Sea turtles' long ocean swim home

A sea turtle lies on the beach in Kailua, Hawaii, on Aug. 7, 2014. Photo: AP Photo/Chris Stewart

LOS ANGELES — It is a mystery that has stumped scientists for decades: How

do female sea turtles manage to navigate back to the same beach where they

were hatched when it comes time to lay their own eggs? After all, sea turtles

travel across thousands of miles of open ocean each year — yet the females

somehow find their way back to where they were born.

Keep in mind that there are no visual guideposts in the open ocean where sea

turtles spend most of their lives. It is vast and featureless — and yet every two to

three years, sea turtles dig their nests at the same location where they once

crawled out of their own eggs.

An Invisible Force

Since the 1990s, scientists have suspected that returning sea turtles are

somehow guided by information supplied by the Earth’s geomagnetic field.

Geomagnetism is an invisible but powerful force. Electrical currents streaming

from the Earth's core, which is mostly iron, create a strong magnetic field. This

force acts like an enormous bar magnet buried in the planet's center. One pole

By Los Angeles Times, adapted by Newsela staff on 01.28.15

Word Count 928

— the North Pole — is in the Arctic, while the other — the South Pole — is in

Antarctica. When sailors and other travelers use a compass to find north, they

rely on geomagnetism. A compass' metal needle is attracted to the North Pole

of the Earth's mega-magnet, just as a hairpin is attracted to an ordinary magnet.

Scientists knew that turtles use geomagnetism to guide themselves in a general

direction, but they did not know how they found the exact beach where they

were born.

Earth's Changing Angles

A study published on Thursday looks at loggerhead sea turtles that bury their

eggs on the Florida coast. It shows that slight changes in the Earth’s magnetic

field affect where turtles travel and where they bury their eggs.

Once again, to understand how geomagnetism works, it helps to imagine that

there is a giant bar magnet inside the Earth. The bar lines up almost vertically at

the center of the Earth, but is skewed slightly. Because the bar continuously

shifts a bit, its angle relative to the Earth changes over time. The changing

angles are known as inclination angles.

Also important to understand is something known as a magnetic signature.

Everything on Earth reacts to geomagnetism in a different way, and the precise

way each thing reacts is known as its magnetic signature. Imagine that you are

pointing a magnet at two objects, and that one is pulled quickly toward the

magnet, while the other is not. The difference in the degree to which they are

attracted to the magnet is part of their magnetic signature.

Magnetic signatures are not fixed, however, but change when the magnetic field

changes in strength and direction.

Turtle GPS System

Scientist J. Roger Brothers was involved in the new sea turtle study.

“Turtles have evolved a way to use the inclination angle of the geomagnetic

field" and the field's strength to guide themselves, he said. It is almost as if they

have "an internal GPS” system similar to what is found in cellphones.

Turtles do not use geomagnetism to just point themselves in the right general

direction, but somehow are able to detect, remember and track their birth

beach's magnetic signature.

Because the geomagnetic field is always shifting, the magnetic signature of a

particular stretch of beach also changes over time. The scientists suspected

that shifts in magnetic signature would be reflected in shifts in where sea turtles

laid their eggs.

To test this idea, the researchers looked at 19 years of data on loggerhead

nesting sites along the Atlantic coast of Florida. They compared the data to the

record of shifts in the geomagnetic field over the same period of time, hoping to

find a connection.

That Spot On The Beach

Magnetic fields move in lines, almost like the lines on a raised map that show

hills and mountains. Scientists knew that when magnetic field lines grow

stronger, the magnetic signatures of beaches next to each tend to move closer

together, like when a scattered group of nails near a magnet all point toward it.

They theorized that in years when the magnetic signatures of adjacent beaches

moved close to each other, or converged, there should be more nests clustered

together on the same stretch of coast. In years when the magnetic signatures

spread away from each other, or diverged, the nests should be more spread

out.

In fact, that is exactly what they found. At times and places where the magnetic

signatures of adjacent beaches converged, scientists found an increase in the

number of nests in the same general area. At times when the signatures

diverged, nests were not as close to one another.

There's No Beach Like Home

However, there are still many more questions to be answered. For example,

researchers do not know what the turtles are using to detect the geomagnetic

field, which can neither be seen nor heard.

“Most likely they have tiny magnetic particles in their brains or in their bodies

that act like a compass," Brothers said. The particles may be scattered

throughout the animals' bodies, which would make them hard to find.

Brothers also noted that while loggerhead turtles may use geomagnetic fields to

help them get back home to lay their eggs, it does not appear they rely on the

information alone.

“We don’t expect that these turtles are coming to the magnetic signature

regardless of what else is going on,” he said. “If a condo is built there, they will

usually decide to go nest somewhere else.”

Quiz

1 Select the option that BEST presents the central idea of the article.

(A) Scientists are researching how and what allows female turtles to

navigate their way across oceans.

(B) Scientists are researching navigation phenomenons using Earth's

magnetic field that are seen in animals.

(C) Scientists are conducting research to know more about the

locations female turtles prefer to lay their eggs.

(D) Scientists are conducting research to find out how powerful Earth's

magnetic field can be at some places.

2 Which of the following sentences from the article is MOST important to include in its

summary?

(A) A compass' metal needle is attracted to the North Pole of the

Earth's mega-magnet, just as a hairpin is attracted to an ordinary

magnet.

(B) Everything on Earth reacts to geomagnetism in a different way, and

the precise way each thing reacts is known as its magnetic

signature.

(C) "Turtles have evolved a way to use the inclination angle of the

geomagnetic field" and the field's strength to guide themselves, he

said.

(D) "We don't expect that these turtles are coming to the magnetic

signature regardless of what else is going on," he said.

3 How does the section "Turtle GPS System" contribute to the article?

(A) It shows that turtles are intelligent beings.

(B) It describes the function of Earth's magnetic field.

(C) It explains the likely role of geomagnetism in turtle nesting behavior.

(D) It draws a connection between geomagnetism and the magnetic

signature of a place.

4 Why does the author include the following sentence in the article?

It is vast and featureless - and yet every two to three years,

sea turtles dig their nests at the same location where they

once crawled out of their own eggs.

(A) to show the intelligence sea turtles exhibit during nesting

(B) to lay stress on the exceptional memory that sea turtles are

believed to possess

(C) to indicate that human beings can learn several things about

navigation from animals

(D) to illustrate how a built-in GPS helps turtles find the nesting sites

where they were born

Answer Key

1 Select the option that BEST presents the central idea of the article.

(A) Scientists are researching how and what allows female turtles

to navigate their way across oceans.

(B) Scientists are researching navigation phenomenons using Earth's

magnetic field that are seen in animals.

(C) Scientists are conducting research to know more about the

locations female turtles prefer to lay their eggs.

(D) Scientists are conducting research to find out how powerful Earth's

magnetic field can be at some places.

2 Which of the following sentences from the article is MOST important to include in its

summary?

(A) A compass' metal needle is attracted to the North Pole of the

Earth's mega-magnet, just as a hairpin is attracted to an ordinary

magnet.

(B) Everything on Earth reacts to geomagnetism in a different way, and

the precise way each thing reacts is known as its magnetic

signature.

(C) "Turtles have evolved a way to use the inclination angle of the

geomagnetic field" and the field's strength to guide

themselves, he said.

(D) "We don't expect that these turtles are coming to the magnetic

signature regardless of what else is going on," he said.

3 How does the section "Turtle GPS System" contribute to the article?

(A) It shows that turtles are intelligent beings.

(B) It describes the function of Earth's magnetic field.

(C) It explains the likely role of geomagnetism in turtle nesting

behavior.

(D) It draws a connection between geomagnetism and the magnetic

signature of a place.

4 Why does the author include the following sentence in the article?

It is vast and featureless - and yet every two to three years,

sea turtles dig their nests at the same location where they

once crawled out of their own eggs.

(A) to show the intelligence sea turtles exhibit during nesting

(B) to lay stress on the exceptional memory that sea turtles are

believed to possess

(C) to indicate that human beings can learn several things about

navigation from animals

(D) to illustrate how a built-in GPS helps turtles find the nesting

sites where they were born