Available online at www.sciencedirect.com
The magnetic map of hatchling loggerhead sea turtles
Kenneth J Lohmann, Nathan F Putman and Catherine MF Lohmann
Young loggerhead sea turtles (Caretta caretta) from eastern
Florida, U.S.A., undertake a transoceanic migration in which
they gradually circle the North Atlantic Ocean before returning
to the North American coast. Hatchlings in the open sea are
guided at least partly by a ‘magnetic map’ in which regional
magnetic fields function as navigational markers and elicit
changes in swimming direction at crucial locations along the
migratory route. The magnetic map exists in turtles that have
never migrated and thus appears to be inherited. Turtles derive
both longitudinal and latitudinal information from the Earth’s
field, most likely by exploiting unique combinations of field
inclination and intensity that occur in different geographic
areas. Similar mechanisms may function in the migrations of
diverse animals.
sea, and then migrate offshore to the Gulf Stream current.
These young turtles become entrained in the North
Atlantic Subtropical Gyre, the circular current system
that flows around the Sargasso Sea [10]. Many loggerheads gradually migrate around the entire North Atlantic
basin before eventually returning to the North American
coast [11,12].
Address
Department of Biology, University of North Carolina, Chapel Hill, NC
27599, United States
Magnetic compasses and maps
Corresponding author: Lohmann, Kenneth J ([email protected])
Current Opinion in Neurobiology 2012, 22:336–342
This review comes from a themed issue on
Neuroethology
Edited by Michael Dickinson and Cynthia Moss
Available online 30th November 2011
0959-4388/$ – see front matter
# 2011 Elsevier Ltd. All rights reserved.
DOI 10.1016/j.conb.2011.11.005
Introduction
The ability of animals to guide themselves unerringly
during long-distance migrations has inspired both awe
and envy in humans, a species that has only recently,
through global positioning technology, achieved a
measure of parity with the skills of elite animal navigators.
Nowhere are the navigational abilities of animals more
stunning than in the marine environment, where various
fish [1–3], reptiles [4,5], birds [6,7], and mammals [8,9]
routinely traverse vast expanses of seemingly featureless
sea. Understanding how these migrations are accomplished has posed a daunting challenge for biologists
who have long struggled to explain navigational mechanisms that appear to border on magic.
One of the longest and most spectacular marine
migrations is that of the loggerhead turtle (Caretta caretta).
Hatchling loggerheads from Florida, U.S.A., emerge from
underground nests on oceanic beaches, scramble to the
Current Opinion in Neurobiology 2012, 22:336–342
During the past decade, evidence has accumulated that
young loggerheads guide their open-sea movements, at
least in part, by exploiting positional or ‘map’ information
inherent in the Earth’s magnetic field [13,14,15,16]. In
this review, we summarize what is known about the
magnetic map of loggerheads.
Among the various sensory cues that ocean migrants
exploit while navigating, the Earth’s magnetic field is
particularly pervasive [17,18]. Unlike most potential
sources of directional and positional information, the
geomagnetic field is present day and night, remains
unaffected by weather and season, and exists throughout
the ocean, regardless of depth. By the late 1980s, when
studies on sea turtle navigation began in earnest, it was
known that diverse animals ranging from molluscs to
birds use the Earth’s field as a compass for maintaining
direction (e.g. toward north or south) [19]. Experiments
soon revealed that a magnetic compass exists in hatchling sea turtles [20,21] and, in combination with visual
cues and ocean wave direction, guides hatchlings as they
swim offshore from the Florida coast to the Gulf Stream
current [22,23].
Once in the Gulf Stream, turtles undertake a transoceanic migration that requires more than just a compass
to complete; turtles also need positional information to
know where to change swimming direction. For
example, the North Atlantic Subtropical Gyre provides
an ideal warm-water nursery habitat for growing loggerheads, but turtles along the northern boundary are at
risk of being carried by currents into fatally cold waters
that lie to the north [10,23]. Similarly, turtles that stray
south of the gyre may be swept into South Atlantic
currents and transported far from their normal range. An
ability to determine geographic position, and to adjust
swimming direction at crucial locations along the
migratory route, might therefore have considerable
adaptive value.
Information about geographic position can potentially be
derived from the predictable geographic variation of
Earth’s magnetic field [24–26]. For example, magnetic
www.sciencedirect.com
The magnetic map of hatchling loggerhead sea turtles Lohmann, Putman and Lohmann 337
field lines intersect Earth’s surface at specific inclination
angles, which become progressively steeper as one moves
from the magnetic equator toward the magnetic poles.
Similarly, field intensity (strength) varies predictably
across the Earth’s surface, but in a direction that differs
from that of inclination. As a consequence, most open-sea
regions have a unique magnetic signature comprised of a
combination of inclination and intensity that exists
nowhere else in the ocean basin [14,27].
The magnetic map of loggerheads
To investigate whether young loggerheads can exploit
positional information in Earth’s magnetic field, hatchlings have been tested in various magnetic fields matching
those that exist in different locations along their migratory
route [13,14,15]. In all of these experiments, turtles
were collected from nests a few hours before they would
otherwise have emerged naturally. Hatchlings were
tested singly in a circular, water-filled arena surrounded
by a computerized coil system (Figure 1), which controlled the magnetic field in which each turtle swam.
Orientation was monitored by tethering the turtle to an
electronic tracking unit that relayed swimming direction
to a computer in an adjacent room.
In the first of these studies [13], hatchlings were exposed
to three magnetic fields that exist at widely separated
locations along the migratory route. Two of the fields
replicated ones along the northern boundary of the gyre,
while the third was like one near the southernmost gyre
edge. Turtles responded to the fields by swimming in
directions that would, in each case, help them remain
within the gyre and advance along the migratory pathway.
These results demonstrated that young loggerheads have
a guidance system in which regional magnetic fields
function as navigational markers and elicit changes in
swimming direction at crucial geographic boundaries.
Because the fields tested initially replicated ones along
the latitudinal extremes of the migratory route, the results
left open the question of whether turtles can also use the
Earth’s field to distinguish among geographic areas that
differ in longitude. To investigate, hatchling loggerheads
were exposed to magnetic fields that exist at two locations
with the same latitude but on opposite sides of the
Atlantic Ocean [14].
Turtles exposed to a field that exists on the east side of
the Atlantic, near the Cape Verde Islands, responded by
swimming southwest. In contrast, turtles exposed to a
field that exists on the west side of the Atlantic, near
Puerto Rico, swam approximately northeast. Both orientation responses appear likely to have functional significance in the migration. Near the Cape Verde Islands,
southwesterly orientation coincides with the southwestflowing Canary current and the migratory pathway. Swimming southwest might also help turtles avoid the southeast-flowing Guinea Current, which can potentially
displace turtles from the gyre. Near Puerto Rico, northeasterly orientation is likely to direct turtles into the
Antilles Current [14]. This current flows swiftly toward
the US coast, where most Florida loggerheads spend their
late juvenile years [23].
These results demonstrated, for the first time, that longitudinal information can be encoded into the magnetic map
of an animal. Before the study, a common view among
many animal navigation researchers was that magnetic
Figure 1
(a)
(b)
Vertical coil
power supply
Horizontal coil
power supply
Computer
Cable from
digital encoder
Current Opinion in Neurobiology
Experimental apparatus used to monitor orientation responses of hatchling loggerhead turtles to regional magnetic fields. (A) A hatchling loggerhead
swimming in a cloth harness. (B) Turtles were tethered to a rotatable tracker arm attached to a digital encoder, which relayed the direction of swimming
to a computer in a nearby building. The arena was enclosed by a magnetic coil system capable of replicating magnetic fields found at different
locations along the turtles’ migratory route. Modified from Lohmann and Lohmann [23].
www.sciencedirect.com
Current Opinion in Neurobiology 2012, 22:336–342
338 Neuroethology
cues, which typically vary more in a north–south rather
than an east–west direction, are likely used by animals as a
surrogate for latitude but not for longitude [28–31]. The
new findings indicate, however, that young loggerheads
exploit the Earth’s field as a kind of bicoordinate magnetic
map from which both longitudinal and latitudinal information can be extracted. This in turn implies that turtles
perceive at least two different geomagnetic features that
vary in different directions across the Atlantic [14].
The precise organization of the loggerhead magnetic map is
not known. Given that turtles detect both field inclination
[32] and intensity [27], a reasonable hypothesis is that the
map is structured around magnetic signatures comprising
unique combinations of these two parameters ([13,14];
see also magnetic compasses and maps). Because isolines of
inclination and intensity do not coincide with either latitude or longitude, and because they intersect at different
angles in different oceanic regions [24], the geographic
areas defined by different magnetic signatures are not of a
uniform size or shape. They also depend at least partly on
the sensitivity of turtles to each magnetic parameter.
The difficulty in relating magnetic signatures to latitude
and longitude raises questions about the conceptual
framework that has guided recent discussions of animal
navigation. Although much attention has focused on
identifying environmental cues that might serve as surrogates for latitude and longitude [29,31,33], sea turtles
appear to have evolved a navigational system largely
independent of these concepts, relying instead on recognition of ecologically important geographic areas defined
by distinctive magnetic fields.
Regardless of how the map is organized, growing evidence suggests that a surprising amount of information is
encoded into it. At present, turtles have been shown to
respond with oriented swimming to fields that exist in at
least 8 different locations along the migratory pathway
(Figure 2). In each case, the direction of orientation is
suitable for helping turtles remain in the gyre and
advance along the migratory route.
Active swimming versus passive drifting
When evidence was first presented that Florida loggerheads undertake a transoceanic migration, it was assumed
that turtles drift passively in the gyre currents until they
return to the North American coast [10]. The existence of
an elaborate set of responses to regional magnetic fields in
hatchling loggerheads suggests a very different paradigm:
that turtles actively direct their migration.
With an increased understanding of ocean currents, it has
become evident that passive drifting alone cannot account
for the migration of Florida loggerheads. For example,
analyses combining ocean circulation models and particle
tracking techniques (Figure 3) reveal that, if turtles were to
Current Opinion in Neurobiology 2012, 22:336–342
drift passively in geographic areas with magnetic fields
known to elicit oriented swimming, they would often
disperse in highly variable directions or, in some cases,
be carried by currents to locations where survival is unlikely. An interesting example of the latter occurs near the
coast of north Florida, where turtles drifting passively are
likely to linger for weeks in a shallow region along the
southeastern U.S. coast where predatory fish and birds are
abundant. Similarly, turtles that drift along the northern
boundary of the gyre may fail to pass reliably through ocean
regions where temperatures drop to lethal levels in winter
(Figure 3) [34]. Simulations of passive drifting at other
points in the gyre currents have also failed to produce
appropriate paths, implying that active swimming is essential to completing the migratory route.
A versatile navigation system
A remarkable aspect of the orientation responses elicited
by regional magnetic fields is that they are expressed by
hatchlings that have never been in the ocean. Thus,
turtles do not need migratory experience to recognize
and respond to fields that mark the migratory pathway. In
addition, hatchlings do not need to pass through a specific
sequence of locations or magnetic fields before responding to fields that they would not, under natural conditions,
encounter for weeks, months, or sometimes even years
after entering the sea.
The ability of young turtles to respond to a variety of fields
independently of when they encounter them may have
evolved because ocean currents exert a strong but unpredictable influence on the path that a young turtle follows.
For example, a hatchling that reaches the center of the Gulf
Stream may be carried rapidly across the Atlantic, whereas
one on the periphery of the current may be spun off into an
eddy and lag far behind. As a result, the time that it takes to
complete the transoceanic migration, or to arrive at a
particular region along the way, probably varies greatly
among individuals. Moreover, analyses of ocean currents
indicate that a range of migratory trajectories is possible,
suggesting that there is no single, uniform migratory route
that all turtles follow and thus no reliable sequence of fields
encountered along the way (NF Putman, PhD thesis,
University of North Carolina, 2011). Given these uncertainties, there are clear advantages to a system comprised of
location-dependent responses, in which turtles respond to
specific regional fields that exist in particular geographic
areas if and when they encounter them.
An interesting question is how turtles can evolve a navigational system based on magnetic fields that exist in different
geographic areas, even though the Earth’s field gradually
changes over time [23]. Because poor navigation in young
turtles often equates to death, a likely answer is that strong
selective pressure acts to maintain an appropriate coupling
between the turtles’ directional swimming responses and
the magnetic fields that exist at crucial geographic locations
www.sciencedirect.com
The magnetic map of hatchling loggerhead sea turtles Lohmann, Putman and Lohmann 339
Figure 2
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Current Opinion in Neurobiology
Orientation of hatchling loggerheads in magnetic fields characteristic of widely separated locations along the migratory route. The fields used in
experiments replicated ones that exist at the locations on the map marked by black dots; data are from [13,14,15], and (NF Putman, PhD thesis,
University of North Carolina, 2011). Generalized main currents of the North Atlantic subtropical 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 indicates the mean angle of the
group. The shaded sector represents the 95% confidence interval for the mean angle. Each group of turtles was significantly oriented at P < 0.05 or better
(see [13,14,15] for statistical details). In each case, the direction of orientation was suitable for helping turtles remain in the gyre and advance along the
migratory route. Near the coast of northern Florida, southeasterly orientation presumably moves turtles farther into the Gulf Stream, reducing chances of
straying into cold water north of the gyre [13]. Midway across the Atlantic in the northern part of the gyre, east-northeast orientation is likely to help turtles
cross the Atlantic and avoid lingering in oceanic regions that become cold during part of the year [15]. Near northern Portugal, swimming south
presumably helps turtles remain in the warm gyre currents [15]. Near southern Portugal, swimming southwest may help turtles move offshore and
distance themselves from the shallow, predator-rich coastal waters of northern Africa [15]. Near the Cape Verde Islands, southwesterly orientation
presumably helps turtles start back toward the west side of the Atlantic [14]. Near the southernmost boundary of the gyre, northwesterly orientation
coincides with the migratory route back toward North America [13]. Near Barbados, swimming north may help turtles remain in the gyre and avoid being
carried into the Caribbean (NF Putman, PhD thesis, University of North Carolina, 2011). Near Puerto Rico, northeasterly orientation is likely to guide turtles
toward the main gyre currents moving back toward North America [14].
www.sciencedirect.com
Current Opinion in Neurobiology 2012, 22:336–342
340 Neuroethology
Figure 3
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Current Opinion in Neurobiology
Likely trajectories of passively drifting turtles derived from an ocean circulation model. In this simulation, virtual turtles (particles) were released from 8
points (black dots) in the North Atlantic that correspond to the locations of regional magnetic fields used in laboratory experiments (Figure 2). Ocean
circulation was modeled using the Global Hybrid Coordinate Ocean Model [48], which is based in part on oceanographic measurements compiled over
a period of years. To accommodate seasonal and annual variation in ocean circulation, one virtual turtle was released from each point on the first day
of the month for the period of 2005–2008. The particle-tracking program Ichthyop (v. 2) was used to calculate the daily positions of virtual turtles [49].
Gray lines indicate the paths of turtles drifting passively for 60 days in the surface layer. These simulations, and others like them, imply that
loggerheads are unlikely to migrate successfully around the gyre by drifting passively as proposed by Carr [10]. Instead, directed swimming at various
locations along the migratory pathway appears essential.
along the migratory route [13,14,15]. For example,
under present conditions, turtles that stray out of the gyre
are probably eliminated from the population, whereas those
with orientation responses that keep them safely within are
favored by natural selection. Similarly, as the field that
exists in a specific region of the migratory pathway changes,
only those turtles that respond to the new field in a way that
allows them to migrate successfully will survive to pass on
their genes.
Mechanism of magnetic field detection
The neural mechanisms that underlie magnetic field
detection have not been clearly established in any animal
[26]. The status of sea turtles as threatened species limits
their utility in neurobiological research. In other animals,
however, three main mechanisms of magnetoreception
have been proposed: electromagnetic induction, magnetite, and chemical magnetoreception [35,36].
Mechanisms based on electromagnetic induction appear
unlikely for turtles. Marine animals swimming through
Current Opinion in Neurobiology 2012, 22:336–342
the Earth’s field induce small electrical currents in the
surrounding water, so it may be possible for sharks or
other animals with a highly sensitive electric sense to
detect magnetic fields indirectly [36,37]. Sea turtles,
however, appear to lack electroreceptors.
A second hypothesis is that crystals of the magnetic
mineral magnetite function in magnetic field detection
[38]. Possible magnetite-based receptors have been
identified in rainbow trout [39] and pigeons [40]. Preliminary evidence for magnetite in sea turtles has also been
obtained [41], but whether magnetite underlies magnetoreception in turtles is not known.
A third hypothesis is that magnetoreception is accomplished through a complex set of chemical reactions
associated with the visual system [42,43]. Some evidence
consistent with this hypothesis has been obtained in
birds, flies, and other animals [44], [Mouritsen, in this
issue], but the idea has not yet been investigated in
turtles.
www.sciencedirect.com
The magnetic map of hatchling loggerhead sea turtles Lohmann, Putman and Lohmann 341
An interesting possibility is that two different mechanisms of magnetoreception coexist in some animals
[25,26,45]. In birds, it has been proposed that a compass
sense based on chemical magnetoreception exists in the
eye, while a map sense based on magnetite exists in the
beak. According to this idea, two different mechanisms
evolved, each to detect different aspects of the field; the
compass receptors detect field direction, whereas the map
receptors detect field intensity, inclination, or both.
Whether dual magnetoreceptor systems exist in turtles
is not known.
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Conclusions
Loggerhead sea turtles embarking on their first migration
possess a magnetic map in which regional magnetic fields
characteristic of specific oceanic areas elicit orientation
responses that help steer turtles along the migratory
route. Couplings of regional fields with directional swimming appear to provide the building blocks from which
natural selection can sculpt responses that guide first-time
migrants along complex migratory pathways. Because the
direction that a young turtle travels depends both on the
direction that it swims and the direction that it is carried
by ocean currents, a thorough understanding of sea turtle
navigation probably requires integration of behavioral
findings with oceanographic analyses.
The magnetic map of young loggerheads is most likely
based on magnetic signatures (unique combinations of
magnetic inclination angle and intensity that exist in
different oceanic regions), rather than on natural proxies
for latitude and longitude. Stepping outside of our own
deeply ingrained human spatial system appears essential
to understanding sea turtle navigation.
Finally, the mechanisms that guide young sea turtles on
their first transoceanic migration may function in diverse
migratory animals. The use of regional magnetic fields as
a system of navigational markers appears particularly
plausible for migrations of salmon [1] and other marine
fishes [2,3], but is also worth considering for other longdistance migrants such as marine mammals [9] and birds
[46,47].
Acknowledgements
We thank Susan Whitfield for assistance in producing the figures. The work
was supported by a grant from the National Science Foundation (IOS1022005) to KJL and CMFL.
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