New publication on geomagnetic bi-coordinate navigation

A bi-coordinate map sense based on geomagnetic information was first suggested by Viguier, almost 130 years ago, but it did not receive much attention until the beginning of this century. The magnetic map hypothesis in animal navigation has attracted an increasing amount of attention during the last decade and magnetic navigation has been demonstrated in several animal species from widely separated taxa.

In a recent paper by Boström et al. 2012, published in Ecography, we investigated the global geomagnetic prerequisites for geomagnetic bi-coordinate navigation in order to pinpoint possible problematic, as well as suitable, areas for migrating animals. We analysed the angular difference between isolines for inclination and total field intensity on a global map divided into 3° lat. × 3° long. squares and produced a map illustrating different regions of the earth defined by different angular intervals. This map revealed four vertical bands of little or no variation between isolines (‘no-grid’ zones) in the northern hemisphere as well as a few areas in the southern hemisphere. Most part of the globe show intermediate angular difference (2-30º), but there are also large regions both at northern latitudes and in the Pacific, Indian and south Atlantic Oceans where the two isolines form a clear grid.

Illustration of the possibility for animals to use a bi-coordinate map based on geomagnetic inclination and total field intensity ona global scale. Areas with an angular difference of 2 ° are shown in red, 2– 30° in yellow and 30 ° are shown in green.

We also analysed how the geomagnetic field varied along four assumed migration routes as well as two fictive routes across the US that may have been followed by birds geographically displaced in this region. For animals migrating long distances across longitudes we discovered a possible cause of difficulty. When moving east-west across one of the ‘no-grid’ zones stretching north-south the animals have to first move against higher values for both total field intensity and inclination, past a peak and then continue towards lower values. For juvenile individuals using an innate navigation system more complex than the clock-and-compass model, this information would have to be incorporated into the endogenous migration program. We also discussed possible problems for animals dealing with the ambiguity that may arise when migrating in regions where mirrored combinations occur. These mirrored combinations often occur on both sides of the north-south stretching ‘no-grid’ zones and will hence also affect animals moving east-west across longitudes.