Magnetic map sense in animals: effects of geographic and magnetic displacements
Sammanfattning: Migration research is a large and diverse field and in my thesis I have focused on the impact of geomagnetic cues on animal orientation, navigation and migratory fuelling. Several animal species from widely different taxa possess the ability to sense and make use of geomagnetic information during migratory or homing events. I used a map showing isolines for magnetic total field intensity and inclination to investigate the prerequisites for geomagnetic bi-coordinate navigation on a global scale. Areas with narrow or no angular difference between isolines (“no-grid” zones) were considered hard or impossible to navigate within, whereas areas with angular differences of 30° or more were considered ideal. For “no-grid” zones stretching in a north-south direction, the same geomagnetic combination can be encountered on either side of the zone, which may potentially cause problems for migrating animals passing these zones. In order to gain a deeper understanding about how the geomagnetic field may influence animals I gathered publications presenting data from experiments displacing animals in geomagnetic, but not geographic, space. I found 20 studies presenting 40 different experiments with eight species from four taxa. Some of the experiments only altered one magnetic parameter, which sometimes resulted in non-existent magnetic combinations or combinations found at distant locations. It was often not possible to determine whether the animals had used uni- or bi-coordinate navigation. Experiments with loggerhead sea turtles (Caretta caretta) suggested that this species might display different orientation in different intervals of magnetic combinations or alter its orientation after passing a certain “threshold” value or “signpost”. Migratory birds can use the geomagnetic field to make fuelling decisions, which seem to be based on the experience of a decreasing or increasing gradient and not on exact magnetic combinations. Why animals sometimes, but not always, respond to displacement to non-existent or distant magnetic combinations needs more investigation. Two of my studies focus on migratory fuelling in juvenile northern wheatears (Oenanthe oenanthe) magnetically displaced along their migration route, or parallel and to the west of their migration route to simulate a situation where the birds have been drifted off course. I found that the juvenile northern wheatears displaced to a position south of Greenland, due west of the experimental site, increased more in body mass than individuals displaced along the migration route. These birds also ended up at a higher body mass than birds kept in the ambient magnetic field in Sweden and wheatears displaced south along the migration route or south of the position close to Greenland. The position south of Greenland had a higher inclination and field intensity than the ambient geomagnetic field in Sweden, which seem to have been the cues that the birds responded to. In the last study we displaced Russian juvenile wheatears geographically in the high Arctic, from their breeding grounds in northeast Russia, across the north pole to Svalbard. During the transect we performed orientation experiments on the sea ice and the inexperienced birds managed to choose a meaningful mean orientation, that would lead them the closest way to their wintering area, in three out of twelve experiments, one at a location with an inclination of 89.3°. The displaced wheatears experienced very challenging conditions with steep inclination angles, magnetic storms and overcast skies and it is remarkable that they managed to orient at least at some of the test locations.
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