Turtles Use Earth's Magnetic
Field As Map & Compass

By Steve Connor
Science Editor
The Independent - UK
The mystery of the migrating sea turtle may have been solved. Scientists have shown that the marine reptile can use the earth's magnetic field as both a compass and a map.
An experiment by marine biologists working off the coast of Florida has demonstrated the phenomenal ability of the green sea turtle to sense subtle variations in the earth's magnetic field.
Turtles migrate thousands of miles over open ocean and the research suggests the animals can sense small perturbations in the north-south direction of the magnetic field - both to work out where they are and where they should be going.
Green sea turtles, Chelonia mydas, are especially accurate in terms of their migration, with some of them regularly commuting from the same feeding and breeding sites off the coast of Brazil to Ascension Island, the tiny dot of land just six miles wide in the middle of the South Atlantic.
The scientists, led by Kenneth and Catherine Lohmann, a husband and wife team at the University of North Carolina in Chapel Hill, investigated the green turtle's homing instincts using a giant, two-storey magnetic coil they built around an aquarium in which the field could be artificially controlled.
The Lohmanns and their colleagues used the coil to reproduce the magnetic fields that are known to exist at various locations along the Florida coast so they could expose captive turtles to fields from two widely different geographical locations.
"We captured juvenile turtles using a 400m net stretched parallel to the coastline. Each turtle was placed in a cloth harness and tethered to a tracking device in the pool of water inside the magnetic coil," Dr Catherine Lohmann said.
The scientists varied the magnetic field of the experiment to mimic the fields that exist at two sites, one just off the north coast of Florida and the other at the state's southern tip. The test site where the turtles were caught was midway between.
"Half of the turtles swam in a magnetic field that exists at a location about 350km (217 miles) north of where they actually were. The other half swam in a field that exists at an equivalent distance to the south," she said.
The study, published in the journal Nature, found that the turtles exposed to the northern field responded by swimming south while those in the southern field swam north.
In each case, therefore, the turtles swam in a direction that would have taken them home if they had been at the place where the magnetic field exists, Dr Lohmann said.
Kenneth Lohmann said the experiment shows the turtles are using the Earth's magnetic field in a more complex way than if it were used as a mere compass pointing north-south.
Scientists have shown other migratory animals, notably birds, use a variety of natural cues that act as a map and compass, and those include the earth's magnetic field.
As yet, biologists have not located the sensory organ that can detect changes in a magnetic field. One idea is that it might be a general property of nerve cells that are full of electrically charged particles.
© 2004 Independent Digital (UK) Ltd



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