And yet another recent paper in reports data which “imply that loggerhead [sea turtles] have a navigational system that exploits the Earth’s magnetic field as a kind of bicoordinate magnetic map from which both longitudinal and latitudinal information can be extracted.”6
Cutting through the technical jargon: scientists are piecing together how salmon know to travel hundreds of miles to the spawning ground of their birth, how homing pigeons know how to find home, how sea turtles know the roundtrip route from Florida to Africa, and how a variety of animals have an intuitive directional sense that has defied explanation for centuries.
This research is revealing that many animals have natural magnetoreceptors that read and interpret the Earth’s magnetic field.
The Planet’s Magnetism
Earth possesses a natural magnetic field that points roughly north-south. In the presence of a strong magnetic field, a material such as magnetite is drawn to align with the field. A freely moving compass needle will thus point north and help explorers find their way. Compasses have been used as navigation tools for centuries, but their use is limited. Although there are variations in the magnetic field—it is weaker the farther one is from the magnetic north pole and the higher one is from Earth’s surface—a compass still points strictly north-south; it provides no indication of east-west longitude.
The Earth’s magnetic field is pervasive, but it is not strong—otherwise our many magnetic instruments (including car door locks) might not work. Magnetic fields are measured in Tessla (T), Gauss (G), or microTessla (µT); 1 G = 0.0001T, 1 µT = 0.000001T. The intensity at the Earth’s surface is about 30–60 µT. By contrast, a small bar magnet measures about 10,000 µT, and an MRI machine produces fields above 1,000,000 µT.7 This is why a small bar magnet will attract a compass needle away from magnetic north, and why patients are told to remove all metal objects before undergoing MRI scans.
Moreover, just as two magnetic objects interact to attract or repel one another, a magnetic object affects a magnetic field: the object distorts the field and causes local variations in direction and/or intensity. A steel pocketknife causes a small distortion in the Earth’s magnetic field, but a mountain of iron ore causes a larger distortion. Hence the field intensity varies across the surface of the planet, both in general8 and in localized areas. (These variations can be scrutinized at the National Geophysical Data Center website, which has an online program to calculate seven field parameters for the magnetic field at a particular time and location.9)
In summary, the Earth’s magnetic field is small, and the subtle variations in the field are even smaller. Nevertheless, certain animals are able to read the field parameters precisely and use this information for migratory purposes. The Science article on pigeons’ neuronal responses suggests:10