Numerous animals can detect the Earth's magnetic field and use it as a cue in orientation and navigation. Two distinct types of information can potentially be extracted from the Earth's field. Directional or compass information enables animals to maintain a consistent heading in a particular direction such as north or south. In contrast, positional or map information can be used by animals to assess geographic location, to alter direction at appropriate locations along a migratory route, and to navigate to specific target areas.The use of magnetic positional information has been studied most extensively in sea turtles. These animals can detect at least two geomagnetic elements: the intensity or strength of the Earth's field, and the inclination angle (the angle at which the field lines intersect the Earth's surface). Both elements vary across the Earth's surface so that different oceanic regions typically have different magnetic fields associated with them. For hatchling loggerhead sea turtles undertaking their fust 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 as a component of a classical navigational map, which permits an assessment of position relative to specific geographic destinations. These "magnetic maps" have not yet been fully characterized. They may be organized in several fundamentally different ways, some of which bear little resemblance to human maps, and they may also be used in conjunction with unconventional navigational strategies. Although the magnetic maps of sea turtles lack the high resolution of human global positioning systems (GPS), they provide an interesting conceptual design for low-resolution positioning systems that can function independently of GPS.
展开▼
