Sea turtles are masters of navigation. It begins when hatchlings, only minutes old, find their way from the beach to the sea. Once in the water, they establish a course that will take them on an epic migration. They make this dangerous journey alone, following complex migratory pathways across huge expanses of open ocean without guidance or training.
A team led by Kenneth Lohmann, a marine biologist at the University of North Carolina at Chapel Hill, has studied orientation and navigation in loggerhead sea turtles (Caretta caretta) for more than 20 years. Loggerheads are broadly distributed, living in the Atlantic, Indian, and Pacific Oceans. Those born on beaches along the east coast of Florida will swim offshore to the Gulf Stream, a current of warm water stretching north along the Florida coast.
Experiments conducted by Lohmann and others show loggerhead hatchlings use a variety of cues to orient and guide themselves from their nests to the Gulf Stream, and later to navigate thousands of miles across oceans. In this part one of a two-part series, I discuss how hatchlings find the sea and set their course to the open ocean. I will cover the amazing navigational feats and transoceanic migrations made by juvenile and adult loggerheads in the upcoming part two.
The spectacular trek begins with a humble first leg: the short but vital scramble from the nest to the sea.
Finding the Ocean
Immediately after digging out of their nests, the hatchlings must race toward the relative safety of the water. If they survive to adulthood, the baby loggerheads could weigh over 300 lbs and reach a length of over three feet. But the newborns, at 2 inches long and weighing only 0.05 lbs, are easy prey for crabs, seabirds, raccoons, and other predators.
The hatchlings are drawn to the ocean, but having never seen it or been in it, how do they know in which direction to crawl? The turtles' environment provides several possible cues. One, the seaward horizon is lower than the landward horizon. This is due to the sand dunes and vegetation forming a dark silhouette in the landward direction. Two, the beach slopes down in the direction of the water, so moving downhill will usually lead to the ocean. And three, since water reflects more light than land, the seaward horizon will be brighter than the inland horizon.
Researchers tested loggerhead hatchlings in an enclosed arena, manipulating the horizon height, degree of slope, and intensity and direction of light. If no light cue was available, the turtles moved down slope. But if light was present, this visual cue seemed to override any slope cues. The hatchlings oriented to the side of the arena where the light intensity was brightest. When dark silhouettes were placed close to the horizon, the turtles oriented away from that region.
Orienting toward the brighter, lower horizon guides the turtles to the ocean. But once they are swimming, they will need to use a different set of cues to maintain their headings as they swim outward and lose sight of land.
Toward the Open Sea
When hatchlings reach the water, they quickly establish a course leading further offshore toward the open ocean. Those that have survived the gauntlet on the beach have not escaped all danger — coastal waters harbor seabirds, fish, and other predators. It is crucial for the young turtles to swim quickly through nearshore waters to reach the open ocean where predators are less abundant. Once they have lost sight of land, however, there are few cues to help them maintain their orientation.
At this early stage of the offshore migration, hatchlings appear to use only one directional cue: the direction of waves. In shallow water, waves refract until they approach the beach directly, so orienting into waves guides the turtles seaward.
Experiments in the field and the lab have shown hatchlings depend on wave direction to maintain their orientation from the shore to the open sea. In tests, young turtles swam into approaching waves whenever waves were present. In the absence of waves, the turtles swam aimlessly or oriented in random directions.
Swimming into waves is a reliable strategy for orienting away from land, but for only so long. Once the hatchlings enter deeper water, waves can move in any direction relative to the coastline. Then they must switch to yet another orientation cue.
Scientists observed hatchling loggerheads maintained seaward headings even after reaching deeper waters where wave direction was no longer a reliable source of direction. Once they had distanced themselves from land, sea turtles had to be using some alternative cue to guide their movements and stay their course.
Loggerhead sea turtles are among the animals that can detect the Earth's magnetic field. Could hatchlings be using this information to maintain their course in the absence of waves?
To answer this question, Lohmann and his colleagues needed hatchling sea turtles, a circular pool, tiny turtle harnesses, and a device that could reverse magnetic fields. Each turtle was fitted with a nylon-Lycra harness. The harness was connected to a monofilament line that tethered the turtle to an electronic tracking system in the center of a circular pool, allowing the turtles to swim in any direction. A large coil system surrounded the pool. The researchers could turn the coil system on to reverse the direction of the magnetic field around the swimming turtles.
Some of the tethered turtles were allowed to swim under normal magnetic field conditions. Others swam in a reversed magnetic field, turned 180° by the coil system. Hatchlings tested in the Earth's normal magnetic field tended to swim east to northeast, the direction they normally follow in their offshore migration. But the turtles tested in the reversed magnetic field swam in the opposite direction, indicating loggerhead hatchlings are able to detect the Earth's magnetic field and use it to orient themselves.
Demonstrating that hatchling sea turtles use the Earth's magnetic field for orientation led to another question: Are turtles born with a magnetic directional preference or do they acquire a preference for moving toward the magnetic direction that coincides with the offshore direction?
In several experiments, researchers demonstrated that hatchling sea turtles can establish a magnetic directional preference at least two different ways. One is by swimming toward a light source. Turtles exposed to light from the east subsequently swam eastward when tested in darkness, while those exposed to light from the west oriented westward. Another group of turtles with no prior light exposure oriented randomly, suggesting the turtles are not born with a preferred magnetic bearing. The position of light cues is enough to "set" their magnetic directional preference.
The other way hatchlings can acquire a magnetic directional preference is by swimming into waves. In Lohmann's experiments, hatchlings were tethered inside a wave tank and allowed to swim into the waves for 30 minutes. Then the waves ceased, and some turtles were exposed to the Earth's normal magnetic field while others were exposed to a reversed magnetic field. Hatchlings that swam in the normal magnetic field continued to orient in the direction from which the waves had previously come, but the turtles exposed to the reversed field swam in the opposite direction. Another group of turtles with no previous wave exposure swam in random directions when tested, supporting the hypothesis that they do no inherit a magnetic directional preference but acquire one based on their exposure to other cues.
Together, these findings indicate the experience of maintaining a consistent course (by swimming toward light or by swimming into waves) is enough to set the turtle's magnetic directional preference. Newly hatched turtles begin their offshore migration by first crawling toward the light of the ocean and then swimming into the oncoming waves. It appears this seaward course is then transferred to the turtle's magnetic compass. Once the directional preference of the compass is set, the hatchling is able to continue on the same heading in the vast open ocean using magnetic cues.
To Be Continued...
In their first day of life, these tiny loggerhead turtles have furiously crawled and swam to reach the open sea, sequentially using three different types of environmental cues to orient. But these amazing navigators are hardly done proving their prowess. In part two, I will look more closely at how sea turtles use their magnetic sense to home back to specific feeding grounds and migrate long distances between feeding areas and nesting beaches. In a lifetime that may be longer than 60 years, they will build a detailed magnetic map of their ocean home.
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