Researchers find evidence that sea turtles navigate with magnetic maps

Among the most accomplished navigators in the animal kingdom, sea turtles often migrate across thousands of miles of open ocean to arrive at specific feeding and nesting sites. How they do so, however, has mystified biologists for over a century.

Now, new findings by a research team headed by Drs. Kenneth and Catherine Lohmann, marine biologists at the University of North Carolina at Chapel Hill, indicate that the navigational ability of sea turtles is based at least partly on a “magnetic map” — a remarkable ability to read geographic position from subtle variations in the Earth’s magnetic field.

Previous work by the group showed that baby sea turtles can use magnetic information as a built-in compass to help guide them during their first migration across the Atlantic Ocean. Their latest studies reveal that older turtles use the Earth’s field in a different, far more sophisticated way: to help pinpoint their location relative to specific target areas, the scientists say. In effect, older turtles have a biological equivalent of a global positioning system (GPS), but the turtle version is based on magnetism.

A report on experiments with juvenile green turtles appears in the April 29 issue of Nature, a scientific journal. Besides the Lohmanns, authors are Timothy Swing, a recent UNC graduate, and biology professor Dr. Llew Ehrhart and graduate student Dean A. Bagley, both of the University of Central Florida.

Working and living for a summer on the coast near Cape Canaveral, Fla., the team built a special cube-shaped magnetic coil system almost the height of a two-story house. They used the coil to reproduce the magnetic fields that exist in different areas along the southeastern U.S. coast. They also placed a pool of water in the center of the coil so that they could expose the turtles to various magnetic fields while observing the direction in which they swam.

“We captured juvenile turtles using a 400-meter net stretched parallel to the coastline,” Catherine Lohmann said. “Each turtle was placed in a cloth harness and tethered to a tracking device in the pool of water inside the magnetic coil.

“Half of the turtles swam in a magnetic field that exists at a location about 350 kilometers north of where they actually were, she said. “The other half swam in a field that exists at an equivalent distance to the south.”

Animals exposed to the northern field responded by swimming south, the biologists found. Those in the southern field swam toward the north. Thus, in each case, turtles swam in the direction that would have taken them home if they had actually been at the place where each magnetic field exists.

“These results imply that turtles have a kind of magnetic map,” Ken Lohmann said. “In other words, they can figure out where they are relative to home using magnetic field information. This is a far more complex use of the field than just having a magnetic compass that gives direction.”

Last year, the UNC couple and one of their graduate students showed that spiny lobsters possess a similar map sense based on magnetism, he said. Their lobster experiments were the first to demonstrate that at least some invertebrate animals — traditionally viewed as primitive biological underachievers — possess navigational skills rivaling those of sea turtles, homing pigeons and other animals with backbones.

Before the work, many biologists assumed that complex navigational skills required a sophisticated brain and nervous system, which are absent in invertebrates.

“This is an exciting area of research right now,” Lohmann said. “The picture that is emerging is that magnetic positioning systems are real, and the fact that they exist in both lobsters and sea turtles suggests that they may be widespread among animals.”

Future research will examine precisely how the magnetic map is organized and what components of the field the turtles detect, he said.

The green turtle is an endangered species, and all turtles tested were released back into the sea unharmed.

“We want them to grow up and have lots of babies, so we’ll have turtles to study and enjoy forever,” Lohmann said.

The UNC College of Arts and Sciences studies were funded by the National Science Foundation.