Loggerhead sea turtles (Caretta caretta) make one of the longest and most amazing migrations in the animal kingdom. In the previous installment, I looked at how the journey begins, with hatchlings digging out of their nests and scrambling to the open sea. But this is only the first step in a long transoceanic journey.
Loggerheads born on the coast of Florida will make their way to the North Atlantic Gyre, a vast circular current system that encircles the Sargasso Sea. Juvenile turtles spend 6-12 years in the gyre, sometimes crossing to the eastern side of the Atlantic Ocean, before returning to the North American coast. During this time, they may cover more than 9,000 miles. The conditions within the North Atlantic Gyre are just right for the young turtles to survive and grow, and they manage to avoid straying too far north or south where they risk being swept up by other ocean currents and ejected out of the gyre.
After spending several years navigating the open ocean, loggerheads return to the North American coast to take up residence in shallow feeding grounds. These turtles are able to reliably return to specific foraging areas after long migrations (and after 'experimental displacements' by scientists). Female sea turtles are also known to travel long distances to return to the specific stretch of beach on which they were born to lay their eggs, year after year.
How do young loggerheads with no prior migratory experience find their way across an ocean and back while also staying within the North Atlantic Gyre? What is behind juvenile and adult turtles' ability to pinpoint specific geographic targets separated by thousands of miles? Years of research by Kenneth Lohmann, a marine biologist at the University of North Carolina at Chapel Hill, and others have revealed a remarkable answer.
Before we answer those questions, it's important to understand the difference between a compass and a map and the role these concepts play in a loggerhead's journey.
Compass vs. Map
The loggerhead's spectacular navigational ability implies that it has both a compass sense for maintaining headings and a map sense to determine its position relative to other locations.
A compass provides directional information. Many animals possess a compass sense; it can be based on the position of the sun or stars, patterns of light polarization, or the Earth's magnetic field. A compass is crucial for navigation, but by itself it is often insufficient. In order to find a specific geographic location or maneuver along a complex migratory route, an animal requires a map sense. A map provides positional information. Animals use a map sense to determine their own location relative to a goal.
The difference between a compass sense and a map sense is similar to the difference between holding a compass and seeing that you are facing east and holding a GPS that tells you where you are and how to get to your house from there.
In part one of this two-part series, I discussed how hatchling loggerheads use a magnetic compass sense to maintain their heading when first swimming to the open ocean. Sea turtles are among many animals that possess a magnetic compass, and its mechanisms have been well-researched. But until recently, little was known about these turtles' map sense.
Since sea turtles can sense magnetic fields, it followed that they might be capable of using magnetic information in their map sense. A magnetic compass allows sea turtles to determine direction; a magnetic map would allow them to assess their own geographic position and figure out where they are relative to other locations. To use such a magnetic map, sea turtles would need to be able to distinguish slight differences in magnetic fields and learn how the magnetic field varies over the geographic area where they live and migrate.
Detecting Magnetic Parameters
Several features of the Earth's magnetic field vary predictably. Thus, different geographic locations have different magnetic signatures and might be used to determine geographic position. One of these features is inclination angle, which is the angle at which the magnetic field lines intersect the surface of the globe. This angle ranges from 0 degrees at the equator to 90 degrees at the poles; in other words, inclination angle varies with latitude. A second geomagnetic feature that varies across the surface of the Earth is the strength of the magnetic field. In general, the field is strongest near the magnetic poles and weakest at the equator.
To determine how loggerheads respond to different magnetic inclination angles and intensities, Lohmann and his colleagues used the same basic experimental design that had first revealed the turtles' magnetic compass. In this setup, each hatchling turtle was fitted with a nylon-Lycra harness connected to a monofilament line. The turtle was tethered to an electronic tracking system in the center of a circular pool of water, allowing it to swim in any direction while the tracking system monitored its movements. A large coil system, consisting of many strands of wire through which electrical current could be run, surrounded the pool. Lohmann and his colleagues manipulated the coil to produce magnetic fields of varying intensity and inclination angle.
In two separate experiments, the researchers showed that hatchling loggerheads can detect both magnetic inclination angle and magnetic field intensity. In these experiments, one of the two parameters was held constant while the other was varied. While this experimental approach was necessary to demonstrate that loggerheads are capable of detecting each parameter, it does not accurately reflect the world. In nature, magnetic field intensity and inclination vary together across Earth's surface.
To get closer to the mystery of sea turtles' map sense, hatchlings would need to be tested in conditions replicating those found along their migratory route in the North Atlantic gyre.
Magnetic Map with Signposts
In the next experiment, Lohmann and his colleagues used the same procedure — with the harnesses, circular pool, and coil system — but they subjected the hatchling loggerheads to magnetic fields replicating those found in three widely separated locations within the North Atlantic Gyre.
The turtles responded by swimming in directions that would, in each case, help them remain within the North Atlantic Gyre and continue along their migratory route. For instance, hatchings exposed to a magnetic field replicating one that exists near the northeastern edge of the gyre swam south, whereas hatchlings exposed to a field replicating one near northern Florida swam east-southeast. These responses appear to be inherited, as the hatchlings had never been in the ocean.
The results confirm that loggerhead turtles can distinguish among magnetic fields found along their migratory route. Moreover, the results imply that hatchlings can use these regional magnetic fields as navigational markers. This means that a turtle swimming within the North Atlantic Gyre is able to determine its position and change its swimming direction appropriately if it should venture off-course. In other words, loggerheads possess a magnetic map.
In another recent experiment, Lohmann and his colleagues exposed hatchling loggerheads to magnetic fields replicating those that exist at two locations with the same latitude but different longitudes (on opposite sides of the Atlantic Ocean). In each case the turtles responded by swimming in the direction that would keep them on their migratory route. This experiment was the first demonstration that longitude can be encoded into the magnetic map of any animal. It would appear that sea turtles obtain both latitude- and longitude-like information from the Earth's magnetic field, and use it to create a bi-coordinate magnetic map.
Loggerhead sea turtles hatch with the ability to read the Earth's magnetic field. They can detect subtle differences in the magnetic fields in different parts of the ocean and use these regional fields as navigational markers to help them remain on their migratory pathway.
Over many years and many thousand miles, sea turtles build up their magnetic maps by learning to recognize the variations in magnetic field in different locations. By the time they reach adulthood, these turtles know the magnetic topography of the places they live and forage. They use these magnetic maps to navigate to specific geographic locations — places to eat, mate, migrate, and nest.
Lohmann, K. J., and C. M. F. Lohmann. 1994. Detection of magnetic inclination angle by sea turtles: a possible mechanism for determining latitude. Journal of Experimental Biology. 194: 23-32.
Lohmann, K. J. and Lohmann, C. M. F. 1996. Detection of magnetic field intensity by sea turtles. Nature 380: 59-61.
Lohmann, K. J., Cain, S. D., Dodge, S. A., and Lohmann, C. M. F. 2001. Regional magnetic fields as navigational markers for sea turtles. Science 294: 364-366.
Lohmann, K. J., Putman, N. F., and C. M. F. Lohmann. 2012. The magnetic map of hatchling loggerhead sea turtles. Current Opinion in Neurobiology. 22: 336-342.
Putman, N. F., Endres, C. S., Lohmann, C. M. F., and K. J. Lohmann. 2011. Longitude perception and bicoordinate magnetic maps in sea turtles. Current Biology. 21: 463-466.