How sea turtles go home again

This month, one of the world's most mysterious oceanic migrations will come full circle. And another will begin.

In June, female loggerhead turtles will return to the N.C. beaches where they hatched to lay new eggs in sandy nests.

Fifty-five to 65 days later, baby turtles will clamber out en masse. The 2-inch-long, 0.65-ounce hatchlings will flipper-flop their way through beach sand, push into the salty water and swim out to sea.

They grow into adults weighing several hundred pounds as they swim 9,000 miles in a migratory route that circumnavigates the North Atlantic Gyre - the clockwise current that encircles this oceanic region. It will take five to 10 years, but one in 5,000 will return to the beach where it hatched.

"No one yet knows how sea turtles navigate back to their natal nesting area," said Ken Lohmann, a behavioral biologist at UNC-Chapel Hill. Lohmann and his wife, Cathy, also a behavioral biologist at UNC, have spent their careers systematically cracking the code of how sea turtles use environmental cues to navigate their trans-Atlantic migration.

When the Lohmanns were University of Washington grad students in the late '80s, the dogma was that loggerhead turtle hatchlings made their way to the Gulf Stream and then passively rode the currents. But the Lohmanns' work has revamped entire chapters on the turtles' migratory behavior.

For Ken Lohmann, it began with a postdoctoral project in 1988 that sought to understand how hatchlings know to head straight to the ocean.

"I naively thought we could figure it out in a summer or two," he said. "It was a two-year position, and I was worried what I'd have left to do during the second year."

"Here we are 20 years later, still working on it," Cathy Lohmann said with a laugh. Like many married couples, they often finish each other's sentences.

Discoveries about turtles

In the 1990s, the Lohmanns published papers describing how loggerhead turtles have magnetic compasses that operate independent of daylight. Then came papers describing how the animals are sensitive to both the intensity of the magnetic field and the angle of inclination of magnetic field lines. An animal that can sense different inclination angles, called isolines, can use this information as a proxy for latitude.

In 2001, they published a groundbreaking paper in Science laying out how the turtles' magnetic positioning systems guide them at certain danger points of their trans-Atlantic voyage. These danger points are places where the current splits or shifts, and the turtles could be shuttled in a wrong direction.

"We think that they are actually inheriting a series of instructions that tell them how to orient their swimming when they arrive at each of these danger points," Ken Lohmann said.

The encoded orientations lead each turtle to stay in the North Atlantic Gyre, Cathy Lohmann added. They call these instructions magnetically based "maps" and "compasses" hard-wired in each sea turtle.

Current work

The Lohmanns' current work, funded by the National Science Foundation at around $400,000, seeks to fill in the gaps on the turtles' migratory route between these main danger points.

"You could look at the previous work and say, well, it may be that there are only a few of these instructions to help them past these danger points," Ken Lohmann said. "So since then, we've been examining locations where the turtles are not in any immediate danger of being swept out of the gyre."

If the Lohmanns are right and sea turtles have genetically based navigational instructions, then it means that not all sea turtles are geographically exchangeable. A sea turtle from Japan, for example, cannot be used to restock depleted populations in Florida because it's hardwired to swim a specific migratory route and only that route.

"The way we're beginning to imagine their migration is a process we call magnetic way-marking navigation," Ken Lohmann said. "We're imagining it as a series of inherited responses that basically guide the turtle in a particular direction up until it encounters another point that triggers a different directional response. So it's sort of a connect-the-dot kind of a migration. It's very exciting because..."

"It's a blueprint for studying other species," Cathy Lohmann finished for him. "Because if it works for turtles, then there is no reason it could not work for other migratory species." She cited salmon, elephant seals, sharks and whales as species that may have similar navigational systems.

The Lohmanns also suspect that hatchlings imprint upon the magnetic signature of their natal beaches and may use this imprint to find their way back to the same beaches to lay eggs.

How they do it

To prove the turtles are orienting themselves based on the Earth's magnetic field, the Lohmanns use simulated magnetic fields that mimic real destinations.

"The best way to think of it is to imagine that we're magically transporting them to Portugal or to Cape Hatteras," Ken Lohmann said.

The Lohmanns built a system that generates a magnetic field around a pool of water. It's known as a Helmholtz coil system, in which the intensity and inclination of the magnetism can be programmed to mimic specific magnetic signatures on Earth. Turtles wearing lightweight harnesses are allowed to swim in the pools. The harnesses are tethered to a centralized system to track movements.

They've demonstrated that adult turtles plucked from the east coast of Florida and "transported" to offshore Georgia or the Florida Keys will use their magnetic compass to orient themselves south or north to swim home.

"They were clearly saying, 'I'm not where I want to be. I want to go back home,'" Cathy Lohmann said. Once adult turtles complete their migration, they take up residence in coastal waters and are very loyal to their feeding grounds, she added.

The Lohmanns have also studied how hatchling turtles orient themselves to waves in order to enter the sea. They built a wave simulator and attached the hatchlings with tiny harnesses. They naturally act out their swimming motions in the air when the simulator mimics a wave's orbital motion.

Ken and Cathy hope to finish work on the connect-the-dot migration patterns within a few years. They are also working out plans to test their hypothesis of magnetic imprinting by using salmon.

"Everything I thought to be true about turtles in the beginning was completely wrong, except that they do have a magnetic compass," Ken Lohmann said. "That's been the pattern: Every time I think we're close to finishing the story on turtles, we see that we've missed a portion of it."