Earth's Magnetism And Migratory Behavior Of Loggerhead Sea Turtles
Many species of animals have remarkable abilities to find their way over thousands of kilometers of land and sea during migration. Homing animals can do many things that humans cannot do themselves. For instance, in one experiment, a specie of seabird, the Manx shearwater (Puffinus puffinus), which had never been more than 17 kilometers from home, was removed from her nest on an island off the coast of Wales, flown to Boston, Massachusetts in the United States, and then released. Later, even before the letter announcing her release reached the scientists in Wales, she was back on Wales! To perform an equivalent record, such as sailing from Boston to Wales across the Atlantic ocean, a human would have to spend hours learning to use navigation instruments to cross the ocean. A map would also be needed to find the nest on the other side. Other animals such as caribou, monarch butterflies, salamanders, and sea turtles all perform similar journeys without mechanical aids.
Scientists believe that these animals have several built-in navigation systems, including electrical currents in some species of fish and signals from sunlight in birds, which appear to fall back on the magnetic field mainly when sunlight is blocked by clouds. To prove that some species of animals indeed use Earth’s magnetic field to find their way during migration, a team of scientists conducted a research using sea turtle hatchlings, particularly the loggerhead species. Loggerhead sea turtles (Caretta caretta) are known for their agility to travel long distances across the warm regions of the sea. They mostly build their nest in the southeastern part of the United States and travel hundreds of kilometers always finding their way back to the same beach to lay eggs. They spend up to 10 years swimming around the North Atlantic before returning to nest.
Headed by Kenneth Lohmann, a marine biologist at the University of North Carolina, the research seeks to prove that sea turtles determine their latitude in the Atlantic by using the magnetic field of the Earth. Lohmann believes that sea turtles may use their magnetic sense to stay within warm regions of the North Atlantic then navigate a return to their home beaches.
The turtle hatchlings were leashed in a harness which was attached to an arm in the middle of a circular tank containing seawater. The arm was connected to a computer so that the turtles could swim for hours while the computer made a continuous record of their movements. A system of coils surrounded the tank that could be used to create different magnetic fields around the turtles. The team observed that the turtles swam toward light no matter what the inclination of the magnetic field around them; so, they tested them in the dark. After varying the inclination of the magnetic field they found that at 57º inclination or less the turtles swam northeast. However, at 60º inclination, the turtles made a right angle-turn until they were swimming south.
What made the turtles go south? Studying the migratory behavior of sea turtles, particularly the loggerhead species, scientists attributed this to the sea turtles’ genetic tendency to turn south when they reach 60º. Such a turn ensures that the turtles end up with Sargasso Sea, a warm region, and not in the cold water of the cold Atlantic.
The navigation pattern of Loggerhead turtles hatching on Atlantic beaches was plotted by scientists. According to their findings, loggerhead turtles always hatch at night around the time of the full moon. They then cross the beach toward the light of the moon, which rises in the east. Upon reaching the ocean, they stop responding to light and swim directly into the waves, which keeps them swimming away from the land. The wave patterns in the sea become irregular. The turtles at some point switch from swimming into waves to swimming northeast. They could find their direction by the sun’s position during the day. However, Lohmann suspects that the turtles use their magnetic sense, which can act as compass at night as well as during the day.