Lasting for decades, you ask how many species like birds of passage are able to orientate themselves in the earth´s magnetic field. The magnetoreception, a sensory perception which connects evidenced ferrimagnetic material (magnetite, maghemite) with neural structures, depending on direction of an external magnetic field, is discussed[1,2,3]. Those biogene minerals are found in species spread about most animal classes and humans[4,5,6,7], even in plants and bacteria[8,9]. Yet, the exact mechanism of magnetoreception remains unclear, existing opinions leave many unsolved questions: The particles, firmly embedded in cellular material, are not able to turn like a compass needle. There remain unsolved problems in hypothetical approvals of moving superparamagnetic crystals causing mechanical stimuli in the magnetic receptor[10,11]. Proven cell material supposed to be a magnetoreceptor is being called into question by found macrophages. Furthermore, estimated magnetic forces are extremely low. Disruptive influences like heat or acceleration forces by moving animals which overlay the minimal amount of the mechanoreceptoric effect may be additional difficulties. Anyhow, the complete neuronal circuitry is not clearly described.
In this hypothesis a possible solution bypassing all these problems is given. It is a magnetic receptor which is based on electrochemical processes: The earth´s magnetic field generates a power field in the area of magnetite particles. Thus, iron atoms pass into solution as cations in an electrolyte and cause neuronal excitations. Finally, the nervous excitation will be processed in the brain and may entail an animal´s reaction to it´s environment.
Starting point for this hypothesis are the proven clusters of biogene magnetite (Fe3O4) and maghemite (Fe2O3). Due to it´s high susceptibility, the magnetic mineral acts as a sort of amplifier. In corners, edges or tips of the particles, as well as in arrangements of clusters, different magnetic flux densities arise caused by alternating direction of the earth´s magnetic field. If the edge region of two certain areas A and B on the mineral (cluster) is combined with an electrolyte, an electrochemical cell results. In the event that area A and B have different values of magnetic flux density, an electrochemical potential will be created between A and B and, consequently, an ionic current flows. This effect has been proved for ferromagnetic material.
Assuming that, a proposal for the ultrastructure of a cell-based electrochemical magnetoreceptor emerges: One biogene magnetite particle is surrounded by cell material. At the tip of this particle in current orientation, the earth´s magnetic field produces a high magnetic flow density, whereas it is low at the magnetic field´s immediate environment. The cell material builds an hermetically enclosed space in the outdoor region from A and B, in which the iron electrolytes are located. Because of the different magnetic flux densities, more iron ions will dissolve at area B than at A. Thus, an electrochemical potential is provided. After activation of the synapse and depolarization of the postsynaptic membrane, the electric voltage difference will response an ion flow through gates (membrane apertures) in the sensory cell. Within the cell, stimuli are transmitted along soma and axon to central nervous system. The current circuit is closed by an electron current e- in the metallic mineral. More than one of those electrochemical units can multiply the tapped electric voltage in the soma supported by several dendrites.
For the electrochemical processes at area A and B lead to minimal deformations of the iron mineral´s surface, cell mechanisms has to intervene to regulate the balance of generation and degradation. This could be done by surrounding cells, for example degradation of the formed iron-rich structures by locally based macrophages.
Considering the electrochemical magnetite-based magnetoreception, it agrees with most existing theories. Mainly, the principle can be transferred to highly developed species having sensory neurons. In other living systems like bacteria, sharks and plants, possibly, the presence of alternative mechanisms or morphologies for magnetoreception should be taken into account.
 Fleissner G, Holtkamp-Rötzler E, Hanzlik M, Winklhofer M, Fleissner G, Petersen N, Wiltschko W (2003): Ultrastructural analysis of a putative magnetoreceptor in the beak of homing pigeons. J Comp Neur 458(4):350-360.
 Mouritsen H. Ritz T (2005): Magnetoreception and its use in bird navigation. Current Opinion in Neurobiology 15(4):406-414.
 Heyers D, Zapka M, Hoffmeister M, Wild JM, Mouritsen H (2010): Magnetic field changes activate the trigeminal brainstem complex in a migratory bird. Proc Natl Acad Sci USA 107(20):9394-9399.
 Kuterbach DA, Walcott B, Reeder RJ, Frankel, RB (1982): Iron-Containing Cells in the Honey Bee (Apis mellifera). Science 218:695-697.
 Hsu C-Y, Ko F-Y, Li C-W, Fann K, Lue J-T (2007): Magnetoreception System in Honeybees (Apis mellifera). PLoS ONE 2(4):e395.
 Walker MM et al. (1997): Structure and function of the vertebrate magnetic sense. Nature 390: 371-376.
 Kirschvink JL, Kobayashi-Kirschvink A, Woodford BJ (1992): Magnetite biomineralization in the human brain. Proc Natl Acad Sci USA 89:7683-7687.
 Størmer FC, Wielgolaski FE (2010): Are magnetite and ferritin involved in plant memory? Rev Environ Sci Biotechnol 9(2):105-107.
 Davila AF, Fleissner G, Winklhofer M, Petersen N (2003): A new model for a magnetoreceptor in homing pigeons based on interacting clusters of superparamagnetic magnetite. Physics and Chemistry of the Earth, Parts A/B/C 28:647-652.
 Treiber CD, et al. (2012): Clusters of iron-rich cells in the upper beak of pigeons are macrophages not magnetosensitive neurons. Nature 484:367-370.
 Hinds G, Rhen FMF, Coey JMD (2002): Magnetic field effects on the rest potential of ferromagnetic electrodes. IEEE Transactions on Magnetics 38(5):3216-3218.
Prof. Dr. Egbert J. Linnebach