Internal Compasses Help Turtles, Mole-Rats Navigate

Hatchling loggerhead sea turtles(Caretta caretta). Courtesy of Kenneth J. Lohmann

Two new studies in the 12 October 2001 issue of the international journal Science shed light on how animals use built-in compasses to find their way. In one study, researchers found that migrating loggerhead sea turtles rely on regional magnetic fields to guide them around the north Atlantic Ocean. By orienting themselves along a circular current system called the North Atlantic Gyre, the turtles seem able to avoid veering into dangerously cold waters, according to the authors.

Baby loggerheads from eastern Florida begin a long-distance migration immediately after entering the sea. They swim to the North Atlantic Gyre, which surrounds the Sargasso Sea, circling it for a period of years. Kenneth J. Lohmann of the University of North Carolina and colleagues studied the young turtles' response to different magnetic fields by placing them in a circular, water-filled tank surrounded by a computerized coil system. Each turtle was tethered to an electronic tracking unit that recorded the turtle's position. The turtles responded to certain changes in magnetic field by changing their swimming direction in patterns that would help them remain safely within the North Atlantic Gyre.

In a second study, on subterranean Zambian mole-rats, Czech and German researchers have discovered that neurons in a brain structure called the superior colliculus are part of the animals' biological compass. Groups of the neurons responded selectively to different
field directions. Pavel Nemec of Charles University in Prague and co-authors suggest that the rats may use the magnetosensory information to compose a mental topographical map of their surroundings, something other animals do with different types of sensory information.

-- Kathy Wren