The strength of Earth's field varies in a predictable way across the planet's surface, allowing migratory animals to use it for position-finding. By learning the strength of a field that exists at a particular destination, the animals can home in on it. That much is pretty well established in the science field; what has remained mysterious is how these animals use magnetic-field changes to navigate. [What If Earth's Magnetic Poles Flip?]
Scientists think Earth's magnetic field might urge migratory animals in the right direction like a guiding hand pressing on them. "I think it is similar to touch or pressure. The magnetite-based magnetic sense is innervated by the trigeminal nerve, which mediates touch (heat, cold and pain). If the inner compass needle of a cell points in a certain direction in space, and the fish makes a 90-degree turn, the cell will fire and tell the brain: 'I am 90 degrees out of my preferred direction,'" Winklhofer told LiveScience.
Kenneth Lohmann, a distinguished professor of biology at the University of North Carolina who studies animals' magnetic sense, said the new results have ramifications beyond the realm of rainbow trout.
"If the authors are correct that the magnetite they have found is involved in detecting magnetic fields (which seems likely), then … this might have important implications for how other animals perceive magnetic fields," Lohmann told LiveScience. "It is quite possible that similar magnetite crystals are involved in detecting magnetic fields in numerous animals." It is also possible that there are two or more types of magnetoreceptors that evolved separately, he said.