Iron containing short nerve branches in the upper beak of birds may serve as a magnetometer to measure the vector of the Earth magnetic field (intensity and inclination) and not only as a magnetic compass, which shows the direction of the magnetic field lines. Several years ago, the Frankfurt neurobiologists Dr. Gerta Fleissner and her husband Prof. Dr. Günther Fleissner discovered these structures in homing pigeons and have, in close cooperation with the experimental physicist Dr. Gerald Falkenberg (DESY Hamburg), characterized the essential iron oxides.
"After we had shown the system of dendrites with distinct subcellular iron-containing compartments in homing pigeons, immediately the question was posed whether similar dendritic systems may be found in other bird species, too," as Gerta Fleissner, the principal investigator, comments. Meanwhile they could describe similar candidate structures in the beaks of various avian species. X-Ray-fluorescence measurements at DESY demonstrated that the iron oxides within these nervous dendrites are identical. These findings were published few days ago in the high-ranking interdisciplinary online journal Plos One.
More than about 500 dendrites in the periphery encode the magnetic field information, which is composed in the central nervous system to a magnetic map. It obviously does not matter, whether birds use this magnetic map for their long distance orientation or do not -- the equipment can be found in migratory birds, like robin and garden warbler, and well as in domestic chicken. "This finding is astonishing, as the birds studied have a different life styles and must fulfil diverse orientational tasks: Homing pigeons, trained to return from different release sites to their homeloft, short-distance migrants like robins, long-distance migratory birds like garden warblers and also extreme residents like domestic chicken," explains Gerta Fleissner.
In order to provide convincing evidence, several thousand comparative measurements were performed. The beak tissue was studied under the microscope to identify iron-containing hot spots as a basis for consecutive physicochemical analyses. At the Hamburg Synchrotron Strahlungslabor at DESY the distribution and quantity of various elements was topographically mapped by a high resolution X-ray device. "Here, the beak tissue can be investigated without destruction by histological procedures concerning the site and detailed nature of magnetic iron compounds within the dendrites," Gerta Fleissner explains and she emphasizes that the cooperation with the experimental physicist Gerald Falkenberg as project leader at DESY was essential for this scientific breakthrough.
Specialized iron compounds in the dendrites locally amplify the Earth magnetic field and thus induce a primary receptor potential. Most probably each of these more than 500 dendrites encodes only one direction of the magnetic field. These manifold data are processed to the brain of the bird and here -- recomposed -- serve as a basis for a magnetic map, which facilitates the spatial orientation. Whether this magnetic map is consulted, strongly depends on the avian species and its current motivation to do so: migratory birds, for example, show magnetic orientation only during their migratory restlessness, as could be shown in multiple behavioural experiments by Prof. Wolfgang Wiltschko, who has discovered magnetic field guided navigation in birds. The cooperation with his research team has suggested that magnetic compass and magnetic map sense are based on different mechanisms and are localized at different sites: The magnetic compass resides in the eye, the magnetometer for the magnetic map lies in the beak.
"The now published results clearly help to falsify the old myths concerning iron-based magnetoreception via randomly distributed sites everywhere in the organism, like blood, brain or skull. They rather deliver a sound concept how to identify magnetoreceptive systems in various organisms," Günther Fleissner happily reports. These clear and well-reproducible data may be used as a basis for further experimental projects that might elucidate the manifold unknown steps between magnetic field perception and its use as a navigational cue.
The project was funded by Frankfurt foundations (Stiftung Polytechnische Gesellschaft and Kassel-Stiftung), by the "Freunde und Förderer" of the Goethe University, by the ZEN-program of the Hertie-Stiftung and the Deutsche Forschungsgemeinschaft. The elaborate measurements at the HASYLAB are based on grants of the Helmholtz-Foundation.
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