6th Grade Assessment – Unit 3 – RI.6.1, RI.6.2, RI.6.3, RI.6.5, RI.6.6
Read this article. Then answer questions 1 through 7.
The Sea Turtle’s Built-In Compass
by Sudipta Bardhan
If you were bringing friends home to visit, you could show them the way. You know the
landmarks—a big red house, a bus-stop sign, or even a pothole in the front of your driveway.
But what if you were swimming in the middle of the Atlantic Ocean, where everything looks
almost the same? Could you find your way home?
5 A loggerhead sea turtle could. It’s born with a magnetic sense that tells it how to find its
way from any place on Earth.
These big turtles swim thousands of miles each year. But somehow, they know which way
to turn to stay in warm waters where there is a lot of food.
Loggerheads also seem to have a good memory for places, even for places they have
10 seen just once before. Each female will lay eggs only on the beach where she was born, even
if she hasn’t returned since she hatched. Each year, she goes back to the same beach. That
means a baby loggerhead must figure out exactly where it is from the moment it hatches.
“We think that the loggerhead turtles have a global-positioning system of sorts,”
15 explains Dr. Ken Lohmann, “and that it is somehow based on Earth’s magnetic field.”
This global-positioning system, or magnetic sense, is important. It helps the turtles locate
what they need to live—from the best spots for finding food to their home beaches.
Understanding the turtles’ magnetic sense will help researchers figure out which areas are
important for the survival of this endangered species.
20 It isn’t such a stretch to think that loggerheads may have a magnetic sense. Scientists
already know of several animals that can detect magnetic fields. Whales, honeybees, birds,
fish, and even some bacteria use Earth’s magnetic field to find their way. Many of these
animals, including loggerheads, have a substance called magnetite in their bodies. That’s what
may give them their magnetic sense.
25 A difference between other animals and loggerheads, though, is the way they learn to use
their magnetic sense. Young whales, honeybees, and birds can learn from adults. Loggerheads
are abandoned as eggs.
With no adults to learn from, how do hatchlings figure out how to use their magnetic
sense? Lohmann thinks they use cues from the environment. One of the cues he tested
30 was light on the horizon.
Baby loggerheads hatch only at night. However, a small amount of light reflects off the
ocean. The light makes that region brighter than the rest of the sky. Heading toward the light
helps loggerheads get quickly out to sea, where they can find food.
Turtles hatching in eastern Florida first swim east, since that is the direction of the
35 light. Lohmann tested whether hatchlings use this light source to set their magnetic
“We outfitted each hatchling with a cloth bathing suit that was attached to a fishing line
and set them free in the tank,” says Lohmann. The fishing line was connected to a tracking
system so a computer could record which way the turtles swam.
40 Around the tank, the scientists set up electrical coils to create a magnetic field that
matched the Earth’s. They set a dim light to either the “east” or the “west” of the magnetic
field. Then they let the hatchlings go.
At first, the hatchlings swam toward the light, no matter where it was. After scientists
turned off the light, the turtles that had seen the light in the “east” always swam toward
45 “east.” When the researchers reversed the magnetic field, these turtles turned around and
swam toward the new “east.” They had learned how to use their built-in compass.
Turtles that had seen the light in the “west” swam toward “west.” In the wild, swimming
west would take them the wrong way—away from the ocean. So the light helped set the built-
in compass, even if it did give the wrong direction.
50 Turtles that had their first swim in total darkness swam in random directions.
These experiments showed that loggerheads use cues from the outside world to set their
magnetic sense. Loggerheads can detect magnetic fields from birth, but at first they don’t
know what they mean. After they follow the cues from their surroundings, they remember the
“correct” magnetic direction.
55 Lohmann’s work has led others to protect the loggerheads’ habitat. For example, if a turtle
hatches on a beach with a bright boardwalk, the turtle may be confused about which lights to
follow. If it turns the wrong way, its magnetic sense may be warped forever. That would make
survival hard for the turtle.
Lohmann is working to find other factors that are important in helping sea turtles find
60 their way around the world. Many questions about these beautiful ocean creatures have still
not been answered, so researchers have a lot of ideas to study.