乌龟具有内置的全球定位系统

Researchers used hatchling loggerhead turtles, like this one wearing a special harness, to see how the turtles responded to changing magnetic fields as guides for migration.

RENEE MONTAGNE, host:A GPS device can make anyone an expert on getting from point A to point B, but sophisticated software has nothing on a baby turtle, and that includes hatchlings.

NPR's Joe Palca has the story.

JOE PALCA: Even before there was a Global Positioning Satellite system, humans were pretty good at navigating the globe. A compass helps a lot.

Professor NATHAN PUTMAN (University of North Carolina): But a compass doesn't really tell you where you are. You need a map.

PALCA: Nathan Putman is at the University of North Carolina（北卡罗来纳州，美国）. Baby turtles can't read paper maps, but they do have a skill that helps them navigate. They can detect the Earth's magnetic field.

Prof. PUTMAN: Turtles can use the magnetic field as a sort of map. It gives them positional information.

PALCA: Here's how it works. There are two components of a magnetic field.

Prof. PUTMAN: One is the intensity or the strength of the field.

PALCA: The stronger the field, the closer to the North Pole. But there's another property of magnetic fields. It's the field's inclination, the angle that the magnetic field lines intersect the earth at.

Huh? Trust me. It's just a property of magnetic field turtles can detect. And using those two properties, a turtle...

Prof. PUTMAN: Can get a good idea as to which side of the ocean it's on, not just whether it's in the northern or the southern part of it.

PALCA: To demonstrate this, Putman and his colleagues collected hatchling loggerhead turtles（赤蠵龟，一种大头海龟，等于loggerhead）.

Prof. PUTMAN: We take an individual turtle, and we place it in a circular, water-filled arena.

PALCA: The arena's basically a water tank four feet in diameter, plenty of room for the inch-long hatchlings to swim around in. Then Putman creates an artificial magnetic field around the tank.

Prof. PUTMAN: And so we just change the magnetic field around the turtle to correspond to magnetic fields that exist along their migratory route.

PALCA: Even though the turtles are in Florida, the magnetic field makes them feel as if they're elsewhere. For the study they report in the journal Current Biology, Putman simulates two magnetic environments, one in Puerto Rico （波多黎各，位于西印度群岛东部的岛屿）and the other near the Cape Verde Islands（佛得角群岛，大西洋岛国）off the coast of Africa.

Prof. PUTMAN: The Cape Verde turtles, which are on the eastern side of the Atlantic, swam southwest. Near Puerto Rico, the turtles swam northeast.

PALCA: There's a reason they swam in different directions.

Professor JAMES GOULD (Princeton University): What they're trying to do is to get into the North Atlantic Gyre（环流）, that big circle of water that encloses the Sargasso Sea(马尾藻海）.

PALCA: James Gould is at Princeton University. He studies how animals navigate. He says you have to swim in different directions to get to the gyre, depending on where you start out. And being able to head in the right direction is a critical skill for a hatchling turtle.

The North Atlantic Gyre takes them to a place filled with a kind of seaweed called sargassum , where they can hide out until they're big enough to avoid predators. The hatchlings don't want to miss the gyre.

Prof. GOULD: You do that, and you lose your sargassum. You lose the places you eat. You wind up in cold water, and you die, basically.

PALCA: The stunning thing is that these turtles are born knowing the right way to go. Nathan Putman says scientists have known for a while that animals can detect latitude - how close they are to the pole. His new study shows they can detect longitude - how far east or west they are.

And turtles aren't the only marine creatures that make long, east-west journeys. Elephant seals do it. So do sharks and tuna.

Prof. PUTMAN: And it seems like those migrations would be really difficult to accomplish if you can't determine your longitudinal position.

PALCA: Putman suspects that other animals use magnetic fields for navigation. Humans probably don't, hence the popularity of GPS systems.

Joe Palca, NPR News, Washington.