Evidence for the use of magnetic map information by an amphibian

The question of whether animals navigate using ‘map’ information derived from one or more spatial gradients in the Earth's magnetic field has been debated for half a century. Although there is evidence that certain animals possess the sensory abilities necessary to perceive at least two magnetic components that vary spatially, there previously has been no direct test of the use of magnetic map information by experienced adult migrants. Magnetic information could provide information about an animal's geographic position along a single axis (‘unicoordinate map’) or could be part of a position-fixing system that provides positional information along two nonparallel axes (‘bicoordinate map’) with the second axis being derived from either magnetic or nonmagnetic cues. Here we report that adult eastern red-spotted newts,Notophthalmus viridescens , displaced approximately 45km NNE of their home ponds oriented in the home direction when exposed either to the ambient magnetic field of the testing site, or to a 2° increase in magnetic inclination (normally found further from the home ponds in the same general direction as the testing site). When exposed to a 2° decrease in inclination resulting in a value that would normally be found on the other side of the home ponds from the testing site, however, newts reversed their direction of orientation. The same changes in magnetic inclination had no effect on shoreward magnetic compass orientation, which does not rely on map information. These findings provide support for two critical predictions of the magnetic map hypothesis, and suggest that information about geographic position along at least one axis relative to home may be derived from the magnetic field.