Evidence for cephalic magnetic map receptors in sea turtles.

Although the ability to sense Earth's magnetic field is phylogenetically widespread, receptors for the magnetic sense have not been identified unequivocally in any animal. Because magnetic fields penetrate biological tissue, magnetoreceptors could hypothetically exist anywhere in the body. To investigate the location of magnetoreceptors in juvenile loggerhead sea turtles (Caretta caretta), we attached a weak magnet, which generated a magnetic disturbance over a small localized area, to three anatomical regions (head, mid-body and posterior). We then observed responses of turtles to magnetic map cues they had been conditioned to associate with food. Responses to the food-associated field decreased significantly when a magnet was attached to the head, but no such disruption occurred when the magnet was placed at the other locations. The results provide the first evidence for cephalic magnetoreceptors in turtles and provide a simple methodology that can be used to help localize receptors in other magnetoreceptive animals.