March 10, 2011

FeatureLoggerhead Sea Turtles Perceive Longitude Magnetically

The ability to navigate is essential to sea turtle survival. (Photo credit: Michael Patrick O'Neill / Photo Researchers, Inc.)

A sea turtle doesn't have it easy. Immediately after hatching from its egg, the hatchling sea turtle digs itself out of its beach nest and heads toward the ocean. Those that make the treacherous journey from the beach to the surf swim—and are swept through—the surf zone out to the open ocean, where they continue swimming for several days. The turtles will stay at sea for five to ten years before returning to their breeding grounds to find a mate. The turtles’ migration is truly astounding—sea turtles that enter the Atlantic Ocean will travel over 15,000 kilometers (9,000 miles) before returning to the coast of North America to breed. During their journey, they will travel via the Gulf Stream to the North Atlantic gyre, a circular, counterclockwise current that flows around the Sargasso Sea and helps transport the turtles east past the Azores, Canary, and Cape Verde Islands, before returning them to the North American coast.

Scientists have known for some time that some animals are able to use Earth’s magnetic field to navigate from one part of their habitat to another. The term magnetoreception refers to an animal’s ability to sense magnetic fields. Through laboratory and field experiments, scientists were able to show that, when exposed to certain magnetic fields, animals such as homing pigeons and sea turtles change their orientation or navigation behavior accordingly. Earth’s magnetic field provides animals with two types of information. Directional information helps a migrating animal to maintain a consistent directional heading, such as traveling north to south. This information is sometimes referred to as the animal’s “magnetic compass.” Some animals are also able to use positional information. This type of information lets animals use magnetic cues to figure out their approximate geographical position, or where they are at a certain time in comparison to their final destination. This type of information is sometimes referred to as a “magnetic map.” This magnetic map is determined by two factors: the strength of the magnetic field and the inclination, or angle at which magnetic field lines intersect with Earth’s surface. Both of these factors vary in a predictable manner across the surface of Earth.

According to Dr. Kenneth Lohmann, a sea turtle researcher and professor of biology at the University of North Carolina–Chapel Hill, this magnetic map can be thought of as “a low-resolution biological equivalent of the Global Positioning System, but one that is based on Earth’s field instead of satellite signals.”

Though it has been well-documented that animals use magnetoreception to determine their location in terms of latitude (that is, the north-south orientation), it was not known whether animals can also use magnetoreception to determine their location in terms of longitude (this is, the east-west orientation). Recent research conducted by scientists in Lohmann’s lab at UNC has for the first time documented that loggerhead sea turtles are able to do just that. The results of their research indicate that these sea turtles are able to extract both longitudinal and latitudinal information from Earth’s magnetic field.

In their lab experiment, the scientists placed hatchling sea turtles in a round, water-filled “orientation arena.” This tank was surrounded by a computerized coil system, which the researchers used to manipulate the magnetic field. Each hatchling was harnessed to an electronic tracking unit that sent data about each turtle’s swimming direction to a computer. The scientists subjected the hatchlings to magnetic fields found in two locations along their migratory pathway, each found on opposite sides of the Atlantic Ocean. Both locations have the same latitude, but have differing longitudes. When subjected to a magnetic field similar to that found near the Cape Verde Islands, the hatchlings swam in a southwesterly orientation. When subjected to a magnetic field similar to that found near Puerto Rico, the hatchlings swam in a northeasterly orientation. Both of these swimming orientations coincided with the directions scientists would expect them to swim during their actual migration.

“This work not only solves a long-standing mystery of animal behavior but may also be useful in sea turtle conservation," Lohmann said in a press release about the research. "Understanding the sensory cues that turtles rely on to guide their migrations is an important part of safeguarding their environment.”

Scientists who contributed to the research include Nathan Putman, Courtney Endres, Catherine Lohmann, and Kenneth Lohmann. The results of their research were published in the February 24, 2011 issue of the journal Current Biology.

More to Explore

• Longitude Perception and Bicoordinate Magnetic Maps in Sea Turtles [pdf]
• Orientation and Navigation of Sea Turtles
• Nature Q&A: Magnetic-field perception
• Magnetoreception, the neurobiology of
• Loggerhead Turtle (Caretta caretta)
• Navigational ‘magic’ of sea turtles explained

Last updated March 10, 2011 | Comments (4) | Permalink | Feature