Sea turtles and salmon may use their sensitivity to Earth's magnetic field to guide them home at the end of their epic coming-of-age journeys, suggest scientists aiming to solve one of nature's enduring mysteries.
The newly proposed theory is one of several ideas being explored under the banner of an emerging scientific field dubbed movement ecology.
According to the field's proponents, the study of movement is central to understanding where animals and plants live, how they evolve and diverge, and why they become extinct.
By making movement central to ecological studies, scientists hope general theories about movement will emerge.
Such theories could, for example, help scientists predict how organisms will respond to global climate change and prevent the spread of pests and diseases.
Kenneth Lohmann, a marine scientist at the University of North Carolina, Chapel Hill, applied the concept of movement ecology to sea turtles and salmon.
His aim was to develop a hypothesis for how such animals navigate to their natal areas from distant oceanic locations.
Juvenile sea turtles and salmon leave their birthplaces with an inescapable wanderlust, swimming hundreds or even thousands of miles away.
But after years on the high seas, the biological urge to reproduce calls the creatures home, and they return to the very spots in which they were born.
How they do this has eluded scientists for decades. Lohmann says the secret to the marine animals' navigational success may lie in the variability of Earth's magnetic field.
Each coastal area has a unique magnetic signature, he said.
Previous studies, including work in Lohmann's lab on sea turtles, indicated both the turtles and salmon are sensitive to the magnetic field.
"What we're proposing is the sea turtles and salmon, when they begin life, basically learn, or imprint, on the magnetic field that marks their home area," Lohmann said.
"They retain this information. And years later, when it is time for them to return, they are able to exploit this information in navigating back to their home area."
Once the animals reach their native coastal areas, other senses, such as vision or smell, may guide them the rest of the way. Salmon, for example, are known to use smell to locate spawning grounds once the fish are nearby.
Lohmann and colleagues propose the theory in a paper published this week in a special package about movement ecology in the journal Proceedings of the National Academy of Sciences.
"We are excited about [the theory], because it really is the first plausible explanation for how sea turtles and salmon might be able to return," he said.
An Ancient Idea
Some 2,300 years ago, the Greek philosopher Aristotle searched for common features that unified animal movements of all types, noted Ran Nathan, an ecologist at the Hebrew University, Jerusalem.
This kicked off a long tradition of movement-ecology research.
But over the years, Nathan said, researchers have focused on different types of movement in specific species or landscapes, without looking at how different patterns impacted each other.
These scientists "never meet each other, they never talk to each other, they never go to the same conference, they publish in different journals," Nathan said.
In an effort to bring the scientific community together, Nathan led a yearlong project to establish a unifying framework for studying movement ecology.
Twelve teams of scientists were asked to address four basic questions: Why, how, where, and when do organisms move?
The methodology, Nathan said, applies to all types of organisms, from animals such as salmon, sea turtles, and elephants to bacteria and plants.
"If you give a legitimate field for the study of movement itself ... then people will study movement-related questions more thoroughly," Nathan said.
Martin Wikelski is a zoologist at the Max Planck Institute of Ornithology in Seewiesen, Germany, who specializes in animal movement.
The initiative to raise the prominence of movement ecology is "absolutely essential" to the understanding of wild animals, especially in an era complicated by a changing climate, Wikelski said.
"Every animal moves around and if we don't know the fate of these animals during movement, and how movement contributes to selection, then I think we are pretty much lost," he said.
For example, by understanding what animals encounter as they move about their environment, scientists may be able to determine the factors that cause some to go extinct.
Birds and Bees
James Mandel, an ecologist at Cornell University in Ithaca, New York, said the new paradigm is ideal for his research, which seeks connections between weather patterns and animal movement.
His team outfitted turkey vultures with GPS tags and two-way radio transmitters to collect data on the birds' hourly and daily movements.
One turkey vulture even carried a heart rate monitor to measure how much energy the bird expended during flight.
The researchers combined this data with information on the wind speed, atmospheric turbulence, and cloud height wherever the birds were.
The team found that turkey vultures soar from one billowing updraft of warm air to the next as they migrate thousands of miles between their summer and winter homes.
While many questions remain, Mandel said the data indicate the birds "are highly dependent on favorable weather conditions from energy source to energy source as they go."
Other teams applied the movement ecology framework to the study of elephants in
Africa, elk in
Canada, lynx in
Spain, and butterflies from
Estonia,
Finland, and
China.
Still other groups tested the methodology on seeds in Panama and various plants in the eastern U.S.
The Max Planck Institute's Wikelski, who is also a 2008 National Geographic emerging explorer (the National Geographic Society owns National Geographic News), is pioneering new tracking technology that allows scientists to study the movement of even the smallest creatures, such as bees.
