Sensing Earth's Magnetism
ABCNEWS.com, July 23, 1999
Maybe humans do it. But lobsters, fish, frogs, sea slugs, even some rats, do it better. These animals find their way by using Earth's magnetic field. Some use their magnetic sense to navigate over enormous distances, while others use it for short jaunts. Bobolinks, otherwise known as "glorified blackbirds," have a hugely long migratory trek. Racking up 10,000 frequent-flier miles during round trips between North and South America, each bird returns to the same breeding spot year after year. The loggerhead sea turtle in the North Atlantic is another long-distance traveler. They mostly nest in the southeastern United States, and journey thousands of miles, always finding their way back to the same beach to lay eggs. The turtles spend five to 10 years swimming around the North Atlantic before returning to nest. Ken Lohmann, a biologist at the University of North Carolina, thinks turtles may use their magnetic sense to stay inside the warm regions of the North Atlantic, then navigate a return to their home beaches.
On a much shorter scale, small salamanders, called newts, need a directional sense to dash across the pond to escape from predators. They can remember where a sloping shore lies, which means safety for them. Newts also magnetically navigate to their home pond, from a distance of usually less than eight miles, after roaming the forest floor in their adolescent stage. Sometimes scientists don't know why an animal has a magnetic sense. "We were astounded when the data showed [sea slugs] have a well-developed magnetic sense," Lohmann says. Sea slugs use it to slither shoreward for mysterious reasons; Lohmann thinks access to food is a big motivation.
Though orientation to Earth's magnetic field is nearly ubiquitous in the animal kingdom, no one knows exactly how animals detect it. "It's like understanding that animals can sense light, but not knowing about the eye," explains Mark Deutschlander, a biologist at the University of Victoria in Canada.