Regional Magnetic Fields as Navigational Markers

for Sea Turtles


 

Previous experiments have shown that hatchling loggerheads can detect magnetic inclination angle and magnetic field intensity, two geomagnetic features that vary across Earth's surface and might, in principle, provide positional information to a migrating turtle.  In these initial experiments, however, one of the two parameters was held constant while the other was varied.  This approach was necessary to demonstrate that turtles can detect each field element.  In nature, however, these field elements vary together across Earth's surface.  As a result, most pairings of inclination and intensity used in previous studies resulted in fields with combinations of parameters that do not naturally occur in the North Atlantic.

 

To determine whether hatchlings can distinguish among the magnetic fields actually found in different oceanic regions, we subjected hatchling loggerheads to fields replicating those found in three widely separated locations along their migratory route in the North Atlantic gyre.  Except for the magnetic fields used, the procedure was identical to that used in the previous studies involving inclination and intensity.

 


Results: Turtles exposed to a field replicating one that exists offshore near northern Florida swam east-southeast.  Those exposed to a field like one found near the northeastern edge of the gyre swam approximately south.  Turtles exposed to a field like one found near the southernmost part of the gyre swam west-northwest.  The data are shown in the diagrams below. 


Orientation of hatchling loggerheads in magnetic fields characteristic of three widely separated locations (marked by black dots) along the migratory route. 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.


Interpretation: The results indicate that loggerhead turtles can distinguish among magnetic fields that exist in widely separated oceanic regions.  In addition, turtles responded to the different fields by swimming in directions that would, in each case, help them remain within the North Atlantic gyre and advance along their migratory route.  We conclude that young sea turtles can in effect exploit regional magnetic fields as open-ocean navigational markers.  The turtles emerge from their nests ready to respond to specific fields with directed movement, and these responses are appropriate for facilitating movement along the migratory pathway.

Reference:

Lohmann, K. J., Cain, S. D., Dodge, S. A., and Lohmann, C. M. F.  2001.  Regional magnetic fields as navigational markers for sea turtles.  Science  294: 364-366.

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