Current Biology 21, 463–466, March 22, 2011 ª2011 Elsevier Ltd All rights reservedDOI 10.1016/j.cub.2011.01.057
Longitude Perception and Bicoordinate
Magnetic Maps in Sea Turtles
Nathan F. Putman,1,* Courtney S. Endres,1
Catherine M.F. Lohmann,1and Kenneth J. Lohmann1,*
1Department of Biology, University of North Carolina,
Chapel Hill, Chapel Hill, NC 27599, USA
Long-distance animal migrants often navigate in ways that
imply an awareness of both latitude and longitude [1–3].
Although several species are known to use magnetic cues
as a surrogate for latitude [4–8], it is not known how any
animal perceives longitude [1, 9–11]. Magnetic parameters
appear to be unpromising as longitudinal markers because
they typically vary more in a north-south rather than an
east–west direction [1, 2, 9, 10]. Here we report, however,
that hatchling loggerhead sea turtles (Caretta caretta) from
Florida, USA, when exposed to magnetic fields that exist at
two locations with the same latitude but on opposite sides
of the Atlantic Ocean, responded by swimming in different
their circular migratory route. The results demonstrate for
the first time that longitude can be encoded into the
turtles also assess north-south position magnetically [4, 8,
12], the findings imply that loggerheads have a navigational
system that exploits the Earth’s magnetic field as a kind of
bicoordinate magnetic map from which both longitudinal
and latitudinal information can be extracted.
Results and Discussion
How animals that migrate long distances determine their
geographic position has been debated for more than a century
[1, 2, 13]. Several animals are now known to determine
in the Earth’s magnetic field [4–8]. Some migrants, however,
can also determine their geographic position east to west
areas varies primarily with latitude, extracting longitudinal
information from the field appears to be difficult or impossible
tude perception, however, have remained enigmatic.
Hatchling loggerhead turtles (Caretta caretta) from eastern
after entering the sea for the first time. Hatchlings initially swim
eastward to the North Atlantic subtropical gyre (the circular
current system that flows around the Sargasso Sea) and then
remain within the gyre for several years, during which they
gradually migrate around the Atlantic before returning to the
North American coast [12, 17].
Sea turtles use magnetic cues to approximate their position
along a north-south axis [4, 8]. To determine whether logger-
positions along an east-west axis, we subjected hatchlings to
along the migratory route but on opposite sides of the Atlantic
Ocean. Each location had the same latitude but a different
longitude (Figure 1A). Turtles were tested in a circular, water-
filled orientation arena surrounded by a computerized coil
system, which was used to control the magnetic field in which
eachturtleswam.Each hatchling wastethered toan electronic
tracking unit that relayed the turtle’s swimming direction to
Turtles exposed to a field like one that exists on the west
side of the Atlantic near Puerto Rico swam approximately
northeast (Figure 1B). Those exposed to a field like one that
exists on the east side of the Atlantic near the Cape Verde
Islands swam approximately southwest (Figure 1B). Both
groups were significantly oriented at p < 0.03 or less (Figure 1),
and the two distributions were significantly different (Watson
test, p < 0.002). Thus, the results show that loggerhead turtles
can distinguish between magnetic fields that exist at different
longitudes along the same latitudinal parallel.
Functional Significance of Orientation Responses
The orientation behavior elicited by the two fields is consistent
with the interpretation that these responses have functional
significance in the migration. Near the Cape Verde Islands,
southwesterly orientation coincides with both the migratory
pathway and the direction of the wide, slow-moving Canary
Current (Figure 1; ). Swimming southwest in this area
presumably helps turtles move back toward North America.
It might also help them avoid the Guinea Current, the south-
east-flowing branch of the Canary Current that can potentially
displace turtles from the gyre and carry them along the coast
Near Puerto Rico, the gyre currents are slowed and diverted
as they meander through the numerous islands and reefs of
the Antilles and Bahama Archipelagos, but in deeper water
to the northeast, the Antilles Current flows unobstructed
toward Florida [18, 19]. Northeasterly orientation near Puerto
Rico is thus likely to help turtles stay within the gyre and
embed in currents that facilitate movement back toward the
North American coast, where most Florida loggerheads spend
their late juvenile years .
These results add to the growing evidence that specific
regional magnetic fields elicit orientation responses that help
young loggerheads remain in the gyre and advance along the
migratory route [4, 12, 20]. The hatchlings that we tested had
never been in the ocean, demonstrating that turtles do not
need migratory experience in order to recognize and respond
to fields that exist along their oceanic pathway. Because the
Earth’s field gradually changes, this orientation behavior is
consistent with the hypothesis that strong selective pressure
acts to maintain an approximate match between the
responses of turtles and the fields that mark critical positions
along the migratory pathway at any point in time [12, 17].
Organization of the Turtles’ Magnetic Map
Our results indicate that, for sea turtles, the problems of
perceiving longitude and perceiving latitude share a common
*Correspondence: firstname.lastname@example.org (N.F.P.), klohmann@email.
solution. In each case, magnetic information can be used to
distinguish among different geographic regions.
The ability of turtles to derive both latitudinal and longitu-
dinal information from the Earth’s field necessarily implies
that turtles exploit at least two different geomagnetic features
that vary in different directions across the Atlantic. Thus, the
results demonstrate that turtles use a kind of bicoordinate
magnetic map in position finding, an ability that has long
before been demonstrated.
The precise way in which the turtles’ magnetic map is orga-
nized is not yet known. Along the migratory route, the four
magnetic parameters that might hypothetically provide a turtle
with positional information all have isolines that trend east-
west and intersect meridians on both sides of the Atlantic (Fig-
ure 2). Thus, although any one of these parameters might be
used as a surrogate for latitude, none of them, by themselves,
appear to be suitable for assessing longitude over the entire
It is not necessary, however, to assume that turtles exploit
one magnetic parameter as a surrogate for latitude and
another as a proxy for longitude. Nearly all geographic regions
along the migratory route, including the two used in our exper-
iment, have fields defined by unique combinations of inclina-
tion and intensity, two magnetic parameters loggerheads
detect (Figure 3) [4, 20]. A reasonable hypothesis is thus that
turtles can distinguish among different longitudes using these
unique ‘‘magnetic signatures.’’ Such a strategy appears
feasible in that the fields that exist in locations with the
same latitude but on opposite sides of the Atlantic always
differ in both inclination and intensity (Figure 3), with the differ-
ences exceeding what turtles are known to perceive [4, 8, 20].
Likewise, use of ‘‘magnetic signatures’’ might also explain
how turtles distinguish among geographic regions that differ
in latitude [4, 12]. Viewed in this way, turtles might have a bi-
coordinate magnetic map based on inclination and intensity,
one that does not encode latitude and longitude per se but
that nonetheless provides turtles with both east-west and
In stating that turtles have a bicoordinate magnetic map, we
use the term ‘‘map’’ in accordance with recent usages [2, 23–
27] that make no assumptions about the nature of the internal
spatial representation (if any) that an animal has. It is possible,
and perhaps even likely, that hatchling turtles lack any real
conception of their geographic location and that they advance
blindly along their migratory route by swimming in particular
directions in response to specific magnetic fields. It is also
possible that other cues besides magnetic fields play a role
in guiding the transoceanic migration and that the navigational
system of young turtles provides a foundation to which addi-
tional strategies or mechanisms needed for the navigational
tasks of older turtles [8, 28] can be added during maturation.
Indeed, the experience of migrating across the Atlantic and
back may provide turtles with an extended opportunity to
acquire information (magnetic and otherwise) that can be
incorporated into later navigational processes.
Whether animals other than sea turtles extract both latitudi-
nal and longitudinal information from the Earth’s field is not
known. In principle, some animals might have bicoordinate
maps in which each of the two axes depends on a different
kind of sensory information; moreover, different ways of as-
sessing longitude might have evolved in different animal
groups. It is interesting to note that human navigators first
solved the longitude problem in a very different way than
turtles: by developing a precise and reliable clock that allowed
time of day at a given location to be compared with that at
a distant site . For an animal to determine longitude in
a similar way, it would presumably need a biological clock
that did not reset to local time (or at least not immediately). A
recent experiment designed to investigate whether migratory
birds might assess longitude using two clocks, one of which
synchronizes to local time more rapidly than the other, failed
to find evidence in support of this mechanism . These
results are consistent with the interpretation that birds, like
turtles, have evolved a way to assess longitude that is inde-
pendent of time-keeping. Other possible mechanisms that
animals might hypothetically use involve olfactory cues [30,
31], infrasound [32, 33], or the use of declination in geographic
areas where this parameter varies longitudinally .
along the migratory
Figure 1. The North Atlantic Subtropical Gyre and Orientation of Turtles to
Magnetic Fields near Puerto Rico and Cape Verde
(A) Schematic of the North Atlantic Subtropical Gyre (after ). Arrows indi-
cate the generalized main currents. Hatchling loggerheads were exposed to
the magnetic fields that exist at two locations (marked by black dots) with
the same latitude but on opposite sides of the Atlantic. The distance
between the two points is approximately 3700 km. The test site and natal
beach of the turtles is marked by the open star on the east coast of Florida,
(B) Orientation of hatchling loggerheads tested in a magnetic field from the
west side of the Atlantic near Puerto Rico (left) and in a field from the east
side of the Atlantic near the Cape Verde Islands (right). Each dot represents
the mean angle of a single hatchling. The arrow in the center of each circle
magnitude of the mean vector r, with the radius of the circle corresponding
to r = 1. Turtles tested in the Puerto Rico field were significantly oriented
(r = 0.39, p = 0.03, n = 22, Rayleigh test), with a mean angle of 50?. Turtles
tested in the Cape Verde field were also significantly oriented (r = 0.34,
p = 0.02, n = 35), but in approximately the opposite direction (mean angle =
217?). Shaded sectors indicate the 95% confidence interval for the mean
angle. Data are plotted relative to geographic north (N = 0?).
Current Biology Vol 21 No 6
Regardless of these considerations, our results provide the
first demonstration that longitude can be encoded into the
ings demonstrate the existence of bicoordinate magnetic
maps, which are capable of simultaneously providing animals
with both latitudinal and longitudinal information. Similar
mechanisms may help to explain some of the most impressive
feats of navigation in the animal kingdom, including those of
diverse long-distance migrants such as insects, fish, birds,
and marine mammals [3, 34–37].
Methods have been described in detail previously [4, 12]. Briefly, each turtle
was tethered to an electronic tracking unit in the center of a water-filled
orientation arena. The arenawas surrounded byacomputerized coilsystem
(description below), which was used to control the magnetic field in which
the turtles swam. Each turtle began its trial in a magnetic field matching
that found at the natal beach (inclination = 57.7?, intensity = 46.5 mT) and
was allowed to establish a course toward a dim light (an LED with peak
wavelength = 550 nm) in magnetic east. After 10 min, the light was turned
off and the magnetic field was simultaneously changed to either (1) a field
replicating one near Puerto Rico or (2) a field replicating one near the
Cape Verde Islands. Turtles were allowed to acclimate to the new field for
3 min. A computer then monitored the direction that each turtle swam
toward during the next 5 min and calculated a mean heading.
Each turtle was tested a single time under one of the two field conditions.
No more than two turtles from the same nest were tested in the same field.
The field used to approximate magnetic conditions near Puerto Rico had an
inclination of 46.4?and a total intensity of 39.0 mT (as assessed by four inde-
pendent measurements with an Applied Physics Systems tri-axial fluxgate
magnetometer, model 520A). The field used to approximate conditions
near the Cape Verde Islands had an inclination of 26.1?and an intensity of
35.0 mT. The experimental fields were selected on the basis of estimates
provided by the International Geomagnetic Reference Field Model
(IGRF-10)  for July 2007 (the time when the experiment began) using lati-
tude20.0?N,longitude 65.5?WforPuertoRico andlatitude20.0?N,longitude
30.5?W for the Cape Verde Islands. The IGRF-10 declination estimates for
the target locations were 213.1?for Puerto Rico and 212.9?for the Cape
Verde Islands. Experiments were conducted in Melbourne Beach, Florida,
USA (declination estimate = 26.0?).
The coil system consisted of two different coils arranged orthogonally.
The coil controlling the horizontal component of the magnetic field
measured 2.41 m on a side, and the coil controlling the vertical component
measured 2.54 m. Both were constructed in accordance with the four-coil
design by Merritt et al. . Turtles were restricted to the center of the coil
in an area defined by a horizontal circle of radius 42 cm and a vertical
area of about 5 cm; in this region, calculated  and measured deviations
from perfect field uniformity were less than 1%.
We thank K. Stapput and E.M. Putman for assistance with experiments.
Funding was provided by National Science Foundation grants IOS-
0718991 and IOS-1022005 to C.M.F.L. and K.J.L.; PADI Foundation and
Lerner-Gray grants were provided to N.F.P.
Received: December 20, 2010
Revised: January 8, 2011
Accepted: January 23, 2011
Published online: February 24, 2011
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