Researchers have learned a great deal about how this curious creature could make such a phenomenal voyage. Some of the most valuable information was gained from a single voyage. Scientist Wallace J. Nichols released the captive loggerhead turtle, Adelita, into the Pacific a decade ago. Over the course of a year, Adelita did what no sea turtle had ever done before, she took researchers and turtle enthusiasts along on her journey, to her beach, to nest. Since then, researchers have shed much light on how sea turtles like loggerheads navigate the astounding trip. One of the more fascinating aspects of this navigation is the turtle’s use of magnetic mapping to chart its course.

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Online content for Voyage of the Lonely Turtle was originally posted April 2007.

Voyage of the Lonely Turtle depicts a loggerhead sea turtle’s remarkable journey across an ocean back to its familial nesting site. Venturing across thousands of miles of open ocean to emerge on the very beach where it was born years earlier, the loggerhead performs one of the most remarkable feats of orientation and navigation in the animal kingdom.

“Turtles do return to the beach where they’re born,” says Wallace J. Nichols “Sometimes it will be hundreds of miles away. Sometimes thousands. For a loggerhead in California it was thousands.” That turtle, and the one the NATURE film is based on, was called Adelita. A captive loggerhead who trekked from her feeding site in Baja, Mexico to her place of birth in Japan, Adelita was the first animal ever tracked across an ocean basin. A decade ago, Adelita made her astounding journey of 9,000 miles, defying exhaustion and death, just to lay her eggs on one very special beach. Though Adelita’s trip shed new light on transoceanic migration, Nichols and other marine researchers have continued to fill in more of the unknowns in sea turtle migration.

Green turtle on the beach

HOW DO THEY DO IT?

“Turtles are master navigators,” says Wallace J. Nichols, “we can barely even imagine it.” With a lack of visible landmarks, strong currents, poor vision and the inability to raise their heads more than a few inches out of the water, navigating through thousands of miles of featureless ocean requires serious skills of another kind. But researchers believe these abilities are instinctual rather than strategic.

Early experiments with hatchlings seem to prove that sea turtles have the ability to detect the earth’s magnetic fields even before they venture into the ocean. Detecting the earth’s magnetic field could enable a sea turtle to determine its latitude and longitude, and thus to plot its course or change its course along its migratory route.

“It’s sort of like an extra sense,” says Nichols. Along with visual cues like sea mountains, which are potentially used as signals as turtles move away from the continent, or other sensory stimuli like smell, detecting magnetic fields is a key feature of a sea turtle’s navigational abilities.

WHY DO THEY DO IT?

Even with these astonishing skills that allow them to find their way across open ocean to a destination, the journey still offers considerable risk to sea turtles. Deadly predators, the raging sea, utter exhaustion-in the face of such threats, sea turtles head out for distant shores when it comes time to nest, even though they are surrounded by a coast of beaches in their feeding grounds. Researchers have been intrigued by this particular aspect of sea turtles for years. A creature that risks life and limb because only one beach will do appears to take fastidiousness to the extreme.

Ancient programming, in essence, explains this mystery. A particular nesting choice may reflect centuries-old conditions when temperature, beach profiles or the lack of predation made some areas preferable to sea turtles. “A turtle beach is a special kind of beach,” says Nichols: “the slope, humidity, temperature, types of predators.” In the world of sea turtle real estate, nesting beaches are prime property. They must offer open-water access, especially for larger sea turtles such as leatherbacks and loggerheads. They should have just the right slope so that a mother turtle can hoist herself to her nesting spot but not worry about flooding. The texture of the sand should be loose enough for gas diffusion, yet dense enough to prevent collapse during digging. Nearby offshore reefs and dune vegetation are both pluses. The location of a nest is critical to the survival and development of eggs and hatchlings. If a nest is too near the water, the eggs will become saturated with sea water and fail to develop. If it is too far up the beach, many threats arise: the roots from vegetation can invade the nest; the nest will be closer to predators; the hatchings will have a longer way to travel to reach the water.

In the end, turtle real estate decisions are not too difficult for humans to understand. The determination to make an exhausting, risky journey through the territories of predators and the daunting unknown comes down to historical experience. “If you were born on a beach,” says Nichols, “and it worked for your mom, it will work for you. Their strategy is to go back to the beach they were born. It’s been proven.” “To nest where they feed would represent a break with tradition,” says Nichols. As contemporaries of dinosaurs, the sea turtles have been employing their migratory strategy for a long time, and it looks like they’ll be sticking with it for the immediate future. “Pick a good beach and hatchlings survive. Clearly over millions of years, turtles have done just that,” says Nichols.

With all their ancient history and remarkable abilities, turtles today are faced with a relatively new danger, and one that may prove their undoing: the activities of humans are the greatest threat to the future success of sea turtles. In the year 2000, 1.4 billion hooks were cast into the world’s oceans through industrial fishing. These hooks snagged more than 200,000 loggerhead turtles and killed tens of thousands of them. Humans are also threatening sea turtle nesting beaches through actions that lead to beach erosion. We are disrupting nesting itself with our artificial lighting. Researchers believe that the damage we cause is likely have lasting effects on future nesting populations. With their life cycle dependent on specific nesting beaches and their need to roam the open ocean freely without the threat of becoming a by-catch, sea turtles must now depend on the consideration of humans to ensure that their epic journeys continue in future generations.

Photo © Mark Fletcher

As master navigators of the sea, sea turtles like loggerheads have confounded scientists for years, setting off a research frenzy to uncover the secrets to their navigational expertise. Research on sea turtle navigation began with acclaimed zoologist Archie Carr in the 1950s. It was Carr’s 1956 book, The Windward Road, that first brought attention to the extraordinary homing skills of sea turtles. In the half century since then, the search has never abated. Through the years, scientists have collected a substantial pool of data on this “extra sense,” as Carr called it. They have tried to understand how two-inch long hatchlings can embark on a solo journey across open ocean, finding their way to a feeding beach thousands of miles away. And how adult turtles are able to navigate back thousands of miles with pinpoint accuracy to their natal beach. Researchers have hypothesized that it is a built-in global positioning system that guides turtles on these epic journeys. Though many questions remain, scientists have found that a sea turtle’s GPS unit is not one guided by satellites. Instead, it relies on one of the strongest forces on earth-the planet’s magnetic field.

Scientists believe loggerheads detect magnetic fields and react to that information.

The Earth’s magnetic field acts as a shield from charged particles that emanate from space, mainly from the sun. The field deflects the speeding particles toward Earth’s Poles. The magnetic field is like an enormous bar magnet that is strongest near the poles and weakest near the equator. The intensity results from the fact that the field lines emerge from the southern pole of the Earth, wrap around the planet, and re-enter in the northern half. The Earth’s magnetic field comes from a very deep ocean of liquid iron and nickel that sits above the planet’s inner core. This “outer core” is an electrically conducting fluid in constant motion. The motion of the conducting fluid generates electric currents and, as a result, a magnetic field.

Magnetic field can be determined at a particular location by knowing its inclination angle, or the isoline of inclination. This angle varies with latitude. Points along the Earth have their own angle of inclination specific to their location. At the equator, where field lines are parallel to the Earth’s surface, the inclination angle is said to be 0°. North from the equator, field lines become progressively steeper. At the northern magnetic pole, the field lines are directed almost straight down into the Earth and the inclination is said to be 90°. Intensity, or strength, of field lines at a given location can also be described.

Research on sea turtles suggest these animals possess a type of compass that enable them to detect both the direction and strength of the earth’s magnetic field, and this allows them to determine their location. As they move from place to place, animals like turtles essentially build a magnetic map in their minds. Just as a compass needle aligns with the north-south magnetic axis, the turtle’s compass relays directional information to it, providing it with a reference for its position. Researchers believe that turtles use such prompts in tandem with other environmental information such as underwater volcanoes or smells. Sensing magnetic signals is not a skill confined to sea turtles. Salamanders, sparrows, spiny lobsters, hamsters and bacteria all use magnetic field to get along in the world.

Recently scientists have shown how loggerheads detect magnetic fields and react to that information. In one recent study, baby loggerheads were observed while exposed to different magnetic fields that simulated three key locations along their migratory route. The direction in which each animal swam was recorded and analyzed. Animals exposed to a simulated northern field responded by swimming south while those in subjected to a simulated southern field swam toward the north. In each case, turtles swam in the general direction that would have taken them homeward, had they been swimming in the ocean and encountering the earth’s actual magnetic field. What’s even more fascinating, these turtles had been hatched in captivity and never had any experience of the open ocean. So it emerges that the loggerhead’s navigational abilities are probably innate and not learned.

Other studies on hatchling loggerheads have shown that even one-day-old newborns detect magnetic inclination angle as well as field intensity to navigate their way across the world. Some researchers suspect that turtles may geonavigate by detecting both magnetic features and then using a form of “bicoordinate” magnetic navigation. Such a multi-variable system might enable turtles to return to a home area from nearly any oceanic location. For example, if a turtle learns the inclination angle at a given feeding area along the coast, it might simply compare that information to the inclination angle that exists at its home area. If the inclination angle is too steep, the turtle knows that it is too far north and therefore needs to travel south to reach home. If the angle is too shallow, then the turtle knows it is too far south. They can also apply the technique to detecting isolines of magnetic field intensity. Researchers believe the geomagnetic abilities of turtles becomes more sophisticated and complex as they grow older.

While there is undoubtedly more knowledge to gain in how sea turtles process magnetic field cues to navigate, researchers have made astonishing leaps since the days of Carr. With luck, sea turtles will be able to make continued journeys of epic proportions and researchers will be able to study those journeys and unravel their mysteries.