When they reach Mexico, easternmonarch butterflies gather in large groups.

(t)Fred Habegger/Grant Heilman Photography, Inc., (b)Fritz Polking/Visuals Unlimited

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THE NATURE OF SCIENCE C ◆ 3

One of the first hypotheses about how eastern monarchsnavigate was that they use the Sun as a guide. Researchersbased this hypothesis on other research, which showed thatsome migrating birds rely on the Sun to guide them. However,this failed to explain how the butterflies find their way oncloudy days.

MagnetismScientists later discovered that the bodies of eastern mon-

archs contain tiny grains of a naturally occurring, magneticsubstance called magnetite. Magnetite was used to make thefirst directional compasses. From this discovery, scientistsdeveloped a hypothesis that butterflies use an internal mag-netic compass to help them plot their route.

University scientists tested this hypothesis by performingan experiment. They caught some eastern monarchs during thefall migration. They divided the monarchs into three groupsand exposed each group to different magnetic fields. Thegroup exposed to Earth’s normal magnetic field flew to thesouthwest, which is the correct direction for eastern monarchsto migrate. Those exposed to the opposite of Earth’s normalmagnetic field flew to the northeast. Finally,those exposed to no magnetic field flutteredabout randomly.

Final ConclusionsAfter analyzing the results, the researchers

concluded that eastern monarchs use an internalmagnetic compass to navigate from Canada andthe eastern United States to Mexico. However,most researchers also believe the butterflies alsouse the Sun and landmarks, such as mountainsand rivers, to make their incredible journey.

Figure 4 A magnet has oppo-sitely charged poles.

Figure 3 Magnetite is a mineralwith natural magnetic properties.

(t)Doug Martin, (b)Richard Megna/Fundamental Photographs

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4 ◆ C Monarch Migration

ScienceScientists learned about the migration of eastern monarch

butterflies by starting with questions such as “Where do mon-archs go each fall? How do they find their way there?” Scientistsuse experiments and careful observations to answer questionsabout how the world works. When you test an idea, you aredoing science.

Life science is the study of living things. In this book, youwill learn about the diversity of animals and their adaptationsand behaviors, such as migration, that enable them to survive.

Scientific MethodsResearchers used scientific methods to learn about how

eastern monarchs navigate. Scientific methods are a series ofprocedures used to investigate a question scientifically.

Identifying a QuestionSometimes, scientific methods begin with identifying a

question, such as “Where do eastern monarch butterflies goevery autumn?” After one question has been answered, others

often arise. When researchers discov-ered eastern monarchs migrate toMexico, the next question was “Howdo the butterflies find their way?”

Forming a HypothesisOnce a question is identified,

scientists collect information anddevelop a hypothesis or possibleexplanation.

They might read the informa-tion available on how birds migrateand use it as a basis for the hypothe-sis that eastern monarchs use theSun to navigate. This idea, however,failed to explain how the butterfliesfind their way on cloudy days. As aresult, scientists developed anotherhypothesis—eastern monarchs usean internal magnetic compass thatenables them to maintain a coursein a specific direction.

Figure 5 Sometimes, a scien-tist must collect data outside ofthe laboratory.

David Woodfall/DRK Photo

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THE NATURE OF SCIENCE C ◆ 5

When eastern monarch butterflies reach Mexico’s mountains,the insects abruptly change direction. Scientists hypothesizethat the butterflies then switch to steering by landmarks,such as mountains. Describe one way scientists could test this hypothesis.

Testing the HypothesisScientists test hypotheses to determine if they are true

or false. Such tests often involve experiments, such as onewhere eastern monarchs were exposed to different kindsof magnetic fields.

Analyzing ResultsDuring experiments, scientists gather information, or

data. Data about the butterfly experiment included thedirection that the butterflies were flying when captured,what type of magnetic field they were exposed to in theexperiment, and how they reacted to that magnetic field.

Drawing a ConclusionAfter data have been collected and carefully analyzed, scien-

tists draw conclusions. Sometimes the original hypothesis isnot supported by the data and scientists must start the entireprocess over. In the case of the eastern monarchs, researchersobserved how the butterflies reacted to the magnetic fields andconcluded they use an internal magnetic compass to navigate.Just how the butterflies use Earth’s magnetic field to find theirway is another question for scientists to answer using scientificmethods.

Figure 6 Data from observa-tions are important in scienceinvestigations.

Figure 7 Scientists hypothe-size that monarchs also navigateby landmarks.

(t)Michael Newman/PhotoEdit, (b)James L. Amos/CORBIS

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