If the magnetic compass functions in the offshore migration of hatchlings, then turtles must inherit or acquire a preference for swimming toward the magnetic direction that coincides with the seaward direction. In initial experiments investigating the magnetic compass, hatchling loggerheads were permitted to swim toward a dim light in the east (their normal migratory direction) before they were tested in darkness. These animals subsequently swam east to northeast in the geomagnetic field. To determine whether the initial course of the turtles influenced their subsequent magnetic orientation, turtles in one experiment were exposed to light from either magnetic east or west before being tested in darkness (Lohmann and Lohmann, 1994). Hatchlings that had been exposed to light in the east subsequently oriented eastward, whereas those that had been exposed to light in the west swam approximately westward. Reversing the magnetic field resulted in a corresponding shift in orientation, demonstrating that the turtles were indeed orienting magnetically in the dark. An additional group of turtles tested in darkness without prior exposure to light cues was not significantly oriented.

These results indicate that the position of light cues, or perhaps just the experience of maintaining a course toward a specific direction, can influence subsequent magnetic orientation behavior. Moreover, because hatchlings without prior light exposure oriented randomly, the results suggest that turtles do not emerge from their nests with a preferred magnetic bearing, but instead must acquire one.

Although light cues can be used in the lab to induce turtles to maintain consistent headings while swimming, hatchlings entering the ocean under natural conditions initially orient seaward by swimming into waves. Experimental results have indicated that hatchlings can establish a magnetic directional preference on the basis of wave cues (Goff, Salmon, and Lohmann, 1998). Loggerhead hatchlings that had never been in the ocean were tethered inside a wave tank and allowed to swim into waves for 30 minutes. The waves were then terminated and the water became still. Half of the turtles were allowed to continue swimming in the local geomagnetic field, whereas the other half were subjected to a magnetic field with a reversed vertical component, a treatment that has the same effect on the turtle magnetic compass as reversing the horizontal component (Light, Salmon, and Lohmann, 1993). Hatchlings that swam in the Earth's field continued to swim in the direction from which waves had previously approached. Turtles tested in the reversed field, however, swam in the opposite direction. A third group of turtles that swam without previous exposure to waves was not significantly oriented. These results provide additional evidence that turtles do not inherit a magnetic directional preference for the offshore direction, but instead acquire one that is based on other directional cues.

Under laboratory conditions, then, hatchlings can establish a magnetic directional preference in at least two different ways: by swimming towards a light source or by swimming into waves. Taken together, the results suggest that the experience of maintaining a consistent course (by whatever means) is sufficient to establish a magnetic directional preference. Under natural conditions, turtles in the ocean initiate offshore headings by orienting into waves. Thus, it appears that the seaward course a turtle initiates while swimming away from land is transferred to the magnetic compass, so that a hatchling can continue on the same heading after swimming beyond the wave refraction zone and into the open sea.