The air was warm as the skies grew dark over Diego Garcia. As the nearly full moon reached its highest point, a green sea turtle scuttled her way onto the sand. The ocean giant was more than a meter wide and nearly as long from nose to tail. Her carapace, mottled with splotches of green and black, was slick with salt water.
Turtles glide through the sea with a certain reptilian elegance, but on land their awkward, plodding movements evoke a wind-up toy in need of a few more cranks. After shuffling a suitable distance from the waterline, the turtle began to excavate a shallow hole, using her arms and legs like spades to fling pebbles and sand through the air. Nearly exhausted, she finally began to relax as she released dozens of ping-pong ball-sized eggs into the ground.
It was likely the first time in years she’d set flipper on dry land. Other than the moments after they hatch and crawl into the surf, sea turtles spend their entire lives in the ocean. Only when females return to lay eggs on the same beaches where they hatched do they leave the water—just briefly, for a few hours, before slipping back into the sea. They may lay several clutches of eggs during the mating season before setting off for their foraging territories. There they stay for several years, regaining energy by feasting on seagrass, before returning to their natal beach, mating just offshore, and beginning the cycle anew.
Having carried out the full extent of her duties as a mother, this turtle had completed a ritual that’s played out countless times on Diego Garcia, a footprint-shaped atoll in the Indian Ocean's Chagos Archipelago. Green sea turtles have used the atoll as an incubator for hundreds or thousands of generations, or perhaps longer. Each generation disperses and returns; precisely where these now-endangered reptiles go, what route they take to get there, and how they navigate, is a mystery.
And so on that moonlit night in October of 2017 volunteers from the U.S. military facility on Diego Garcia helped Nicole Esteban, a marine biologist and sea turtle conservationist at Swansea University, fasten a GPS transmitter to the top of the turtle's shell while she laid her eggs. The volunteers nicknamed the turtle Serenity and watched as she and the computer on her back crept back into the waves and disappeared.
Three months later, Serenity reached her foraging waters along a small island called Farquhar Atoll in the Seychelles archipelago. It is some four thousand kilometers west of Diego Garcia, but the GPS signals traced a circuitous route that wound through more than six thousand kilometers of open ocean. Had she had taken a more direct path, she could have accomplished the entire journey in under a month.
Precisely where these now-endangered reptiles go, what route they take to get there, and how they navigate, is a mystery.
It was an unusual trip in other ways, too. Typically when biologists track turtles from their nesting beaches on small islands, most wind up in coastal territory, having paddled across the open ocean until hitting a continental shelf and then turning left or right. But Serenity ended up on a flyspeck island, and many others of her cohort—Esteban’s team tagged a total of 35 turtles over five years—followed suit.
"A number of these turtles migrated to very, very small island targets, some not more than a couple of hundred meters square,” says Alex Rattray, a biologist at Deakin University who was also involved in the research. A few did travel more than 5000 kilometers west to the coastlines of Somalia and Mozambique, but others pulled up short of the coast, joining Serenity elsewhere in the Seychelles archipelago; still others swam north to the Maldives islands.
Their destinations underscored the extraordinary nature of sea turtle migration. It’s astonishing enough that a sea turtle can navigate across thousands of miles of open ocean, with no discernible landmarks, and wind up in the correct place. Even more astonishing is when the correct place is a dot of sand with nothing but blue until the horizon in every direction.
It's a feat that bewildered Charles Darwin. "Even if we grant to animals a sense of the points of the compass, of which there is no evidence," he wrote, "how can we account, for instance, for the turtles which formerly congregated in multitudes, only at one season of the year, on the shores of the Isle of Ascension, finding their way to that speck of land in the midst of the great Atlantic Ocean."
Since Darwin wrote those words in 1873, scientists have tried to understand just how turtles make these awesome journeys. Before the invention of GPS technology, sailors crossing the globe relied on a combination of complex mechanical instruments and accurate timepieces. Much remains unknown about how turtles accomplish the same task, the only tools at their disposal resting within their own brains and bodies—but biologists have come a long way in understanding how sea turtles find their way.
The animated travel tracks of 35 green sea turtle migrations tracked from nesting beaches in the Chagos archipelago. Hays et al./Current Biology
In the first month after slipping off the beaches of Diego Garcia, Serenity had logged nearly four thousand kilometers. She was still in the open ocean, north and a little east of Madagascar. Rattray wouldn't know it yet, but when he switched his computer on to check on her progress, he would find Serenity at almost precisely the longitude of her final destination in Farquhar Atoll. There was just one problem: was she was at the wrong latitude.
The turtle had missed her target by some two hundred kilometers. That would be like walking from New York City to Los Angeles, but accidentally winding up in Tijuana, Mexico instead. But rather than take a right turn and swim north, she kept heading west, further and further away from her goal.
So how did she get back on course?
Whether you're a sea turtle or a ship's captain, finding your way around the planet requires two tools: a map and a compass. A map tells you where you are relative to some other location: where you started out, for example, or where you want to go. A compass helps to keep you moving in a reasonably straight line.
"There's all sorts of ways to set and maintain a heading," says marine biologist Nathan Putman, who studies navigation in sea turtles and salmon. Animals who have good vision can orient based on the polarization of sun light, as some birds are thought to do, or based on the position of stars in the night sky, as in dung beetles. If animals can marry their vision to an internal clock, then they can use the sun's position as a compass, accounting for its movement across the sky throughout the day. It's thought that a time-compensated sun compass, as it is called, is one of the tools that migrating monarch butterflies use to maintain their headings.
Other animals might orient based on the direction that persistent winds blow or the direction waves travel through the ocean. Indeed, when loggerhead sea turtles first hatch on Florida beaches, they know to swim directly into oncoming waves, a strategy that deposits them into the Gulf Stream, part of a larger network of currents called the North Atlantic Sub-tropical Gyre. These currents, which stretch from the eastern seaboard across to southern Europe and northern Africa, encircle a region known as the Sargasso Sea. By staying within the gyre, vulnerable young sea turtles can remain relatively safe.
In 1989 a meteorological fluke helped Ken Lohmann, a biologist at the University of North Carolina at Chapel Hill, confirm that hatchlings use waves as a guide. That year, Hurricane Hugo temporarily caused waves to travel towards the open ocean rather than towards Florida's Atlantic coast. When Lohmann dropped newly hatched turtles into these conditions, they swam into the waves, just as their innate programming instructed them to—and as a result, they went the wrong way.
But what if turtles were to hatch on a calm, windless night with no waves to point the way? Lohmann brought hatchlings into a laboratory to find out. In complete darkness and with no other cues available to guide them, they swam towards the northeast, which in their natural environs would have safely launched them into the Gulf Stream. When he then induced an artificial magnetic field around the tanks, the turtles continued in what they thought was a northeasterly direction. In reality, thanks to Lohmann's magnetic misdirection, they were swimming in precisely the opposite direction. The results confirmed that they really did use Earth's magnetic field.
Sea turtles hatch with at least two strategies for finding their way pre-programmed into their brains: the movement of waves and the Earth's magnetic field.
Taken together, Lohmann’s experiments revealed that sea turtles hatch with at least two strategies for finding their way pre-programmed into their brains: the movement of waves and the Earth's magnetic field.
Currents around Diego Garcia are of course different from those off the Florida coast. Turtles born in Diego Garcia are delivered into the South Equatorial Current, which is part of a larger system called the Indian Ocean Gyre. The parameters of Earth's magnetic field differ as well. But the underlying principle remains the same: turtles are born with a set of instructions that, at least most of the time, safely delivers them into the open ocean.
"Offshore migration is really just the first part in a longer transoceanic migration,” says Lohmann, one that occupies the first five to eight years of a sea turtle’s life—a period sometimes called “the lost years,” spent in the open ocean, where sea turtles were once thought to lazily drift wherever the currents took them. But Lohmann realized that this strategy could be deadly.
If turtles in Florida floated passively on the North Atlantic Sub-tropical Gyre, for example, then after they cross the Atlantic and approach the Iberian Peninsula, they could be caught in currents that flow towards the United Kingdom, where the ocean becomes lethally cold for a sea turtle. Likewise, where the gyre glances off the coast of Africa and returns to the Americas, a second current flowing the opposite direction continues south past the equator and away from the rest of the population. They might survive such a journey, but would likely never participate in passing their genetic heritage to the next generation.
Lohmann suspected that something more sophisticated than a magnetic compass alone keeps sea turtles on a safe trajectory. He scooped more hatchling turtles off Florida beaches and brought them into his laboratory for a new series of magnetic displacement experiments.
When he exposed the turtles to artificial magnetic fields that mimicked those found off the coast of Portugal, they began swimming south, which would decrease their chances of ending up in frigid northern waters. When he adjusted the field to imitate those in the open ocean west of Africa, the turtles swam towards the northwest, likewise ostensibly enabling them to remain within the gyre.
"We used turtles that had never been in the ocean before," Lohmann emphasizes. They had only hatched a few hours earlier, "and we just exposed them to these faraway locations." Sea turtles are born not just with a magnetic compass, but also with a magnetic map.
Despite these sophisticated systems, however, adult turtles like Serenity do make mistakes. "They have these amazing feats of navigation but they're not perfect at it," says Rattray. From the ocean north of Madagascar, she would travel almost 600 kilometers further west before apparently realizing she was off course.
“One limitation of tracking studies is they can provide you with the track that a turtle followed but they can never tell you exactly what the turtle was thinking or the reasons it chose to go along that particular route," says Lohmann when I ask him to review the courses taken by Serenity and the other turtles in Esteban's study.
If turtles are born with a map of their natal ocean basin, I ask him, how can they wind up so many hundreds of kilometers off course? Even the few turtles whose foraging territories are located not in faraway waters but on the Great Chagos Bank, just a few dozen kilometers from Diego Garcia, didn't swim in anything resembling a straight path.
"Turtles may sometimes choose not to go the most direct route," Lohmann offers. "There may be foraging areas along the way, they may wish to detour to areas that are rich in food. There may also be areas that have predators in them. And there sometimes are oceanographic reasons that they may not go on the most direct route."
One by one, Esteban's team had already ruled most of those explanations out. There was no correlation between wind or current and when turtles arrived at their destination, or even when they changed direction. As for stopping for food, says Rattray, “96 percent of their time in migration is spent in water deeper than two hundred meters, so there's certainly no seagrass. They're not going off course to feed."
A more likely explanation is that the turtles' inherited magnetic map is simply so crude that deviations of dozens or even hundreds of kilometers are to be expected. That evolution endowed them with a fairly coarse-grained map isn't very surprising, given the imperfections of Earth's magnetic field.
The field is formed by movements of liquified iron and nickel within the planet’s core, a phenomenon called magneto-hydro-dynamics that geophysicists are still working to fully understand. Because those metals swirl around somewhat erratically, the resulting magnetic field winds up being somewhat uneven as well.
Despite localized unevenness, though, Earth’s magnetic field is still broadly predictable. It varies in strength, with a weaker pull near the equator and a stronger pull near the poles. It also varies in its inclination, or the angle at which magnetic field lines intersect with the planet’s surface. Near the equator the inclination angle is quite shallow. Near the poles it becomes almost perpendicular.
If sea turtles were sensitive to just one of these parameters—strength or inclination—then they could derive their position along a roughly north-south axis. Because sea turtles can detect both features, they can also derive their position on an east-west axis. The bi-coordinate grid utilized by sea turtles is not exactly the same as latitude and longitude, but it works in a similar way. "And that seems to be pre-programmed, that's the software that comes with the computer," says Putman. "Which is kind of wild."
Before long, Serenity had reached Aldabra Atoll, famous for its giant land tortoises. "For a month, this turtle just sat here west of Aldabra and just went around in circles basically," Rattray recalls. "Every morning I would come into my office, have a coffee, and just see where the turtle had gone." He assumed that Aldabra was her target, and that she was having a bit of trouble sorting out its precise location relative to her own.
Then a day came when Serenity abruptly reversed course and began moving in almost a straight line directly to Farquhar Atoll, some six hundred kilometers to the east. She finally reached her destination on New Year’s Eve, nearly three months after leaving Diego Garcia.
Thanks to extremely precise satellite technology, it was easy to see exactly where Serenity was at any given moment. But since the turtle herself relied on a mental map built around an irregular and constantly changing set of magnetic parameters, Putman suspects that it might simply have taken her a while before she realized that she was in the wrong spot.
"I can't make any logic out of it," says Rattray. He can't identify any oceanographic or atmospheric cues that the turtle might have used to infer that a course correction was needed. "Our evidence shows that they don't find their targets with pinpoint accuracy. They struggle to find very isolated island targets," adds Esteban. "But they do eventually find them, even if they overshoot them by a thousand kilometers."
One thing is clear: from the beginning, Serenity and the other turtles all had a particular destination in mind. "Often they were bypassing great foraging sites, and just continuing on," says Esteban. There's no particular reason, for example, that Serenity couldn't have spent her time foraging at Aldabra rather than continuing on to Farquhar Atoll. Rattray suspects that the turtles are likely returning to areas they identified during their juvenile "lost years" as good habitats with few predators and lots of food.
"The real redeeming thing about that is they always manage to work out where they are and how to get to where they're going,” says Rattray. “It reinforces their reputation as these amazing marine navigators.” It also offers valuable insight for protecting sea turtles, nearly every species of which is now threatened with extinction.
"If a population of loggerheads were to become extinct, we will not be able to rescue them by taking loggerhead turtle eggs from elsewhere and moving them," says Lohmann. Sea turtles in Florida, for example, hatch with specific navigation instructions for the north Atlantic Ocean. Were they relocated to, say, beaches in Japan, their instincts could lead them towards catastrophe. What that means is "each of these populations is unique and, from a conservation perspective, has to be approached as a separate entity," he says. Rather than think of endangered species, we should think of endangered populations.
Esteban adds that sea turtles can also lead researchers to critical habitats that were previously unknown. The Seychelles are now committed to setting aside thirty percent of their exclusive economic zone for marine conservation, she says, and they're using sea turtle tracking data to identify which locations are most important to protect. Turtle protections also have an umbrella effect: the seagrass meadows they rely on are an important refuge for juvenile fish, thus helping to maintain fisheries, and are valuable for their role in carbon sequestration.
Meanwhile, the battery inside the GPS unit attached to the back of Serenity’s shell died long ago, around four months after she first arrived at Farquhar Atoll. Nobody knows exactly where she is right now, but chances are good that she's still in the area, lazily munching on seagrass. At some point in the next several years she will conclude her round trip back to Diego Garcia and begin the entire process again. Whether she undertakes an equally meandering migration, or swims along a straighter path, is anybody's guess.
Lead image: A green sea turtle at Po‘olenalena Beach, Maui, Hawai’i. Credit: J. Philipp Krone