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Thursday, 11 October, 2001, 18:13 GMT 19:13 UK
Turtles steer a steady course
Loggerhead sea turtle, Kenneth J Lohmann
Straying off course can be fatal for a young turtle
Image: Kenneth J Lohmann

Ivan Noble

Pity the poor loggerhead sea turtle, Caretta caretta, on its long migration around the Atlantic Ocean.

Loggerhead sea turtles, Kenneth J Lohmann
Hatchling turtles already have sophisticated navigational ability
Image: Kenneth J Lohmann

If it strays off course, away from the circular current system known as the North Atlantic gyre, the consequences are usually fatal.

But researchers in the United States are now confident that they know how most young turtles do manage to stay on course during their years at sea.

The creatures have an internal "compass" which detects local variations in the Earth's magnetic field and points them in the right direction.

Magnetic fields

Kenneth J Lohmann, of the University of North Carolina, and his colleagues studied hatchling loggerhead sea turtles, an endangered species, to try to discover whether they could track changes in the Earth's magnetic field.

When they start their migration they are about two inches long and completely defenceless

Kenneth J Lohmann
University of North Carolina
They found that when young turtles were placed in a magnetic field which corresponded to a particular location on the North Atlantic gyre, they would swim in the appropriate direction.

This was despite the fact that the young turtles in the experiment had not yet been to sea.

"Our results provide direct evidence that young sea turtles can in effect exploit regional magnetic fields as open-ocean navigational markers," Dr Lohmann and his colleagues write in the journal Science.

"The turtles emerge from their nests ready to respond to specific fields with directed movement; these responses are appropriate for keeping young turtles within the gyre system and facilitating movement along the migratory route," they write.

The Earth's magnetic field varies from place to place in two main ways:

  • It is stronger near the poles and weaker near the Equator;
  • The plane of the field is roughly parallel to the Earth's surface at the Equator and becomes steeper and steeper towards the Poles.

A turtle's life

Besides navigation, loggerhead sea turtles have plenty of problems with which to deal.

If they make it out of the egg without being eaten by ghost crabs, raccoons, skunks, foxes, or dogs, the young hatchlings are preyed upon by mammals, sea birds, crabs, and carnivorous fishes.

Scientists believe this is what prompts them to migrate in the first place.

"The waters very close to shore are probably a very dangerous place for turtles to be," Dr Lohmann told BBC News Online. "When they start their migration, they are about two inches long and completely defenceless.

"Most people who have written about the problem think that probably the turtles have evolved this strategy to get them away from shallow areas. They spend five to 10 years at sea," he said.

Unknown workings

Larger turtles fare better and can deal with a shark attack by using the flat side of their carapace to ward off the predator.

"Sharks are one of the few predators that large turtles still have," Dr Lohmann said. Turtle survivors can make it to the age of 50 or beyond.

Dr Lohmann and his colleagues believe they have proved that the turtles do respond to the Earth's magnetic field, but they are still unclear about how the internal compass works.

"We don't know that yet and we certainly would love to know the answer," he said.

See also:

19 Aug 01 | Sci/Tech
Technology hope for turtles
04 Jul 01 | Northern Ireland
Tourists 'driving species to extinction'
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