Inherited Magnetic Maps in Salmon and the Role of Geomagnetic Change.

Migration in animals has evolved as an adaptation to environmental variability across space and through time. The availability of reliable sensory cues and guidance mechanisms used in navigating among disparate locations is an essential component of this behavior. An "inherited magnetic map" is navigational solution that has evolved in some marine animals that, without prior experience or guidance from older conspecifics, migrate to oceanic foraging grounds. Laboratory experiments demonstrate that navigationally naïve salmon encountering magnetic fields characteristic of certain regions along their migratory route will bias their swimming in a particular direction. Simulations of this behavior within realistic models of oceanic circulation suggest that such behavior is highly adaptive, making the migratory route more predictable and facilitating movement into favorable oceanic regions. Such behavior is possible due to the spatial gradients of components of the geomagnetic field (e.g., the inclination angle of field lines and the total field intensity) that provide a bicoordinate grid across much of the Earth's surface. However, this environmental feature is not static, but experiences gradual and unpredictable changes that can be substantial over successive generations. Thus, drift of the geomagnetic field, in addition to variable oceanic conditions, could play a major role in shaping the distribution of marine taxa that are dependent upon such mechanisms for migratory guidance. Several possibilities are discussed for how animals might mitigate the effects of geomagnetic drift, such as calibrating their inherited magnetic map relative to the field in which they develop. Further exploration of the dynamics of the geomagnetic field in context of animal navigation is a promising avenue for understanding the how animals deal with an ever-changing environment.