Hatchlings that have just entered the ocean appear to orient exclusively on the basis of waves; no evidence presently exists for the involvement of other directional cues at this early stage of the offshore migration. Because waves entering shallow water refract until they approach the beach directly, orienting into waves leads turtles seaward.
In deeper water farther from land, waves no longer provide a reliable indicator of offshore direction. Nevertheless, hatchling loggerheads tracked from a Florida beach continued on the same offshore headings even after entering areas where wave direction no longer coincided with their established courses. The ability to maintain headings seemingly independent of wave direction implies that, after they have distanced themselves from land, hatchlings use one or more alternative sources of directional information to guide their movements.
Loggerhead and leatherback hatchlings are known to orient to the Earth's magnetic field, a cue that might potentially provide sea turtles with a way to maintain a heading without relying on waves. The experiments that first demonstrated that sea turtles can detect magnetic fields involved monitoring the directions that turtles swam toward under various magnetic fields in the lab. For these tests, each hatchling was placed into a nylon-Lycra harness as shown below.
Posted by Jay Roberts at 03:54 AM | Permalink