Certain tissues in some species grow crystals of magnetite, a magnetic material whose crystals are similar to miniature magnets. The Earth's magnetic field, which is present everywhere, interacts with these crystals. If a magnet were free to move, it would point point north like a compass. However, when it is constrained and made to move through the Earth's field, it generates a minute electric current proportional to the direction of motion. Since the biogenic magnetite is integrated with the animal's nervous system, it can sense direction because of the electrical pulses emanating from the magnetite in its body.
Pigeons and some other birds use the Earth's magnetic field for navigation. In one experiment, pigeons whose vision had been artificially blocked found their way back to their loft. Magnetite is found in the head in the region above the beak. Magnetic-sensitive birds can not only navigate by sensing the magnetic field, but also determine their position in space. Bees have rings of magnetite around their abdomens. It is believed that they use magnetic navigation to travel between their hives and sources of food.
Chains of magnetite crystals are found in the cartilage in the front of the skull in sockeye salmon and some other species of fish. Magnetite is also found in dolphins, and it is believed that dolphins often ground themselves when they encounter an abrupt change in magnetic field that interferes with their navigation. A class of sea mollusks known as chitons have magnetite in their teeth. Marine magnetite-bearing bacteria follow the Earth's magnetic lines of force -- it is speculated that some magnetite ore deposits were formed in geologic times by magnetic concentration of such bacteria.
The presence of magnetite in human brain tissue, healthy as well as diseased, has been established by rigorous scientific studies. Certain diseases, such as epilepsy, appear to be related to the presence of magnetite. Studies of magnetite in brain tissue are also highly significant for evaluating the effects of electromagnetic radiation on human health; our environment is awash with low-level electromagnetic radiation emanating from transmission lines, cell phones and a host of other electronic devices.