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Unlocking the mysteries of migration: millions of creatures make yearly, record-breaking treks across the globe. Why--and how--do they do it?


Each year, a giant herd of wildebeests makes a sweeping circle of Africa's Serengeti region. Millions of plate-size red crabs set out to cross an island in the South Pacific. And a seemingly infinite number of monarch butterflies soar thousands of miles in order to congregate in Mexico.

Similar feats of nature are repeated by countless other mammals, birds, insects, fish, and reptiles. But why do they do it? And better yet: How? Scientists are studying these animals to learn more about their travels and the tools that help them cover such great distances year after year.


The ground rumbles as more than 1 million wildebeests stampede across the plain. This massive herd is leaving its winter home on the southeastern edge of Serengeti National Park in Tanzania. The large antelopes will spend the next 4 to 12 weeks crossing the African plains as they migrate north. They are following the rains to their summer grazing area in the northwest corner of the park (see map, p. 15). In autumn, the wildebeests will complete the 2,900 kilometer (1,800 miles) migratory loop back south.

The wildebeests' movement is one of the most-studied migrations in the world. Even so, scientists still do not understand what triggers the annual trek. "We don't know what goes on in the animals' minds, but we do know what the consequences of migration are," says John Fryxell, a biologist at the University of Guelph in Canada. By keeping on the move, wildebeests visit the freshest grasses and watering holes. During their trek they also take a break from predation as they leave behind forests filled with meat-eating lions, hyenas, and cheetahs.


Like the Serengeti's wildebeests, the red crabs that live on Christmas Island, just south of Indonesia, have an annual migration that follows the rains. The blood-red crabs inhabit inland rainforests, rarely leaving their burrows and moving for only about 10 minutes each day. Once the rainy season begins, however, the crabs ramp up their activity levels and scuttle up to 1 km (0.6 mi) each day as they head to the coast to breed (see map, p. 12).


For these critters, the 4.8 km (3 mi) trek to the sea is like running a marathon. How do the crabs go from being couch potatoes to long-distance champions? Researchers have found that the secret lies in a special hormone within the crabs' muscle fibers. Levels of this chemical messenger spike right before the migration, allowing the crabs to efficiently convert stored energy in their muscles into glucose, the sugar that fuels motion. This way, the crabs don't run out of energy during the weeklong trip to the coastal breeding grounds.


Sometimes a migration is more complicated than following the rains or heading to a particular breeding site. Monarch butterflies' migration takes several generations to complete. In the fall, the insects fly as many as 3,200 km (2,000 mi) south to reach a pine forest, just west of Mexico City, where they spend the winter (see map, above). Once spring arrives, the females begin the journey back north--but they make it only partway, laying the eggs of the next generation, which will continue the journey north. The butterflies that return to Mexico the following winter are the great-great-grandchildren of the original butterflies.

How do monarchs that have never been to Mexico make the journey without getting lost? The insects follow the sun. But there's a hitch: "The problem with using the sun [to navigate] is that it moves throughout the course of the day," says Steven Reppert, a neurobiologist at the University of Massachusetts who studies monarch migration.


To compensate, a monarch's "sun compass" is in sync with its circadian clock. A circadian clock regulates an animal's daily bodily functions. It's what tells your body to wake up in the morning, eat lunch around noon, and go to bed at night. As a butterfly flies south, it uses its circadian clock to correct its path as the sun rises and sets.


Monarchs aren't the only species with a GPS-like internal navigation system to help them migrate. Baby loggerhead turtles swim from beaches along the southeastern coast of the United States where they hatched to the North Atlantic gyre (see map, right). This region of rotating ocean currents has warm temperatures and rich feeding grounds. "Over the next five years, the turtles follow that enormous loop around the gyre," says Kenneth Lohmann, a biologist at the University of North Carolina.


How do the turtles find and stay within the bounds of the gyre? Hatchlings have an internal compass that detects Earth's magnetic field and helps them stay on course (see Earth's Magnetism, p. 13). As they move around the gyre, the turtles are also able to sense how the magnetic field varies from place to place. "The turtles use the magnetic field as a kind of map," says Lohmann.

Exactly how loggerheads sense Earth's magnetic field remains a mystery. Scientists have found fragments of a magnetic mineral called magnetite in their brains. Magnetite has been shown to help some birds, such as pigeons, navigate, but it's unclear ff the mineral plays a role in the turtles' trips. Another theory is that magnetic navigators like loggerheads have special photoreceptors in their eyes that might do all of the work. These photoreceptors could have chemicals that detect the magnetic field, pointing the turtles in the right direction.


Many species sense Earth's magnetic field to find their way home.


Molten iron churning in Earth's outer core generates electric currents that in turn create Earth's magnetic field. Lines of magnetic force extend from the South Pole and curve around to rejoin at the North Pole. The field is strongest at the poles and weakest at the equator. Animals can use the variation in magnetic-field strength to navigate,


* What is migration?

* Why do animals migrate?

* What tools do some animals have to help them stay on course while migrating?


* Monarch butterflies' circadian clocks, which help them follow the sun, are located in their antennae.

* The word "wildebeest" means "wild cattle" in the South African language of Afrikaans.

* A single female Christmas Island red crab will release 100,000 fertilized eggs into the sea that immediately hatch into larvae. The larvae spend the next month in tile sea. They then moll into baby crabs and head to the inland forests to find a new home.


* Do you or anyone you know head south each winter? Or do you stay home throughout all of the seasons? What technologies have humans developed that allow them to comfortably stay year-round ill places like frigid Antarctica and steamy Brazil?


MATH: Wildebeests spend the months of December through May in their southern winter grounds and August through October ill their summer grounds in the north. What percent of tile year do they live in each area? (Answer: December through May is 6 months. 6 months / 12 months in o year = 50 percent of the year in the south. August through October is 3 months. 3 months / 12 months = 25 percent of the dear in the north.)


You can access these Web links at

* VIDEO EXTRA: Watch a video about migration at:

* Check out National Geographic's comprehensive Web site that accompanies its Great Migrations mini-series at: /great-migrations.

* Learn more about monarch butterfly migration at:

* Find out more about the Christmas island red crab migration here: /visitor-activities/migration.html.
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Author:Hamalainen, Karina
Publication:Science World
Article Type:Cover story
Geographic Code:1USA
Date:Mar 21, 2011
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