Sea turtle navigation
A "Turtle Positioning System" helps reptiles on their fantastic voyage.
"There's no off-the-shelf Speedo turtle bathing suit that we know about," laughed biologist Ken Lohmann, as he attached a soft cloth harness, or bathing suit, to a three-month-old loggerhead turtle.
About a dozen of these young turtles "work" in Lohmann's University of North Carolina (UNC) laboratory. With help from the National Science Foundation (NSF), he's learning how these and other sea turtles use the Earth's magnetic field for a journey of thousands of miles around the Atlantic Ocean.
Loggerheads have no interaction with their parents. Females bury eggs on the beach that hatch 50-60 days later. But the hatchlings seem to be programmed with an astonishing "Survival To Do List."
Item One: Get out of Dodge
"The hatchling turtles start life with a supply of yolk attached to their bellies," said Lohmann. "And during the first several days of their lives, they don't stop to eat. All they do is migrate offshore. Their entire survival strategy seems to be predicated on the idea that the faster you get away from land, the better," he said.
In these first few frenzied days, the yolk sac provides nutrition, and the hatchlings guide themselves offshore by instinctively swimming into waves. This gets them away from shallow waters and numerous predators.
Item Two: Follow the Magnetic Field
"We've found over the years that one of the turtles' main tricks for navigation is that they are exquisitely sensitive to the Earth's magnetic field," said Lohmann.
Knowing when and where to turn is critical in the five-to-10-year journey that takes them from the east coast of the United States to the coast of Portugal, south toward the west coast of Africa, then back toward the beach where they were born.
For example, there's a magnetic field off Portugal that triggers the turtles to turn south. If they don't, they'll likely be swept into frigid waters off Great Britain and Scandinavia, and die.
"In one experiment, we exposed young turtles that had never before been in the ocean to magnetic fields that exist at three widely separated locations along their migratory route," said Lohmann.
The large majority of the South Florida-hatched turtles in that experiment--whether exposed to the fields they would encounter off Florida, Portugal or Africa--swam in the proper direction to complete their circular route around the Atlantic.
"We were astonished! It really was a very surprising and exciting finding," said Lohmann.
"The results implied that the turtles can actually distinguish among the magnetic fields that exist in different geographic locations and, what's more, they seem to use these fields as a system of navigational markers that help them complete their migration," he said.
Lohmann said the turtles go beyond using the magnetic field as a compass. They also use it as a source of positional and geographic information.
"In effect, sea turtles have a crude global positioning system that is based on the Earth's magnetic field," said Lohmann.
A better understanding of this turtle ability could help in research of both animal and human navigation. Some other animals, from salmon to spiny lobsters to migratory salamanders, might also use the Earth's magnetic field in similar ways.
Another experiment in the Lohmann lab tests how the turtles respond to wave motion. Looking at these turtle "athletes" is rather odd. Outfitted in their "bathing suits" and suspended on a mechanical wave simulator that moves them in circles, the turtles "swim"--without any water!
"The turtles will actually act out their swimming behavior in air," said Lohmann. "When a turtle first enters the ocean, the signal that tells it to begin swimming is the absence of any contact with the substrate. Young turtles are positively buoyant, so they float up when they enter the water, and as soon as nothing is touching the underside of a turtle, it begins to swim."
"In shallow coastal areas, waves reliably approach the land from the deep water, so swimming into waves reliably guides turtles offshore," said Lohmann.
As hatchlings, the turtles in the UNC-Chapel Hill lab are tested for about 15 minutes on the "wave simulator" in a typical experiment. Older turtles might spend an hour to an hour and a half swimming (or "air swimming") for this navigation research.
It's a pretty good gig. None of the turtles in this lab would have survived without human intervention.
Of about 100 baby turtles that hatch out of eggs in a typical sea turtle nest, some are too weak to crawl to the surface of the sand. The turtles in this UNC study were all rescued from nests on North Carolina beaches. Once rehabilitated and healthy, they will work in Lohmann's lab for a couple of years and then be released into the Atlantic.
Lohmann's study of animals and magnetism began when he was studying neurobiology, working with sea slugs as a graduate student. Two decades later, he is still helping to unlock answers.
The dramatic connection between turtles and magnetic fields is already altering conservation efforts for these reptiles.
"When we think about preserving habitats for animals, we normally gravitate toward traditional descriptions of the environment: What sorts of plants are there, what the humidity is, what sort of climate exists. But when you learn more about particular species, you discover that the list of criteria for what constitutes an environment may be somewhat different. In the case of the turtles, it has become evident that the magnetic field has to be taken into consideration," said Lohmann.
There's already been a change in procedure for some sea turtle rescue groups who monitor turtle nests. Historically, these scientists and volunteers would put wire mesh screens over the nests to keep predators like raccoons from eating the eggs. But now that it is known that these metal coverings could alter the developing turtle's magnetic "imprint," the conservation practice has changed. South Carolina, for instance, has replaced metal protective cages with plastic ones.
Loggerheads, leatherbacks and other sea turtles are threatened in many parts of the world. As someone who has cared for lizards, snakes, frogs, toads and turtles since he was a kid, Lohmann believes his work will be part of the solution.
"The people who are involved in our research usually are passionate about both science and conservation. They enjoy the challenge of trying to figure out things that are very difficult to learn about. At the same time, they are deeply devoted to the welfare of animals. We really like sea turtles and want to see them prosper in the future. Part of the fun for us is using our scientific expertise to do research that promotes sea turtle conservation," said Lohmann.