Where on earth can animals use a geomagnetic bi-coordinate map for navigation?
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Many animal taxa have been shown to possess the ability of true navigation. In this study we investigated the possibilities for geomagnetic bi-coordinate map navigation in different regions of the earth by analysing angular differences between isolines of geomagnetic total intensity and inclination. In no-grid zones where isolines were running almost parallel, efficient geomagnetic bi-coordinate navigation would probably not be feasible. These zones formed four distinct areas with a north-south extension in the northern hemisphere, whereas the pattern in the southern hemisphere was more diffuse. On each side of these zones there was often a mirror effect where identical combinations of the geomagnetic parameters appeared. This may potentially cause problems for species migrating long distances east-west across longitudes, since they may pass areas with identical geomagnetic coordinates. Migration routes assumed for four populations of migratory passerine birds were used to illustrate the possibilities of geomagnetic bi-coordinate map navigation along different routes. We conclude that it is unlikely that animal navigation is universally based on a geomagnetic bi-coordinate map mechanism only, and we predict that the relative importance of geomagnetic coordinate information differs between animals, areas and routes, depending on the different conditions for bi-coordinate geomagnetic navigation in different regions of the earth.