Clusters of superparamagnetic magnetite particles in the upper-beak skin of homing pigeons: evidence of a magnetoreceptor?

Previous electrophysiological studies on bobolinks, an American migratory s ongbird, and on homing pigeons suggested that the skin of the upper beak ma y be involved in magnetic-field perception, which makes this tissue likely to contain a magnetic field receptor. In the upper-beak skin of homing pige ons, we localised high concentrations of Fe3+, which form distinct coherent elongated structures extending up to 200 mum in length. Rather than being randomly distributed over the tissue, these structures always occur in the same skin layer, the stratum laxum of the subcutis. Using transmission elec tron microscopy (TEM), we identified the material as aggregates of magnetit e (Fe3O4) nanocrystals, in the grain-size range of superparamagnetism at am bient temperatures. The nanocrystals (with grain-sizes between 2 and 5 nm) form densely packed, encapsulated clusters of 1 to 3 mum in diameter. It is demonstrated that such a cluster undercroes shape changes as the magnetic field changes, and thus could represent the core of a magnetic-field recept or. This interpretation is supported by the fact that the found structures are adjacent to nervous material.