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First published online October 19, 2007
Journal of Experimental Biology 210, 3697-3705 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.001313
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Magnetic maps in animals: nature's GPS

Kenneth J. Lohmann*, Catherine M. F. Lohmann and Nathan F. Putman

Department of Biology, University of North Carolina, Chapel Hill, NC 27599 USA


Figure 1
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  		Fig. 1. (A) Diagrammatic representation of the Earth's magnetic field illustrating how field lines (represented by arrows) intersect the Earth's surface, and how inclination angle (the angle formed between the field lines and the Earth) varies with latitude. At the magnetic equator (the curving line across the Earth), field lines are parallel to the Earth's surface. The field lines become progressively steeper as one travels north toward the magnetic pole, where the field lines are directed straight down into the Earth and the inclination angle is 90°. (B) Diagram illustrating four elements of geomagnetic field vectors that might, in principle, provide animals with positional information. The field present at each location on Earth can be described in terms of a total field intensity and an inclination angle. The total intensity of the field can be resolved into two vector components: the horizontal field intensity and the vertical field intensity. (Whether animals are able to resolve the total field into vector components, however, is not known.)

 

Figure 2
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  		Fig. 2. Orientation of hatchling loggerhead turtles in magnetic fields characteristic of three widely separated locations (marked by black dots on the map) along the migratory route. Generalized main currents of the North Atlantic gyre are represented on the map by arrows. In the orientation diagrams, each dot represents the mean angle of a single hatchling. The arrow in the center of each circle represents the mean angle of the group. Broken lines represent the 95% confidence interval for the mean angle. See text for details. (Modified from Lohmann et al., 2001Go.)

 

Figure 3
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  		Fig. 3. Orientation of young pied flycatchers held in captivity during the time of their first migration. Results are shown from birds maintained in the magnetic field of Frankfurt, Germany (left), and birds subjected to a sequence of magnetic fields approximately matching those that they encounter along the migratory pathway (right). During the time corresponding to the first leg of the migration (Leg 1), both groups of birds were significantly oriented toward the south-southwest, corresponding with the direction that they initially travel. Only the birds exposed to the magnetically simulated migration, however, adopted appropriate southeasterly orientation during the time corresponding to the second leg of the migration (Leg 2). The arrow in the center of each circle represents the mean angle of the group. Arrows that extend across the first inner circle (dotted line) denote distributions that are significantly oriented at P<0.05, whereas those that extend across the second inner circle (solid line) denote P<0.01. (Modified from Beck and Wiltschko, 1988Go; Wiltschko and Wiltschko, 2005Go.)

 

Figure 4
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  		Fig. 4. (A) A juvenile green turtle swimming in a magnetic navigation experiment. Turtles were placed into soft cloth harnesses and tethered to an electronic tracking device that monitored their orientation as they swam in a water-filled arena surrounded by a magnetic coil system (Lohmann et al., 2004Go). (B) Evidence for a magnetic map in green turtles. Juvenile turtles were captured in feeding grounds near the test site in Melbourne Beach, Florida, USA. Each turtle was exposed to a magnetic field that exists at one of two distant locations along the coastline (represented by stars). Turtles exposed to the field from the northern site swam approximately southward, whereas those exposed to the field from the southern site swam approximately north. Conventions are as in Fig. 2. (Modified from Lohmann et al., 2004Go.) See text for details.

 

Figure 5
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  		Fig. 5. A magnetic map in spiny lobsters. Lobsters from the middle Florida Keys were subjected to magnetic fields that exist in locations north or south of the location where they were captured. As with juvenile green turtles, lobsters subjected to the field from the northern site oriented approximately southward, whereas those exposed to the field from the southern site crawled approximately north. The open triangle outside each orientation diagram indicates the actual direction to the capture site from the test site. In each case, lobsters responded as if they had been displaced to the locations marked by the stars rather than orienting in the direction that was actually toward the capture site. Conventions are as in Fig. 2. (Modified from Boles and Lohmann, 2003Go.)

 

Figure 6
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  		Fig. 6. Isoclinics (lines of equal magnetic inclination) and isodynamics (lines of equal magnetic field intensity) in the Indian Ocean, along the east coast of Africa. Isoclinics run approximately east–west in this region and are shown in 1° contours. Isodynamics run approximately north–south and are represented in 1000 nT increments. Thus, in this geographic area, the two sets of isolines form a nonorthogonal grid that birds, sea turtles, fish or other animals might, in principle, exploit as a kind of bicoordinate magnetic map. Whether any animal does this is not known. (Modified from Lohmann et al., 1999Go.) Isolines were derived for the year 1995 from the International Geomagnetic Reference Field (IGRF) 1995 model.

 

Figure 7
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  		Fig. 7. Map of magnetic field inclination along the southeastern coast of the United States. The isolines represent isoclinics (lines of equal magnetic field inclination). In this part of the world the isoclinics trend east–west, while the coastline is aligned approximately north–south. As a result, each area of coastline along the eastern seaboard is marked by a unique inclination angle. A similar pattern exists for the isolines of total intensity. A sea turtle navigating along the east coast to a particular coastal feeding or nesting area might thus hypothetically do so by detecting a single magnetic element such as inclination or intensity (see text for details).

 

Figure 8
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  		Fig. 8. A possible strategy for locating a specific target area using a single magnetic element such as inclination or intensity, illustrated here with an example of a sea turtle using the strategy to approach an oceanic island. The turtle needs to `know' the value of one magnetic element at the target and might also need some minimal information about the pattern of isolines in the region. Instead of attempting to steer directly toward the island, the animal swims on a path that is deliberately offset from the target by enough that the turtle will arrive at the appropriate magnetic isoline on a known side of the target. In this example, the turtle adopts a course that takes it well west of the island. Thus, when it arrives at the appropriate isoline, it knows to turn right and swim along the isoline toward the southeast rather than turning left and following the isoline northwest. Because the isoline intersects the target, the turtle locates the goal.

 

Figure 9
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  		Fig. 9. Strategy for traveling along a complex migratory route using magnetic waymark navigation. In this hypothetical example, a migratory shark has learned to return to a feeding area on the west side of a peninsula. It began life with the ability to perceive magnetic inclination and intensity, as young sea turtles do (Lohmann and Lohmann, 1994Go; Lohmann and Lohmann, 1996Go), but as it gained migratory experience, it learned that the easiest way to complete its route is to ignore the regional pattern of isolines and instead change direction at several crucial locations, each marked by a distinctive magnetic field. Eventually, the shark learns to associate each magnetic waymark with a direction of swimming, and the migration is completed as a series of sequential steps, with each magnetic waymark triggering the appropriate direction for the next segment. (In reality, whether sharks can derive positional information from the Earth's field is not known.)

 

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© The Company of Biologists Ltd 2007