Take a Left at Aruba: How Do Animals Navigate the Underwater World?

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You’re a baby loggerhead sea turtle. A few days ago, you were tied up in knots in a dark, warm, although somewhat cramped, shell in the sand. Since then, you’ve been working your way to the surface of your nest, absorbing the last of your yolk sack and waiting with your siblings for the magic moment to boil over in a hatch onto the Florida beach. Now you’re free in the cool night air, staring at the hulking black shape of the shore to one side of you and the flat line of the ocean glimmering in the moonlight to the other.

Which way do you go?

If you’re reading this, then obviously you chose correctly. You followed the instinctual desire to head into the light and swim toward the open water, away from dry land and predators, such as crabs and shorebirds. But more importantly, you ultimately headed for the North Atlantic gyre, the great circular flow of ocean current in the central Atlantic that surrounds the Sargasso Sea and connects Florida, the northern coast of Africa, and the southern coast of Europe. These are the nutrient-rich waters where you spent your first five to 10 years, growing from a mere pup to a stunning, foot-and-a-half-long example of adolescent sea turtle.

RELATED READ: UNDERSTANDING THE GULF STREAM

So, have you ever wondered how you got back here? And do you have any idea how you’ll find your way home after your annual walkabouts throughout the Caribbean? Certainly you’ve thought about these things during your circuitous swims — 9,000 miles is a lot of time to kill — we all do, our minds naturally chewing on such weighty subjects on numbingly long road trips.

Fortunately, biologists are working to answer this very question: How do animals make major migrations each year, thousands-of-miles-long epics across open oceans, through ferocious storms? What they’ve found is that animals probably use a combination of senses, including sight and sound. But they’re also likely to use a remarkable, albeit controversial, “sixth sense” — the ability to detect and ride the magnetic energy of the Earth. In other words, they don’t need to ask directions. They’ve got a built-in compass showing which way to go.

Going My Way?

Say you hesitated a moment too long on that beach — after all, this is a big decision — and you were scooped up by one of those scientists, a particular Kenneth Lohmann at the University of North Carolina-Chapel Hill. You’re in good hands; his specialty is investigating how sea turtles navigate. Never mind that you’re going to have to wear a little vest over your turtle neck; fashion aside, it’s for a good cause. By the way, that vest will attach you by a monofilament strand to a stick that won’t allow you to swim away, in spite of your best efforts. Just as well, you’re in a pool.

As luck would have it, you were born with a bizarre behavior. When you lose contact with solid ground, you start swimming madly, flippers flapping in a frenzy. That’s because you’re positively buoyant, and somewhere in the thousands of generations before you, one of your ancestors started swimming as soon as he or she started floating instead of being battered back onto shore over and over like driftwood. This has turned out to be a handy adaptation, especially for Lohmann, because you’ll do it even if you’re suspended in mid-air — by, for example, that string attached to your vest.

When you left shore, you swam toward deep water on a course perpendicular to the beach. As you paddled, the surf approached you head-on, first lifting you up, then pushing you back, then pulling you down, then pushing you forward as you swam through each wave crest. Lohmann wondered if that movement was a navigation cue, so he replicated the motion up, back, down, forward. When he threw in a left or right instead of back and forth, you tried to turn your body to orient yourself into the “wave.” You’d also occasionally stop and lift your head — out of the “water” — to breathe.

That orientation to the wave motion suggests that turtle hatchlings use it as their escape route, but eventually they get so far off shore — outside the wave refraction zone — where waves don’t point directly in toward shore anymore. For young loggerheads, finding the gyre and staying within it is a life-or-death situation. One wrong turn, and they will end up in the terminally cold water off the coast of England, or they’ll be swept into the south Atlantic, with a similarly ominous result.

RELATED READ: HOW WAVES ARE FORMED

Before we dive any deeper into your journey, let’s talk about magnetism. There are two things to consider when talking about the Earth’s magnetic field: inclination angle and intensity. To understand the former, think back to junior high school science class. Drop iron filings on a bar magnet, and the metal arranges in a pattern that makes the magnet look like it’s grown a set of ears, attached at its top and bottom. That’s essentially what the Earth’s magnetic field looks like. Move up or down in latitude and the angle of the field relative to the Earth’s surface changes. Near the equator, it’s parallel. At the poles — the tips of the ears and the earlobes — it’s perpendicular. In theory, if you could sense that inclination, you’d know how far north or south you were.

Second, there’s field intensity. Suffice it to say, it’s stronger at the poles, weaker in the middle. Put the two together and you might — might — have X and Y coordinates.

At first, Lohmann just wanted to know whether you can detect the presence of a magnetic field. He attached you to the vest string and placed you in the pool to record what direction you tried to swim. As it turns out, you wanted to swim exactly the direction that would have led you toward the open ocean had you been a little quicker back on the beach — east. When he reversed the magnetic polarity of the pool by turning on an electrical coil around it, creating a big electromagnet, you turned around, too, thinking you were still pointed east.

And here’s the kicker. When he used the setup to change the magnetic field to represent approximately that found off the coast of Africa, you turned northeast — toward Portugal. If he changed it to mimic Portugal, you turned back in the direction that would take you to Florida, navigating like a diver kicking a compass bearing to a wreck site.

“The navigational task before them is they’ve never been in the ocean before, but they have to swim all the way around the Atlantic to get back,” Lohmann says. “What we’ve been able to find out is that the turtles seem to hatch out programmed to respond to specific magnetic fields that exist in different parts of the ocean. So they actually seem to inherit a system of magnetic landmarks, so to speak, and when they hit these landmarks they respond by swimming, for example, to the south or to the west.”

As you grow up, results from Lohmann’s follow-up studies, published in the journal Nature last April, suggest your sense of direction will improve significantly. While a little tyke knows instinctively to swim east, then north, then southwest, as you age you’ll start filling in blanks and developing a map of your world.

For the next 10 years, you’ll spend your time as a juvenile turtle in estuaries and lagoons along the coast of Florida, biding your time and growing another foot or two in length until you reach sexual maturity. If you have any questions about that last part, ask your mother. Anyhow, while you may occasionally travel into open ocean as an adult and you could travel as far north as Nova Scotia and as far south as South America if food and water temperatures permit, this is how you’ll spend the rest of your life, although every two or three years you’ll come together with other turtles to mate near the beach where you hatched. In the greater scheme of things, you’ll be taking navigation to an entirely new biological level. Instead of just a compass telling you to swim east, you’ll have a navigation sense that’s more of a map. It will tell you that you’re west, or south, or north of your destination, and, therefore, how to get back home.

“The difference between the hatchlings and the older turtles is that the hatchlings don’t navigate to highly specific locations, they’re just out in the ocean, and they’ve just got to keep on a general path,” Lohmann says. “But by the time these guys are juvenile turtles, when they come back to the North American coast to settle down in a feeding ground, then they develop a real attachment to their feeding area. It’s been known for some years that if you catch one of these turtles, and you move it 50 or 100 miles along the coast and let it go, more often than not the turtle will swim back to that place where it was living before.”

To test this idea, Lohmann took juvenile green sea turtles (Chelonia mydas) and placed them in a pool surrounded by a larger coil system. He ran two scenarios. First, he replicated the magnetic field that would be found more than 180 miles (288 km) north of the area where the turtles were found. Second, he replicated the field found more than 180 miles (288 km) south. In each case, the animals aligned themselves to swim the direction that would take them “home.”

“The turtles, in effect, have the equivalent of a GPS system that’s based on magnetism,” he says. “They can actually figure out where they are relative to their feeding ground using the variation in the Earth’s magnetic field.”

Animal Magnetism

You — the turtle — are not the only creature that can sense these magnetic fields. Researchers say whales, sharks, tuna, salmon, trout, spiny lobsters, sea slugs, birds, and some terrestrial animals also use a sixth sense in one of two ways: as either a compass or as a magnetic map. What we don’t understand is how the system works or even the mechanism that makes it possible.

“I thought it would take us about two years to figure out how [turtles] guided themselves during their migration,” Lohmann says. “That was about 15 years ago. We’re still working on it, and we still don’t know a lot.”

When scientists try to explain why and how animals do what they do, they try to take a three-step approach. First, they study the animal’s behavior: What does the animal do that’s interesting, when does it do it, and why? Then, they study its physiology, the input and output of the senses that cause the animal to do whatever it’s doing. Then, they study its anatomy, the inner workings of its microbiology to figure out what structures or organs cause the physiological changes.

RELATED READ: THE UNDERWATER SCIENCE OF SOUND

Lohmann accomplished the first part with the pool in his lab, and for the turtle work, that’s probably where it will stay. For once, you’re in luck, because as a loggerhead turtle, you’re a threatened species, according to the U.S. Endangered Species Act of 1973, so no one’s going to study your physiology and anatomy — steps decidedly more invasive than making you dress funny. But research is under way on other animals, both in Lohmann’s lab, using pink nudibranchs (Tritonia diomedea) and Caribbean spiny lobsters (Panulirus argus), and elsewhere.

In fact, Lohmann’s spiny lobsters have turned out to be puzzlers. While they have a relatively simple neurological system, compared with a turtle, whale or human, they possess the ability to navigate as if they have a mental atlas, just like turtles. In the wild, that gives the lobsters the ability to migrate to deeper water each fall — over nearly 50 miles — when Caribbean weather tanks. Progressions of the lobsters move in single-file nose-to-tail columns, some in groups of more than 50 individual animals. In the spring, they turn around and navigate home alone. By capturing the lobsters, tagging them, taking them on a drunken stumble of a boat ride to “disorient” them, and relocating them between seven and 25 miles from their den site, he learned that they, too, could find their way back home. This crowns the lobsters as the first invertebrates with a sense of “true navigation,” that is, the ability to find their way home from a location they’ve never been without seeing how they got there.

Compare that with one of the nudibranchs: “The sea slugs have what we call a ‘magnetic compass,’” Lohmann says. “They can distinguish between north and east and south, but we know they don’t have any kind of a map. If you pick one up and move it 10 kilometers, it’ll be lost, and it won’t be able to find its way back.”

At the physical level, his team identified six cells in the nudibranch’s brain that seem to be involved with magnetic orientation. “We’re interested in finding out how the brain of this animal senses magnetic fields,” he says. “That’s something that can’t be done in sea turtles. If we’re able to figure out how it works in sea slugs, there’s a good chance it will work the same way in a turtle.”

Scientists speculate there are a couple of possible explanations for animals’ navigational prowess: Electromagnetic induction, chemical induction, and biogenic induction. The first could be used by sharks, rays, and other elasmobranch fish. These animals possess a sensory system called the ampullae of Lorenzini, which are tiny pores on the fishes’ head that detect electrical energy from their prey. According to the hypothesis — it has only been lightly tested, although it does appeal to some notable shark experts — sharks detect a voltage change by moving (swimming) through a conductor (salt water) over a magnet (the Earth). In essence, they’re becoming little electrical generators, and they use that information to make global migrations along magnetic highways. How they process that information is anybody’s guess.

The second depends on a chemoreceptor somewhere in the animal’s body. Smell and sight are two other senses regulated by chemoreceptors, and research suggests that the eyes or nose are possible locations for the direction-finding one, too. Homing pigeons — probably the world’s most studied navigators since Magellan — have receptors in their eyes. Cut the optic nerve, and they lose their sensitivity to magnetic fields. However, if you only blind them with opaque contact lenses, they’re homing ability will bring them close, but not exactly, to their roost. They do, apparently, need eyesight to thread the needle.

Lastly, researchers in New Zealand studying rainbow trout (Oncorhynchus mykiss) have found that the fish seem to have receptors in their noses. Several years ago, biologist Michael Walker trained captive trout to press a bar to receive food when they detected a magnetic field emitted from a probe in their tank. He looked closer — much closer — and found that they had a tiny amount of iron-rich crystals called magnetite, or lodestone if you’re making a hand-held compass out of it, in several cells within the lining of their noses. Several species of whales, birds, fish, and possibly sea turtles and sharks appear to have a directional sense based on some biogenic origin. Walker and colleagues at the University of Auckland determined that these crystals are polarized like bar magnets and are strung together in chains — a simple, biological compass.

But what they haven’t done yet is connect the proverbial dots — they’re still looking for evidence that these magnetoreceptors are actually connected to nerve endings and the rest of the animal’s sensory system. Minus that link, the magnets aren’t plugged in to anything. Of course, they might also do different things in different animals, and some animals could use more than one system to cross oceans, so this won’t be simple. Welcome to the vagaries of experimental biology.

Indeed, magnetoreception and magnetic-based navigation are somewhat controversial in science, at least in part because there’s no similar ability in humans and it’s hard to imagine, much less explain, what having these senses must be like. And it does taste suspiciously like extrasensory perception or psychic shenanigans. Just to complicate matters, a team of scientists at the University of Wales thinks Lohmann’s sea turtles aren’t guided by magnetism at all. They published a study in “Proceedings of the Royal Society of London” in 2003 that suggested, based on satellite tracking data, that green sea turtles smell their way to islands more than 750 miles (1,200 km) away. Those released downwind, the scientists say, were able to find the six-mile-wide Ascension Island faster than those released upwind. (Though they admit they’re at a loss to explain how the turtles get close enough to the island to smell it.) And while some studies have correlated whale groundings with areas of magnetic anomalies, most marine mammalogists aren’t convinced.

One thing’s for sure, with more than a century of research into how animals migrate, no one’s going to answer the question anytime soon. It remains one of the big unanswered mysteries in biology, and scientists aren’t even positive they’re headed in the right direction.

Not that you’ve ever had that problem. You know where you’re going, even if we can’t explain how, or why. And the rest of us will just have to go along for the ride.

ADDITIONAL RESOURCES

Department of Biology University of North Carolina at Chapel Hill – www.unc.edu/depts/geomag/

The Phillips Lake Biology Department University of Virginia Tech – www.biol.vt.edu/faculty/phillips/

By Greg Laslo