Migratory guidance mechanisms in marine turtles

Hatchling sea turtles emerge from underground nests, scramble to the ocean, and migrate to the open sea. Loggerhead Caretta caretta hatchlings from eastern Florida, U.S.A., appear to use three different sets of orientation cues sequentially as they migrate offshore. On the beach, hatchlings crawl seaward by orienting toward the low, bright oceanic horizon. In the ocean, turtles initially orient offshore by swimming into waves, which can be detected as wave surge motion or orbital movements. As a hatchling crawls across the beach, swims offshore, or both, it apparently transfers the initial seaward heading to a magnetic compass. This transfer of directional information may allow turtles to maintain offshore courses in deep water where waves no longer move reliably toward land. Sea turtles may use the earth's magnetic field not only as a cue for compass orientation but also as a source of positional information. Results have demonstrated that loggerheads can detect inclination angle and field intensity, two geomagnetic features that vary predictably across the earth's surface. Hatchlings responded to magnetic features found along their migratory route by swimming in directions that would presumably favor retention within the North Atlantic gyre, an oceanic region favorable for growth and development. How adult turtles navigate to their natal regions to nest is not known. We speculate, however, that adults may exploit geomagnetic features in one of several different ways to guide themselves into the vicinity of a nesting area.