Detection of Magnetic Field Intensity

by Hatchling Sea Turtles


 

Previous experiments have shown that hatchling loggerheads can detect magnetic inclination angle. However, inclination is not the only field element that might provide turtles with positional information. A second geomagnetic feature that varies across the surface of the Earth is field strength or intensity.

 

To determine whether hatchlings can perceive differences in magnetic field intensity, turtles were exposed to magnetic fields with different intensities but the same inclination angle. Except for the magnetic fields used, the procedure in these experiments was identical to that used in the previous study involving inclination angle.

 


Results: Turtles exposed to a field with an intensity of 52,000 nT swam eastward. This field is about 10.6% stronger than the field that exists at the natal beach in Florida and corresponds to the intensity that turtles encounter near North and South Carolina as they progress along their migratory route. Turtles exposed to a 43,000 nT field (a field 8.5% weaker than the natal beach field, and one that the turtles first encounter on the eastern side of the Atlantic near Portugal) swam westward. The data are shown in the diagrams below. 


Orientation of hatchling loggerheads in magnetic fields with two different intensities. The first intensity (52,000 nT) exists in a location near North and South Carolina (marked by "a"). The second intensity (43,000 nT) is one that turtles first encounter on the far eastern side of the Atlantic near the coast of Spain and Portugal. In the orientation diagrams, each dot represents the mean angle (average direction) of a single hatchling.  The arrow in the center of each circle represents the mean angle of the group.  The dashed lines represent the 95% confidence interval for the group mean.  Standard tests for circular statistics indicated that each group of turtles was significantly oriented, meaning that the turtles had a directional preference as a group and did not orient randomly. Turtles tested in a field with an intensity of 52,000 nT oriented (on average) approximately east to northeastward. Those tested in a field with an intensity of 43,000 nT oriented (on average) approximately westward.


Interpretation: These results demonstrate that hatchling loggerheads can distinguish between field intensities that occur in different locations along their migratory route. Moreover, because eastward orientation near South Carolina and westward orientation near the coast of Portugal would both function to keep young turtles within the gyre, the results imply that young turtles can, in effect, derive positional information from field intensity.

Reference:

Lohmann, K. J. and Lohmann, C. M. F.  1996.  Detection of magnetic field intensity by sea turtles.  Nature  380: 59-61.