Testing for the presence of magnetite in the upper-beak skin of homing pigeons.

We carried out magnetic and nonmagnetic experiments on fresh, upper-beak skin tissue samples isolated from six pairs of homing pigeons to test whether the tissue contains magnetite particles. Results of (1) room-temperature isothermal remanent magnetization (IRM) acquisition and alternating field (AF) demagnetization, (2) low-temperature demagnetization of saturation IRM acquired at 5 K in a field of 5 tesla (T) (SIRM(5 K)) after zero-field cooled (ZFC) and field cooled (FC) treatments, and (3) cycling of the saturation IRM acquired at 300 K in a field of 5 T (SIRM(300 K)) between 5 and 300 K, indicate the presence of magnetite in the measured samples. A significant loss of SIRM(5 K) below 20 K suggests the dominance of superparamagnetic (SPM) particles. The SIRM acquisition capacity of the female pigeon is stronger than that of the male pigeon in all four measured pairs, suggesting for the first time that the magnetite concentration is probably sex dependent. Light microscopic observation on the histological sections stained with Prussian Blue detected the presence of some tiny, dotted, dark-blue staining Fe3+ aggregates (size 1-4 microm) located directly beneath the subcutis within strands of connective tissue, nearby the rim of the regions full of red nuclei. The results of this study support the idea that homing pigeons may have a magnetite-based receptor, which potentially could be used for sensing the Earth's magnetic field during navigation.