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Natural voyagers

Jan 1, 2003

Sailing within sight of the Azores during a transatlantic crossing some years ago I noticed a small sea turtle, inches long, swimming just below the ocean's surface. I imagined it was simply heading home to the islands after a day of feeding but was nonetheless impressed by the young animal's determined paddling in those lonely seas.

In reality, that turtle was probably only partway through a spectacular ocean voyageone far longer than my own. In all likelihood, it had departed the southeastern coast of the U.S. after hatching some months before and riding the Gulf Stream thousands of miles north and east; the Azores served only a waypoint along a route that would eventually lead southward past Europe and Africa, then all the way back to the coast of North America.

Sea turtles routinely undertake such extraordinary journeysdespite looking, to the untrained eye at least, as if they wouldn't be totally out of place sunning themselves on a log next to a common snapping turtle. New research indicates that sea turtles guide themselves with a remarkable ability to sense Earth's magnetic field and may even create mental "magnetic maps" on which to plot their positions. This enviable ability to determine global position in the mindsans GPS or even sextantsurely ranks sea turtles among the ultimate navigators. Yet despite their navigational skill the world's seven species of sea turtle will need human aid to avoid the biggest hazard of all: extinction.

Scientists hoping to learn about turtle migration routes recently mounted a satellite tracker onto a turtle's shell as the animal fed in the Sea of Cortez, near Baja California. The biologists were not disappointed, as the animal subsequently embarked on a migration that proved nothing short of phenomenal. As the satellite tracker relayed fixes, and the turtle made its way across the Pacific to Japan, one amazing aspect of the animal's route stood out: The course was virtually beeline-straight for the entire 6,500-mile trip.

Blue-water voyagers

Such lengthy blue-water voyages are common among sea turtles as they migrate between feeding and breeding grounds, where females crawl onto beaches to deposit eggs. Some turtles paddle from the Sea of Cortez to Australia, rather than Japan, to breed, and Kemp's ridley turtlesthe most endangered sea turtlesconverge from around the Atlantic, Caribbean, and Gulf of Mexico to lay eggs on a single beach in Mexico. Most spectacularly, green turtles that feed along the Brazilian coast regularly travel more than 1,400 miles to nest on Ascensiona desolate island lost in the middle of the South Atlantic.

Streamlined bodies, ample fat-storage capability, and a propensity for swimming persistently (if not especially quickly) serve sea turtles well during their journeys. But the real mystery lies in how they manage to find their way. Particularly when the destination is so often a small, isolated island.

Recently Kenneth J. Lohmann, a professor of biology at the University of North Carolina at Chapel Hill, and his colleagues have gained major insights into this phenomenon.

"We've studied extensively a group of loggerhead turtles that nest along the east coast of Florida," says Lohmann. When turtles in this group hatch, they immediately migrate offshore from their home beaches to the Gulf Stream, where they find food and refuge in floating clusters of sargassum seaweed. For the next five to 10 years the loggerheads live in the open ocean. Many cross the Atlantic, showing up in areas along Spain, Portugal, and Africa. But eventually they manage to navigate a complete circuit of the Atlantic to take up feeding grounds back along the North American coast.

Adult turtles are exceedingly difficult to studyin part because they can weigh three or four hundred poundsso Lohmann's team has so far focused its attention on hatchlings, and the early stages of the navigational puzzle. "We've made a tremendous amount of progress understanding how turtles guide themselves from Florida to the Gulf Stream," Lohmann explained, "and in the last few years we've begun to address the question of how they navigate around the North Atlantic gyre, the circular current system of which the Gulf Stream forms the western edge. It turns out that hatchling loggerheads can detect a couple of different features of the Earth's magnetic field. These couldat least hypotheticallybe used to determine position lines, and we've begun to speculate that adults may use this magnetic information as a sort of GPS system."

Coastal piloting

Hatchling loggerheads use three kinds of sensory cues to pilot their way offshore. Their first hurdles lie in the scramble from land to seaa short but perilous journey fraught with dangers such as being eaten by a raccoon or crab or crawling the wrong way and ending up in a parking lot.

"Visual cues are of primary importance during this brief initial phase," said Lohmann. "In particular, turtles pay attention to light intensity. Because the ocean always reflects more light than the landon dark, undeveloped shorelines, at leasta turtle can reliably get from its nest to the water by crawling toward the brighter, lower horizon."

As soon as they enter the ocean, however, hatchlings apparently pay little attention to visual cues, navigating instead based on the direction of ocean waves, not unlike traditional Polynesian navigators. "For the first part of the offshore migration, close to shore, the waves are a good cue, because of the phenomenon of wave refraction," said Lohmann. "As waves enter shallow coastal areas, they are refracted until they approach the beach straight-on. Consequently turtles in very shallow waters can usually successfully guide themselves offshore simply by swimming into the waves."

The real navigational challenge begins beyond the wave refraction zone. There the waves can move in any direction relative to the coastline, losing their usefulness as a navigational signpost. Yet in Lohmann's tracking experiments hatchlings maintained a straight course for several days.

"What we think they do," he said, "is establish the initial course using wave cues, then transfer the course over to an internal 'magnetic compass.' We've shown through laboratory experiments that turtles can sense the Earth's magnetic field and establish and maintain courses relative to it."

As for getting around the North Atlantic gyre, the traditional view has held that turtles ride the current system passively. But Lohmann and his colleagues realized that it would benefit the turtles to influence their paths actively, in order to avoid pitfalls.

Though the gyre offers warm water and abundant food, turtles straying too far north enter dangerously cold water. Especially hazardous is the region offshore from Spain and Portugal where the Gulf Stream divides after flowing eastward across the Atlantic basin. The northern branch of this split current, called the North Atlantic drift, shoots off toward Great Britain, and water temperatures decrease rapidly. Young loggerheads caught in this north-flowing current invariably die. Sidestepping this danger would of course be a good thing."We became interested in the possibility," said Lohmann, "that turtles might actually approximate their latitude by detecting certain features of the Earth's magnetic field that vary with latitude and based on this knowledge swim in the necessary direction to avoid cold waters. We first zeroed in on inclination angle as a possible proxy for latitude." Inclination angleAt every location around Earth, magnetic field lines form an angle with respect to the planet's surface. At the magnetic equator, inclination angle (also called dip) is zero, in Florida it's roughly 57°, in North Carolina approximately 65°, and so on to the north until near the pole it's 90°magnetic field lines at the top of the world are directed almost straight downward.

"The idea we wanted to test," said Lohmann, "was whether young turtles could actually distinguish between inclination angles, because if they have that ability then in essence they can approximate latitude. These experiments were quite fun. We designed a little bathing suit for the hatchlings, attached a string to the suit, and tethered the animals in a circular tank. They didn't seem to notice they weren't going anywhere, so they would swim indefinitely." This miniature "ocean" was surrounded by a set of coils that could be used to artificially alter inclination angle.

"We found," says Lohmann, "that if we exposed the young loggerheads to a magnetic inclination angle found naturally near the northern boundary of the gyre system, they turned and swam toward the south. But if the inclination angle was one found near the southern border of the gyre system, they swam roughly north or northeast. So it seems the turtles respond to different inclination angles with swimming behavior intended to keep them within the favorable area of the North Atlantic gyre."

Using inclination angle alone, however, a turtle would be at a loss to determine whether it was on the east or west side of the ocean basin. A second coordinate would be needed, just as humans use both latitude and longitude to plot position. Conceivably a turtle could resolve this quandary by sensing the strength of the magnetic field as well. This second feature of Earth's magnetic field happens to vary across the planet's surface in a different direction than inclination, making it potentially useful as the other coordinate in a two-coordinate mapping system. Lohmann then set out to test whether loggerheads can distinguish between different magnetic field intensities that they would encounter in the North Atlantic gyre.

The results were compelling. "We found that if we exposed turtles to an intensity found on the western side of the Atlantic, off the Carolina coast," he said, "they swam approximately eastward. On the other hand, if we gave them an intensity that occurs naturally at roughly the same latitude but over on the east side of the Atlantic, they paddled more or less westward." Here was another feature of the magnetic field to which the turtles responded with apparent efforts to stay within the favorable waters of the gyre, opening up the real possibility that turtles might use information about the magnetic field to determine their global position.

An internal GPS

"The gradients of magnetic field inclination and intensity form a grid like latitude and longitude," Lohmann said, "though not as cleanly. As a result virtually all locations within the Atlantic are marked by a unique combination of inclination and intensity. It seems plausible that adult turtles could use this fact to, in effect, create a magnetic map and actually pinpoint their location."

This "mental map" might develop through experience, said Lohmann, rather than being something the animals are born with: "Hatchlings might imprint on the magnetic features of their home beach but may not really know until some years later how to get back to that location. It's analogous to when we teach our kids what their address is when they're very young, so if they're ever lost they can tell someone that. The kid doesn't really understand what that address means, but an older person with experience can get them home. Then later, through experience, the child learns the neighborhood and as an adult is able to get back to that location from anywhere in the world."

As turtles wander the oceans, they may "learn" the gradients of inclination and intensity, finding that if they swim northward, inclination angle becomes steeper. And if they swim east, intensity becomes weaker.

"Eventually they may come to possess a rough grid map that enables them figure out, for example, I want to go to somewhere where the inclination angle is 57°, and I'm up here at 61°, so I need to go south," said Lohmann.

No one knows how turtles manage to sense the magnetic field. One possibility is that particles of magnetite found in their bodies play a key role.

"Magnetite is the same mineral found in compass needles," said Lohmann, "and it's conceivable that these little magnetic particles might act like tiny compass needles."

Another theory is that animals capable of detecting the Earth's magnetic field may have specialized receptors in their eyes. However the unusual navigation system works, it's quite effective. Researchers have captured adult turtles at nesting beaches, transported them hundreds of miles away, and dumped them back in the ocean. The beasts swam directly back to the nesting beaches. Aside from a magnetic sense there seem no other adequate way to explain the navigational ability of turtles. Some birds, for example, use a form of celestial navigation, relying on star patterns. But turtles are highly adapted to underwater vision and apparently are extremely nearsighted once they lift their heads out of the water.

"Most scientists think turtles are not able to see star patterns at all," said Lohmann. "If they do, it's almost certainly from underwater, and you can imagine how difficult navigation would be with waves distorting the image. Also, satellite tracking studies have shown that turtles can navigate quite effectively on nights with complete overcast, so celestial navigation doesn't seem to play a role."

It's possible, however, that turtles might be partially guided by "smelling" waterborne chemical signals. But chemical cues disperse haphazardly, making them effective only over short distances. Even so, turtles might well "smell" their way through the final stages of the migration, after they've already drawn close to the target to be within range of any dissolved odors.

Turtles in danger

Despite their remarkable evolutionary successes as ocean voyagers (if only humans had evolved a similar position-finding ability) sea turtles now seem headed for oblivion. All seven species of sea turtles are currently listed as threatened or endangered, and at least a few species are in imminent danger of going extinct.

The Kemp's ridleyconsidered most endangerednests almost exclusively at one location in Mexico. Its population may have historically numbered in the hundreds of thousands, but today the Kemp's ridley has declined to only a few hundred nesting females. These magnificent animals frequent the same Gulf of Mexico waters as a sizable fleet of shrimping vessels, and each year a significant number of the turtles become tangled in shrimp nets and drown.

"The leatherback turtle is also doing especially badly," said Lohmann, "even though it's just an astonishingly large turtle. They can grow up to nine feet long and weigh 2,000 pounds, feeding exclusively on jellyfish. Unfortunately, their eggs are eaten by people around the world, and also the turtles often mistake plastic bags for jellyfish, so their guts become clogged. Leatherbacks are decreasing at an alarming rate."

Green turtles, traditionally the main ingredient in turtle soup, have also declined dramatically. "Go back and read some of the accounts from Columbus's voyage," says Lohmann, "and there's a mention of the crew being kept awake at night because the ship was banging into so many turtles. Now, there are far, far fewer. I've worked in Florida for the better part of a decade now, and I've only seen one adult nesting."

A major problem for turtles is loss of suitable nesting habitat. Coastal development is invariably accompanied by increased artificial lightingin buildings and parking lots and along streetscreating confusion for hatchlings, which often head for these new bright lights on land rather than the safe glow of the ocean's horizon. Construction of sea walls and other structures also makes it impossible for females to crawl ashore to lay eggs. Sea turtles continue to be severely harmed by disruption of their brief but vital land-based reproductive activities.

Despite their phenomenal navigational abilities at sea, marine turtles will need a course correction in human activities to put them on the path to recovery.