"Movement Ecology" to Explain Salmon Homing?

John Roach
for National Geographic News

December 2, 2008

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.

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 them home. And so they return to the very spots in which they were born.

Exactly how the marine animals find their way home is an enduring scientific puzzle.

Kenneth Lohmann, a marine scientist at the University of North Carolina, Chapel Hill, says the secret may lie in the variability of Earth's magnetic field.

Each coastal area has a unique magnetic signature, he said.

"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," he 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.

Lohmann and colleagues propose the theory in a paper published today in a special package about the field of movement ecology in the journal Proceedings of the National Academy of Sciences.

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 in Jerusalem.

This kicked off a long tradition of what Nathan dubbed "movement ecology" research.

But over the years, he said, researchers focused on specific types of movement in certain groups of species, rather than coming up with a unifying theme.

"They 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 year-long project to establish a unifying framework, or paradigm, for movement ecology research.

Twelve teams of scientists were asked to address four basic questions: why, how, where, and when organisms move.

The framework, Nathan said, applies to all types of organisms, from animals such as salmon, sea turtles, and elephants, to bacteria and plants.

He introduces the paradigm in the PNAS package, which includes the results from 12 teams.

"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.

He said 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.

"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.

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 to unify weather patterns with animal movement.

His team outfitted turkey vultures with satellite-reporting 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 get a measure for how much energy it expended during flight.

The researchers coupled 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, elks in Canada, lynx in Spain, and butterflies from Estonia, Finland, and China.

Yet other groups tested it on seeds in Panama and various other 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.

(Related story: Tiny Radio Tags Offer Rare Glimpse into Bees' Universe [November 14, 2008].)

He said the combination of Nathan's movement ecology initiative and new technologies will open "a new era in biology."