Magnetoreception.

Animals can use the direction of the magnetic field as a compass and the intensity of the magnetic field as a component of the navigational 'map'. Two fundamentally different mechanisms of magnetoreception have been discussed: (1) light-dependent reactions in specialized photopigments lead to radical pairs, with the ratio singlet/ triplet depending on the molecule's alignment with respect to the ambient magnetic field and (2) reactions involving small crystals of magnetite, a specific iron oxide of biogen origin. The first mechanism provides birds and possibly amphibians and insects with compass information; the second, which can theoretically provide animals with information on direction and intensity, appears to mediate intensity information in birds and compass information e.g., in mammals. Little is known about the magnetoreception mechanisms in other animals.