The osmotic magnetometer: a new model for magnetite-based magnetoreceptorsin animals

Homing pigeons and migratory birds are well known examples for animals that use the geomagnetic field for their orientation. Yet, neither the underlyi ng receptor mechanism nor the magnetoreceptor itself is known. Recently, an innervated structure containing clusters of magnetite nanocrystals was ide ntified in the upper beak skin of the homing pigeon. Here we show theoretic ally that such a cluster has a magnetic-field-dependent shape, even in fiel ds as weak as the Earth's magnetic field; by converting magnetic stimuli in to mechanical strain, the clusters can be assumed as primary units of magne toperception in homing pigeons. Since the orientation of the strain ellipso id indicates the direction of the external magnetic field, a cluster of mag netite nanocrystals also has the potential to serve as the basis of the so- called inclination compass of migratory birds. It is quantitatively demonst rated that the magnetic-field-induced shape change of a cluster can be ampl ified as well as counterbalanced by means of osmotic pressure regulation, w hich offers an elegant possibility to determine the magnetic field strength just by measuring changes in concentration.