Swimming the isoline
The strategy of using a single magnetic parameter to locate a nesting beach has usually been
discussed in the context of continental coastlines (Lohmann and Lohmann 1994, 1998;
Lohmann et al. 1999). A similar strategy, however, can potentially be used by turtles
or other animals to locate any oceanic target, including remote islands. As with locating
a nesting area along a coastline, the animal must know at least one magnetic element
(such as intensity or inclination) that exists at the island, so that it can recognize the
magnetic isoline on which the island lies. All that is then required is for the animal to adopt
a heading that is sufficiently to one side or the other of the target that, when the isoline
is intersected, the animal knows which direction to swim along the isoline to intersect the
island (Figure 5, top panel).
A similar strategy was used by mariners at a time when latitude, but not longitude, could
be measured. Instead of steering directly toward an island, a ship’s navigator would
deliberately chart a course that would cause the ship to arrive at the appropriate latitude
either significantly east or west of the target. The ship would then sail along the latitude
in the direction that the target was known to lie until the island was reached.
Mariners referred to this strategy as ‘‘sailing the latitude.’’ For turtles, it might be called
‘‘swimming the isoline.’’ Whether the strategy is used is not known.
Oceanic magnetic maps 59
Figure 5. Two different hypothetical strategies of magnetic navigation for sea turtles and other
marine animals. (Top) A possible strategy for reliably finding a specific oceanic region or target area
(such as a remote island) using a single magnetic element. The animal (turtle in this example) would
need to ‘know’ the value of one magnetic element such as inclination or intensity at the target and
might also need some minimal information about the pattern of isolines in the region. Instead of
attempting to steer straight 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
60 K. J. Lohmann & C. M. F. Lohmann
discussed in the context of continental coastlines (Lohmann and Lohmann 1994, 1998;
Lohmann et al. 1999). A similar strategy, however, can potentially be used by turtles
or other animals to locate any oceanic target, including remote islands. As with locating
a nesting area along a coastline, the animal must know at least one magnetic element
(such as intensity or inclination) that exists at the island, so that it can recognize the
magnetic isoline on which the island lies. All that is then required is for the animal to adopt
a heading that is sufficiently to one side or the other of the target that, when the isoline
is intersected, the animal knows which direction to swim along the isoline to intersect the
island (Figure 5, top panel).
A similar strategy was used by mariners at a time when latitude, but not longitude, could
be measured. Instead of steering directly toward an island, a ship’s navigator would
deliberately chart a course that would cause the ship to arrive at the appropriate latitude
either significantly east or west of the target. The ship would then sail along the latitude
in the direction that the target was known to lie until the island was reached.
Mariners referred to this strategy as ‘‘sailing the latitude.’’ For turtles, it might be called
‘‘swimming the isoline.’’ Whether the strategy is used is not known.
Oceanic magnetic maps 59
Figure 5. Two different hypothetical strategies of magnetic navigation for sea turtles and other
marine animals. (Top) A possible strategy for reliably finding a specific oceanic region or target area
(such as a remote island) using a single magnetic element. The animal (turtle in this example) would
need to ‘know’ the value of one magnetic element such as inclination or intensity at the target and
might also need some minimal information about the pattern of isolines in the region. Instead of
attempting to steer straight 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
60 K. J. Lohmann & C. M. F. Lohmann
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