'Dancing' turtles prove use of magnetic field as a map

PARIS — Dancing turtles have proved for the first time that some animals use Earth's magnetic field to create a personal map of their favorite spots, scientists said on Wednesday.

Some animals that migrate across the globe, such as birds, salmon, lobsters and sea turtles, are known to navigate using the magnetic field lines that stretch from Earth's North to South Pole.

Scientists knew the animals used this magnetic information as a compass to establish where they were. Now, they increasingly believe turtles are also able to plot a magnetic map featuring important places such as nesting or feeding spots.

This would require the migratory animals to "learn the magnetic coordinates of the destination", according to a study in the journal Nature led by Kayla Goforth of the University of North Carolina.

The study said the research provides the first "direct evidence that an animal can learn and remember the natural magnetic signature of a geographical area".

The researchers found that the turtles' talent for mapmaking was separate from their inner compass, suggesting that the two forms of "magnetoreception" work in different ways.

For the experiment, the scientists put young loggerhead turtles in a tank surrounded by a magnetic coil that replicated the magnetic field of the Atlantic Ocean.

Every day over two months, the scientists changed the magnetic field of the tank between the North American coast and the Gulf of Mexico. The turtles, however, were only fed when they received the magnetic information from one of the areas.

When the turtles anticipated food, they flapped around, opening their mouths and spinning in circles in the water. The researchers filmed this behavior, dubbed the "turtle dance".

The turtles danced with the most enthusiasm in the tank when they knew they would get food.

This was "strong evidence" that turtles can learn the magnetic signatures of "specific geographical areas", the researchers said.

Even when tested four months later, the turtles still knew where they should dance.

No one knows exactly how animals tune into this magnetic information. One theory is that some can detect the magnetic field's influence during a chemical reaction between light-sensitive molecules.

However, when the researchers tried to interfere with this process by using what is called radiofrequency fields, the turtles kept on dancing in their spot, undisturbed.

A separate experiment testing the turtles' inner compasses was more successful.

In a tank replicating the magnetic conditions of the West African archipelago Cape Verde, the radiofrequency emissions seemed to scramble the turtles' compasses, sending them off in random directions. The researchers concluded that "a reasonable working hypothesis is that the compass sense relies on chemical magnetoreception, whereas the map sense relies on an alternative mechanism".

This hypothesis is backed up by signs that other migratory animals, such as birds may also have dual magnetic field receptors.

Agencies via Xinhua