Regional Magnetic Fields as Navigational Markers for Sea Turtles

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The coil system consisted of two different coils arranged orthogonally (7). Each coil measured 2.27 m on a side and was constructed in accordance with the four-coil design of Merritt et al. (26). Turtles were restricted to an area in the center of the coil defined by a horizontal circle of radius 25 cm and a vertical area of about 5 cm; in this region, calculated (26, 27) and measured deviations from perfect field uniformity were less than 1%. For details about the coil, arena, and tracking system, see Science Online (www.sciencemag.org/cgi/content/full/294/5541/364/DC1).

Methods were described in detail in (7). Briefly, hatchlings were collected from their nests on the night when they would otherwise have emerged. Each was tethered in the arena and permitted to swim for 10 to 30 min in the field of the natal beach (inclination 57.5°, intensity 47 μT) toward a dim light in the east, a process that serves to set the initial offshore magnetic heading of the turtles (28). The light was then turned off and the field immediately changed to one of the three experimental fields (see below). After an acclimation period of 3 min, a computer monitored the direction toward which each turtle swam in darkness under the new field condition (7). Each turtle was tested only once under one of the three field conditions; no more than four turtles from the same nest were tested in any given field. The field used to approximate magnetic conditions near northern Florida had an inclination of 59.3° and a total intensity of 49.1 μT (as assessed by five independent measurements with a Schoenstedt digital fluxgate magnetometer, model DM-2220 R). The field used to approximate conditions in the northeastern gyre had an inclination of 59.1° and an intensity of 45.2 μT; the field simulating the southern border of the gyre had an inclination of 16.7° and an intensity of 31.0 μT. The experimental fields were selected on the basis of estimates provided by the International Geomagnetic Reference Field (IGRF) model, 1995 revision, for July and August 1995 (when the data were collected) using latitude 29.0°N, longitude 80.0°W for northern Florida; 43.0°N, 20.0°W for the northeastern gyre region; and 10.0°N, 39.0°W for the southern gyre boundary. The field measured within the arena was within ±0.4° (inclination) and ±0.4 μT (intensity) of the IGRF estimates for each target location. The IGRF declination estimates for the target locations were –5.2° for northern Florida, –10.4° for the northeastern gyre, and –18.7° for the southern gyre. Experiments were conducted in Boca Raton, Florida (declination estimate = –4.6°).

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Secular variation refers to the change in field elements (such as inclination and intensity) that occurs over time. See (9) for more information.

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An interesting speculation is that some of the variation in responses of different turtles to the same field (Fig. 1) might reflect diversity in genes affecting factors such as which fields elicit responses or which direction a turtle swims when a given field is encountered. By producing offspring with variable responses, adult turtles might increase the likelihood that some progeny will survive even if Earth's field changes significantly.

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We thank S. Johnsen, M. Salmon, and three anonymous reviewers for helpful critiques of the manuscript. Supported by NSF grants IBN-9419993 and IBN-9816065 (K.J.L.).