�����鶹��Ƶ

Baby loggerhead turtles “dance” when they are expecting food, a behaviour that researchers have used to investigate their navigation abilities. By learning to associate a magnetic field with a food, this cute display has helped indicate that the sea turtles have two distinct geomagnetic senses to help them navigate during their epic ocean journeys.

“The turtle dance is a strange pattern of behaviour that emerges quickly in young captive sea turtles when they figure out that food comes from above,” says at the University of North Carolina at Chapel Hill. “They would get very excited and raise their heads up out of the water and come swimming over, and often if the food wasn’t dropped in immediately, they would begin to flap their flippers and spin around.”

Lohmann and his colleagues realised that there might be a way to use this behaviour to reveal how turtle navigation works. They put juvenile loggerhead sea turtles (Caretta caretta) in tanks surrounded by coil systems that created magnetic fields in the water, replicating those in their natural habitats.

The juveniles spent an equal amount of time in two magnetic fields, but were only fed in one of them. Soon, when they were in a magnetic field they associated with food, the turtles started to dance in anticipation, a learned behaviour reminiscent of Ivan Pavlov‘s famous dog experiment. “We demonstrated that the turtles can learn to recognise magnetic fields,” says team member at Texas A&M University.

The researchers then reproduced a magnetic field near the Cape Verde islands, an area where loggerheads tend to when migrating. The team demonstrated that the juvenile turtles also did this. Then the researchers trained other turtles to associate the Cape Verde field with food.

One of the ideas about how some animals sense magnetic fields is that there is a complex set of chemical reactions, possibly taking place in the eye, that are influenced by Earth’s magnetic field.

To try to affect any such system, the team used an additional magnetic field that oscillates at a radio wave frequency, which should interfere with that cascade of chemical reactions.

Regardless of whether the oscillating field was turned on, the turtles could detect the underlying Cape Verde magnetic signature and would dance, which suggests their map sense isn’t dependent on this chemical reception mechanism. But the oscillating field did make them turn in random directions, rather than south-west.

“This is good evidence that there are actually two different magnetic senses in the turtles: one that is used for the map sense, one that is used for the compass sense,” says Lohmann. “The simplest explanation would be that the magnetic map sense does not depend on this chemical magnetoreception process, but the magnetic compass sense does.”

“The magnetic map sense is a positional sense, kind of like a GPS, and their compass sense tells them which way to go,” says Goforth. “This is probably how they’re getting back to important ecological locations such as feeding grounds and nesting areas.”

“It’s a new way of thinking about how turtles are using the magnetic field to navigate,” says at the University of Massachusetts Amherst. “What’s really fascinating is we still don’t understand how they’re even perceiving the magnetic field. So, this is getting at what is going on mechanistically.”