Most people wouldn't attempt a transoceanic voyage without the aid of a map, compass, GPS or at least a keen ability to navigate using the sun and stars¿even if they had made the trip several times before. Baby sea turtles, however, embark on just such a journey all by themselves shortly after hatching. Scientists have long wondered exactly how the creatures find their way across the Atlantic ocean and back during their first migration. Now findings reported today in the journal Science provide the strongest evidence yet that the young turtles possess a built-in compass, enabling them to chart their course according to the earth's magnetic field.
Previous studies, led by Kenneth J. Lohmann of the University of North Carolina, had shown that hatchling loggerhead turtles can detect features of magnetic fields known as inclination angle and field intensity. The new research, conducted by Lohmann and his colleagues, demonstrates that this information can elicit changes in the swimming direction of migrating Florida loggerheads that keep them on track. On track, for these turtles, means following a warm, food-rich current system known as the North Atlantic gyre.
The team tested the tiny turtles' ability to distinguish among the regional magnetic fields in a seawater-filled pool. A computerized coil system surrounding the pool enabled the researchers to control the magnetic field in which each hatchling, tethered to an electronic tracking unit, swam. The results were striking. Turtles exposed to a field replicating the one that exists off the coast of northern Florida, for example, swam southeast. Subjecting them to a field akin to one near the southernmost part of the gyre, in contrast, prompted the creatures to paddle west-northwest. "Our results provide direct evidence that young sea turtles can in effect exploit regional magnetic fields as open-ocean navigational markers," the team writes. "The turtles emerge from their nests ready to respond to specific fields with directed movements. These responses are appropriate for keeping young turtles within the gyre system and facilitating movement along the migratory route." The researchers further note that similar mechanisms might guide other ocean migrants, such as fish, marine mammals and some migratory birds.
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Along those same lines, a second report in this week's Science indicates that internal compasses may help one creature find its way not in the ocean but underground. A team of Czech and German scientists working on the Zambian mole-rat found that a brain structure known as the superior colliculus (SC) contains neurons that respond to magnetic stimuli. The mole-rat SC also appears to be involved in processing the magnetic information. These findings, the researchers conclude, suggest that the subterranean mole-rats form mental maps of their dark environs using magnetosensory input¿something other animals do using visual and auditory information, among other kinds.
