Sea turtles can extract two types of navigational information from Earth's magnetic field: (1) directional or compass information; and, (2) positional or map information useful in assessing geographic location. Within the latter category, turtles perceive at least two geomagnetic elements: the intensity of Earth's field and its inclination. Both vary across the globe so that different oceanic regions are typically marked by different magnetic fields. For hatchling loggerhead turtles undertaking their first transoceanic migration, regional magnetic fields function as open-sea navigational markers, eliciting changes in swimming direction at crucial points in the migratory route. Older sea turtles use magnetic information in a classical navigational map, allowing them to assess position relative to specific geographic destinations. Although the magnetic maps of sea turtles lack the high resolution of human global positioning systems, they may provide an interesting conceptual design for low-resolution positioning systems that can function independently of GPS.
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