My Cousin: The Loggerhead Turtle

Check this out! Young Loggerhead turtles have internal GPS systems. Amazing! They read the Earth's magnetic field to adjust the direction in which they swim. It seems they hatch with a set of directions, which, with the help of their magnetic sense, ensures that they always stay in warm waters during their first migration around the rim of the North Atlantic.

Over time they build a more detailed magnetic map by learning to recognize variations in the strength and direction of the magnetic field lines. It isn't known how the Loggerheads sense magnetism. Part of the problem is that magnetic fields can pass through biological tissues without being altered, so the sensors could, in theory, be located in any part of the body.  Many researchers think that magnetic receptors probably exist in the head of turtles and perhaps other animals. These might be based on crystals of magnetite, which align with the Earth's magnetic field and could pull on some kind of receptor as it changes polarity. The mineral has been found in some bacteria, and in the noses of fish like salmon and rainbow trout, which also seem to track the Earth's magnetic field as they migrate.