Migrations through the eyes of a sea turtle

If you are reading this, you are likely a human. We humans are terrestrial beasts—we live on land and we can learn to swim, but we require air to breathe. Through our skin, we can feel changes in air or water temperature, the wind blowing, and even water currents nudging or pushing against us. We can hear a range of sounds, taste with our tongues, and communicate with our voices. We are also visual creatures—we can see a spectrum of color, and use color and light to communicate with each other. As humans, we experience the world through sensory windows unique to us; hence, it is easy for us to forget that other creatures may experience and sense the world in very different ways.

Now imagine that you are a sea turtle hatchling, smaller than a deck of cards. You are so small you can easily fit in a human hand. You slowly become aware of your surroundings—you are in a closed dark space. You feel movement around you. The temperature drops a little and that movement becomes more frantic. Something gritty gets in your eyes and many leathery appendages hit your face and body. The temperature drops a little more and you feel a sense of urgency—to move, to climb up through a collapsing substrate. As you climb, it feels like you are swimming through something thick that you can’t quite grasp, like trying to climb up an escalator that is going down. You suddenly emerge from your collapsing hole with other small beings just like you, all boiling out onto a beach at night.

You are on your own now.

Everything looks blurry; there is darkness behind you and a faint glow of light in front of you. You are drawn to the light. You also develop a sense of place—you imprint on the Earth’s magnetic field, taking note of this location because it will be important sometime in the future. You may have a sense of where you must now go. You start to move and crawl, crawl, crawl towards this brighter horizon. The moon and the stars are reflecting in the ocean. There is where you must go. To get there you may encounter rocks, hills, and valleys you must traverse—some of these are far, far larger than you. It takes time and every second you spend crawling towards the bright horizon you are in danger. Large strange creatures may try to attack and drag you away. They have claws and beaks. They are much stronger and faster than you. If you are lucky, if you are quick, and if you persist, you reach the ocean.

Enormous walls of water crash on top of and around you, tumbling you in the surf, relentlessly pushing you back towards the beach and the predators you need to avoid. But you are energized and keep moving. You orient into the onslaught of each wave, using the force of the wave to direct your movements. Your vision clears when you are underwater and what was fuzzy on land becomes clear in the ocean. You swim through the surf into surface waters that rise and fall with less urgency. You keep swimming, swimming, swimming at the sea surface. Your lungs are so small that you can’t afford to remain at depth for long. You swim through the sunrise and a day of sunshine that heats your shell and your body, helping to quicken your movements. Then sunset, cooling through another night of darkness. As you swim, large creatures appear below you or swoop down at you from above. You use up your energy reserves, energy from the yolk that gave you life in your dark egg, to swim, swim, swim away from these new predators. Swimming into deeper waters that hopefully offer you safety. You must reach those waters before your energy reserves are exhausted.

As you swim, swim, swim, you encounter a change in the water—a physical force that pulls at you. You are near the end of your energy reserves and you can’t fight the force. When you finally give in to this pull, allowing your body to move and drift with the water, you find that you travel faster and you can rest a little. You have encountered a surface current that helps you move away from shore, away from the near-shore predators, the hungry birds and fishes that are interested in eating you as an afternoon snack. As you ride the current, this oceanic highway, you encounter some floating algae—brown and tangled, it traps water at the surface of the ocean, wrapping you in warmth. You climb on top of this algal mat and finally rest.

You are safe.

This new home buoys you as you travel. The sun rises and sets, rises and sets. At night, overhead you are surrounded by a bowl of darkness with the twinkling lights of the stars, and the glow of the moon. This new home continues to slowly drift along the oceanic highway. Sometimes, in the middle of the ocean, the winds start to blow, causing the waves to get larger, crashing on top of you and your algal home. When this happens, your home starts to break up—bunches of algae float apart from each other, diminishing your safe, warm, food-filled algal home until it is scattered over kilometres of ocean.

You realize that you generally know where you are in the world—you were born with the ability to sense the Earth’s magnetic field—you have an internal “map” sense, similar to an internal GPS or compass. So, when you feel the temperatures dropping too much and your body and limbs start to slow down due to the cold, or when you start traveling to places that are too far to the north or south, away from waters where you are most comfortable, you know you must move, and travel back to the safety that is ingrained in your understanding of where you are and where you should be. And if you are lucky, you are able to find more of the brown, floating algae that provides warmth, food, and safety from predators lurking—swimming and circling—below your perch on top of the tangled habitat. While you drift with the algae, you bask in the warmth of the sun. This warmth makes you hungry and you find plenty of food lurking in the tangled algal mats you call home. You are cold-blooded; the more you eat and the warmer you are, the faster you grow— outgrowing the jaws of those predators who live and wait, hungry, just below you.

The years pass.

You have grown larger than a dinner plate or even a toilet seat. You find you can dive deeper and deeper, relying less and less on the sea surface as a place of safety. You can now hold your breath longer and you find that you are able to outswim and outsmart some of those predators that lurk in the waters beneath. You need more and more food to sustain your larger size—food that isn’t available in the quantities you require in the open ocean. These resources are thousands of kilometres away, in those treacherous, predator-filled coastal waters that you first encountered as a hatchling. So, you slowly make your way back. Perhaps you use your innate compass sense and your “GPS superpower” on your return journey, or perhaps you simply follow the currents. You are bigger now, no longer the snack-size of a deck of cards, making it harder for other creatures to eat you.

As you transition back to coastal habitats, your diet changes and you start spending more time at depth, diving through different temperature layers searching for food on the seafloor. You no longer nibble on the small creatures found hiding in floating algal habitats offshore. If you are a green sea turtle, you may become a vegetarian—a coastal lawnmower, grazing on seagrasses and algae found growing in shallow coastal waters. If you are a loggerhead, then you become the terror of the same creatures that once would have eaten you! You develop a taste for crabs, those scary creatures that once chased you as a hatchling. You discover that your powerful jaws are built to crush and you start feeding on whelks, horseshoe crabs, and other crunchy creatures. But the abrupt changes to your diet combined with diminished resources or polluted waters can stress your body. You may get sick, or become infected with a virus that causes tumors to grow on your skin. You might have hitchhikers join you on your travels. Algae or barnacles may grow on your back, or little crabs, leeches, and other small creatures may make a home on your body. Too much algae or too many creatures may slow you down, making it harder to swim through the water.

From your oceanic home, you have migrated to coastal foraging grounds—areas along the shallower continental shelf waters or closer to land within bays, lagoons, and rivers. Some of these foraging grounds may be too cold for you in the winter, so when the temperatures drop at the end of the summer and the days grow shorter, you become restless and feel the need to swim, swim, swim out of the cooling waters. You may spend the winter on the edge of warmer currents within deeper shelf waters. You bide your time, waiting for the seas to warm in the spring so you can follow the warming temperatures back to the productive foraging grounds you visited before. Back to the bays, lagoons, and rivers that provide a banquet of prey for you during the warm months, making the winter wait worthwhile. You face a number of threats in these coastal habitats. Human activities are everywhere; any time you travel between habitats, you swim through a gauntlet of fishing gears, boats, and unhealthy waters that assault you and your senses. Over time, the once-abundant food in your foraging areas may become scarce, causing you to spend more and more energy searching for the resources that will help you grow, grow, grow to maturity.

Decades pass.

If you are clever and fast enough to avoid the increasing human presence in your coastal home, you mature into an adult female, ready to reproduce. Your growth slows, your hormones change, and you feel the urge to migrate back to those beaches that you crawled down decades before as a hatchling. You feed, feed, feed in anticipation of your reproductive migration—fuelling up for the long journey back to your natal beaches, because you know you won’t eat again until much later. You might use your “GPS superpower” once again to return to the region you “bookmarked” in your brain when you were a hatchling. To the same beaches that your mother, your grandmother, sisters, and cousins all return to every few years, to find mates and to lay eggs.

During your journey, you bump into male turtles who will court you by gently biting the back of your neck and rear flippers. Those that you like, you will choose as your mate. You have your pick; more than one is successful in getting your attention! Those that you don’t like, you treat them like any other predator and try to swim away or hide from them on the seafloor. And then, when you are “home”, back in your natal waters, you prepare to leave the ocean and return to the beaches where you once hatched. When the sun goes down, you swim towards shore, getting pushed by the waves towards the beach, where you emerge from the water and slowly drag your now enormous body across. Your vision blurs and things are not as clear to you as they are underwater. The beach is different than you remember. More lights. You get a bit confused, unsure of where to go once on the beach. There are many artificial lights glowing on the beach and you aren’t sure if you should be scared of or attracted to them.

The beach is different than you remember.

There are more signs of humans and the creatures that humans attract—raccoons, dogs, coyotes, even armadillos. Your perception of the beach environment has changed now that you are an adult female turtle. What once were rocks, hills, and mountains are now small shells or ripples in the sand made by human footprints or human vehicles. The beach is smaller, due to human development and storm erosion. You need to find a suitable place to lay your eggs. Somewhere that the eggs will be safe and protected from terrestrial predators, high tides, and beach erosion. You will dig, dig, dig your nest patiently, slowly. You carefully lay your eggs, then cover, cover, cover them. You will leave them there on the beach, incubated by the sun and the warmth of the sand, until your hatchlings emerge, just like you did many decades ago. You will repeat this process for more than a month, returning to lay more eggs every week or two.

When you lay your last nest of the season, you start the long migration back to the foraging grounds you know, where you will find food to recover from the excitement of the last several months. You will remain on these foraging grounds for a couple of years. Banking more energy to make the long migration—the remigration—back to your natal, and now nesting beaches to mate, and lay more eggs before once again returning to your foraging grounds to refuel.

Over the decades, it becomes harder and harder to find the food and energy you need to make this mating and nesting migration. You once only needed two years to refuel, but now it may take you three or four years to bank the energy needed for the long trip home to your natal beaches. And over the decades, you notice that there are more and more changes to those nesting beaches—more erosion and habitat loss, obstacles like seawalls appear, blocking your path up the beach to lay your eggs. It now takes you even more time and energy to find safe places to lay your eggs; you may not have the energy to nestas many times as you once did.

Sea turtles have been following this life-long pattern of migration and movement across varied habitats for millions of years and you are no different. You continue to repeat the cycle of foraging, migrating, mating, nesting, migrating, foraging until you are too old, or until the threats from humans make it impossible for you to continue. Or, perhaps, until humans realize how their actions and activities may make you work harder, harder, and harder to achieve your basic life goals: to survive, to thrive, and to reproduce.

There is a human idiom that states before you judge someone, you should walk a mile in their shoes. What if we humans tried to swim a mile, or thousands of miles with your flippers? We humans are terrestrial creatures and experience the world differently than sea turtles; we lack the same sensory “window” and capabilities to sense and experience the world as turtles do. Sadly, humans can’t sense the Earth’s magnetic field and we don’t have an innate sense of place. Like the young, oceanic stage sea turtles, we, too, must remain close to the sea surface to breathe. We can sometimes feel layers of temperatures when we wade into the ocean, or be buffeted by currents. But we do not have to swim thousands of kilometres to reach our destinations, so we may not realize that there are hidden highways, currents, within the oceans that can help a turtle travel from one place to another. And we may not realize that our actions add up over time, over the lifetime of sea turtles, making their lives so much harder. Maybe, if humans could better understand and imagine what sea turtle lives are like, it is then possible to define effective conservation measures to better protect them from the gauntlet of human activities that turtles encounter over the course of their long, long, long lives.


Further Reading

Lohmann K., C. Lohmann, L. Ehrhart, D. A. Bagley and T. Swing. 2004. Geomagnetic map used in sea-turtle navigation. Nature 428, 909–910. https://doi.org/10.1038/428909a

Mansfield K. L. , J. Wyneken, W. Porter, J. Luo. 2014. First satellite tracks of neonate sea turtles redefine the “lost years” oceanic niche. Proceedings of the Royal Society B 281: 20133039. DOI: http://dx.doi.org/10.1098/rspb.2013.3039

Mansfield K. L. and N. F. Putman. 2013. Oceanic Habits and Habitats. Caretta caretta. In: The Biology of Sea Turtles, Volume III (eds. Wyneken, J., K. J. Lohmann and J. A. Musick). Pp 189–205. CRC Press.

This article is from issue

15.4

2021 Dec