Magnetic Map of Juvenile Green Turtles
Turtles exposed to the magnetic field that exists north of the testing site oriented approximately southward (see figure below). In contrast, those exposed to the field that exists south of the test site swam approximately northward. Thus, turtles responded to each field by swimming in a direction that would have led them back to their home area had they actually been at the location where each field naturally occurs.
The figure to the right shows the orientation of juvenile green turtles tested in magnetic fields replicating those that exist at two different locations (marked by stars on the map). In the orientation diagrams, each dot represents the average swimming direction (mean angle) of a single turtle. The arrow in the center of each circle represents the average direction of the group. The dashed lines represent the 95% confidence interval for the mean angle (in other words, we can be 95% certain that the average direction of the group falls within the sector of the circle demarcated by the dotted lines). The two distributions are statistically different, indicating that turtles can distinguish between magnetic fields that exist in different geographic locations within their environment.
Previous experiments have shown that juvenile green turtles home to their feeding areas if displaced and released in their natural habitat. In this experiment, turtles responded to each field by swimming in a direction that would have led them back to their home area had they actually been at the location where each field naturally occurs. These results provide strong evidence that juvenile sea turtles have a magnetic map that functions in navigation and assists them in moving toward specific destinations.
The precise feature or features that turtles detect and exactly how the magnetic map is organized remain to be determined. Turtles may have a map in which magnetic cues provide only one coordinate, while another environmental feature (perhaps in this case the shoreline) provides the second. Thus, turtles might swim along the coast until encountering a magnetic parameter that marks a specific coastal location. Alternatively, turtles might detect two different magnetic elements (such as inclination and intensity) and rely on bicoordinate magnetic navigation. Although the regional isolines of the various magnetic elements have similar patterns near Florida, no two sets are exactly parallel; thus, bicoordinate magnetic navigation is hypothetically possible if turtles are sufficiently sensitive to two parameters. Regardless, however, the results demonstrate for the first time that, as sea turtles mature, they acquire the ability to exploit magnetic information in a more complex way than do hatchlings, using it as a component of a classical navigational map that permits an assessment of position relative to specific geographic goals.
Lohmann, K. J., Lohmann, C. M. F., Ehrhart, L. M., Bagley, D. A., and T. Swing. 2004. Geomagnetic map used in sea turtle navigation. Nature. 428: 909-910. [Download pdf]
last edited 04/28/2004