Magnetic field distortions produced by protective cages around sea turtle nests: unintended consequences for orientation and navigation?

The Earth’s magnetic field plays an important role in the orientation and navigation of sea turtles. Galvanized steel wire cages are often placed over turtle nests to protect them from predators, but the material typically used in cages has a high magnetic permeability and might therefore affect the nearby field. Here we report magnetometer measurements indicating that standard nest cages do indeed significantly alter the local magnetic field in the area where eggs develop. The mean change in total intensity was 26% at a level corresponding to the top of the egg chamber and 5% at a level corresponding to the bottom. Similarly, the mean change in field inclination was 20% for the top level and 4% for the bottom. In principle, the altered magnetic environment might affect subsequent magnetic orientation and navigation behavior in several ways, although whether turtles that develop in an unnatural magnetic field actually suffer navigational impairment has not yet been studied. Constructing protective cages out of magnetically inert materials provides a way to deter predators without risking unintended behavioral consequences of distorting the ambient field.