Magnetic Navigation by an Avian Migrant?

Abstract

Spatial variation in the geomagnetic field provides a potential source of information about geographic position (map information) that has been suggested to play a role in animal navigation (Yeagley 1947; Gould 1980; Moore 1980; Walcott 1980). To use a magnetic map, an animal would have to learn the alignment, and perhaps the steepness, of the gradient of at least one magnetic component within its normal range of movement. It would then have to extrapolate this gradient or gradients beyond its area of familiarity. A comparison of the value of a magnetic field component at an unfamiliar site with the home value of that component would provide information about the animal’s position along the gradient (Phillips 1996; Wallraff 1991). Nonparallel gradients of two different components (either or both of which could be magnetic) could then provide the basis for bicoordinate position-fixing (true navigation; Griffin 1952). Spatial gradients in the geomagnetic field are, however, extremely weak. The average rate of change in magnetic total intensity is only 3–5 nT km−1, ranging from values of approximately 25 000 nT near the magnetic equator to 65 000 nT at the magnetic poles. The average rate of change in magnetic inclination is only about 0.01° km−1, ranging from 0° at the magnetic equator to 90° at the magnetic poles. The accuracy of map information derived from the geomagnetic field depends in part upon the sensitivity of the underlying magneto-reception mechanism. For example, a map with a resolution of 100 km would require total intensity measurements accurate to approximately 1% (500 nT), whereas a resolution of 1 km would require measurements accurate to approximately 0.01% (5 nT). The use of a magnetic map, therefore, may require an extremely high level of sensitivity to variation in the intensity and/or inclination of a static magnetic field and, thus, may help to establish the lower limits of such sensitivity in a biological system (Kirschvink and Gould 1981; Semm and Beason 1990; Walcott 1991). The possibility of a magnetic map or geographic position sense, therefore, raises fundamental questions not only about bicoordinate navigation, but also about the biophysical mechanism(s) that mediate sensitivity to the geomagnetic field and about the limits of sensory perception.