Other Environmental Themes

GPS Onboard

Baby loggerhead sea turtles begin a solo, transoceanic migration almost immediately after their birth on the shores of eastern Florida. They navigate the ocean waters for several years, following a well-defined circular course called the North Atlantic gyre, before returning to the American coast. How do young hatchlings manage such an ambitious journey on their own? The answer, according to a 2001 Science article, is a built-in compass that enables turtles to keep their bearing by adapting to changes in the Earth's magnetic field.

A similar capacity for "true navigation"—the ability to sense one's location within the Earth's magnetic field, rather than just choose the right direction—has been observed in green sea turtles and, more remarkably, in spiny lobsters. As described in a 2003 Nature report, researchers captured more than 100 lobsters, put them in opaque containers, shipped them via circuitous routes to new sites, blindfolded them, and released them in the sea. The lobsters invariably set off on a homeward path, unless they were fooled by artificially-produced magnetic fields. Many scientists did not think a lowly invertebrate was capable of such sophisticated navigation.

A 2001 study found that thrush nightingales can also discern their location from magnetic cues—a talent crucial to survival during long migrations from Sweden to southern Africa. In particular, the nightingales need to bulk up on food before the nearly 1,000-mile flight across the Sahara desert. Birds confined to a laboratory gained three times more weight than control animals when exposed to magnetic field changes similar to what they'd encounter upon arriving in northern Egypt.

One question still remains: Just how do animals sense magnetic fields? In recent experiments on the Zambian mole rat, magnetic stimuli caused heightened activity in a brain region called the superior colliculus. Further studies will show whether this brain structure is the main site of magnetic processing in invertebrates, perhaps holding the key to navigation in general.